30

Disorders of thyroid function

30

Disorders of thyroid function

30

Disorders of thyroid function

30

Disorders of thyroid function

  30 Disorders of thyroid function

Lothar Thomas

30.1 Physiological function of the thyroid

Thyroid hormones influence numerous physiological and biochemical events in the cell. Throughout the body, they are the main regulators of the basal metabolic rates and influence the metabolism of every class of foodstuff. They regulate the growth and differentiation of cells and are especially active in liver, heart, kidney, skeleton, gastrointestinal system, and the central nervous system. However, each tissue responds in a characteristic fashion. Thyroid hormones influence intermediary metabolic processes such as the activities of other hormones or the metabolism of drugs. They are the main regulators of obligate heat production in homothermous creatures due to their effect on the basal metabolic rate and they stimulate respiration. Thyroid hormones influence cellular functions in the long term range via nuclear receptors and time enhanced via extra nuclear binding proteins /1/.

30.1.1 Hypothalamic-pituitary-thyroid axis

The hypothalamic-pituitary-thyroid axis is a classic system sub serving control of the hormone secretion by the thyroid /23/:

  • Thyrotropin releasing hormone (TRH) at hypothalamic level
  • Thyroid stimulating hormone (TSH) at pituitary level
  • Tetraiodothyronine (thyroxine, T4), triiodothyronine (T3), and reverse T3 (rT3) at peripheral cellular level.

Refer to Fig. 30.1-1 – Hypothalamic-pituitary-thyroid axis

Pulsatile and non pulsatile secretion of TRH by the hypothalamus leads to the release of TSH from the pituitary, which in turn stimulates the thyroid to release the pro hormone T4 into the circulation. T4 is converted peripherally into metabolically active T3 and metabolically inactive rT3. T3 and T4 feedback inhibition controls TRH and TSH secretion.

Whereas all of the circulating T4 is secreted directly by the thyroid, the same is true for only 20% of the circulating T3. The other 80% results from the peripheral mono iodination of T4 to T3. Most circulating T4 and T3 is bound to plasma proteins such as thyroxine binding globulin (TBG), thyroxine binding pre albumin (transthyretin), and albumin. Only about 0.03% of T4 and 0.3% of T3 exist in free form (FT4 and FT3). Only FT4 and FT3 are metabolically active.

The serum levels of thyroid hormones are generally maintained within a narrow range by means of feedback inhibition of hypothalamic TRH and pituitary TSH. Circulating thyroid hormone is registered by the cells of the para ventricular nucleus of the hypothalamus, where T4 is converted to T3 by mono iodination. If there is a decrease in intracellular T3, TRH is released into the hypothalamic pituitary circulation and stimulates anterior pituitary thyrotropic cells to produce TSH. TSH levels increase in the plasma and the thyroid responds by releasing T4 and T3. The resulting increase in the concentration of thyroid hormones exerts feedback inhibition on the secretion of TRH and TSH.

The feedback control mechanism is dependent upon the presence of thyroid hormone receptors, which bind to the promoters of TRH and TSH subunit genes and regulate their expression. In the presence of their ligand, T3, thyroid hormone receptors mediate ligand dependent repression of the transcription of these genes. In the absence of T3, the transcription rate is not simply returned to baseline, but ligand independent over expression occurs. This results in a disproportionate increase in TSH with a log-linear relationship between the concentration of TSH and FT3 or, for example, FT4.

Refer to:

In a system like this, small linear decreases in thyroid hormone serum level lead to an exponential increase in TSH and small increases in thyroid hormones lead to a sharp decline in TSH. This is why TSH determination is much more sensitive than thyroid hormone measurement for diagnosing thyroid dysfunction. Early measurement of TSH can detect minor changes in T4 and T3 levels that are not yet clinically apparent.

FT4 concentration within the reference interval and in association with:

  • Elevated TSH level indicates latent (subclinical) hypothyroidism
  • Decreased TSH level indicates latent (subclinical) hyperthyroidism.

The hypothalamic-pituitary-thyroid axis responds relatively slowly. In a hypothyroid patient receiving L-thyroxine treatment, for example, it takes 4–6 weeks for TSH to return from elevated level to normal. It also takes several months for low TSH level to return to the reference interval in patients treated for hyperthyroidism. These situations as well as extra thyroidal factors such as critical illness, medication use, abnormal T4 and T3 protein binding, or dysproteinemia can result in discordance between TSH levels and concentrations of T3 and T4.

30.1.2 Thyroid disorders

Thyroid disorders are the second most common type of endocrine disorders worldwide, second only to diabetes mellitus. For instance, the prevalence of overactive thyroid gland (hyperthyroidism) in women is 2% while the prevalence of underactive thyroid gland (hypothyroidism) is 1%. The prevalence is 5–10 times lower in men.

Hyperthyroidism and hypothyroidism are classified as:

  • Primary: due to thyroid disease or thyroid dysfunction mediated by autoantibodies
  • Secondary: due to pituitary dysfunction or thyroid involvement in systemic disease
  • Tertiary: due to dysfunction of the hypothalamic-pituitary-thyroid axis.

According to the clinical symptoms and laboratory results /4/:

  • Subclinical means in the first instance no more than that the result of thyroid stimulating hormone (TSH) has changed but that thyroid hormones remain normal. TSH is the main indicator of thyroid dysfunction.
  • Overt describes functional disorders of the thyroid of endogenous or exogenous causes and levels of TSH, free T4 or free T3 outside of their reference intervals.

Thyroid enlargement and thyroid nodules are common in the general population, with a prevalence of 20–36%. In almost all cases, they are caused by dietary iodine deficiency with subsequent low iodine content of the thyroid (Section 10.11 – Iodine). A compensatory increase in the synthesis of paracrine and autocrine growth factors modulated by TSH leads to thyroid hypertrophy. In Germany, 10–20% of goiters and thyroid nodules are functionally autonomous and account for half of all cases of hyperthyroidism /5/. Thyroid nodules caused by clonal aberrations are another group of thyroid disorders. The frequency increases with age and nodules can be detected on ultrasound in approximately 50% of individuals over the age of 60 years. Most, however, are benign. In the case of nodules with a diameter of greater than 1 cm, functional autonomy and medullary thyroid cancer must be ruled out by determining TSH and calcitonin and performing a fine needle aspiration biopsy.

Unless treated promptly, maternal hypothyroidism due to iodine deficiency or other causes can result in congenital hypothyroidism (cretinism) in the newborn. Neonatal screening for hypothyroidism by means of TSH determination is mandatory in many countries.

Disorders of thyroid function may represent adaptive responses to physiological states or be due to primary thyroid disease, systemic non-thyroidal illness, or dysfunction of the hypothalamic-pituitary-thyroid axis.

Adaptive physiological responses occur in the thyroid during childhood, old age, and pregnancy (Tab. 30.1-1 – Thyroid function and physiological states).

Reactive thyroid dysfunction in systemic disease is often due to a dysfunction of the hypothalamic-pituitary-thyroid axis (Tab. 30.1-2 – Adaptive response to non-thyroidal illness).

Primary functional disorders of the thyroid (hyper- and hypothyroidism) are common all over the world. Their incidence is dependent on a number of factors, the most important of which is inadequate iodine intake. In areas with high iodine supply, hypothyroidism is more common than hyperthyroidism, whereas hyperthyroidism dominates in areas with mild and moderate iodine supply. Primary hyperthyroidism and hypothyroidism can be divided into a number of nosological subtypes with different etiology, clinical presentation, prognosis, and outcome of therapy (Tab. 30.1-3 – Primary thyroid disorders associated with hyper-, hypo- and euthyroidism).

Overt thyroid dysfunction denotes the presence of disease with clinical symptoms. Latent or subclinical disease is defined biochemically because clinical symptoms are absent /67/. Refer to Tab. 30.1-4 – Subclinical thyroid function disorder.

Reference intervals for TSH and thyroid hormones in pregnant women are depending on the country /8/. Refer to Tab. 30.1-5 – Reference intervals for TSH and thyroid hormones in pregnant women.

Diagnosis of functional disorders of the thyroid is based on the sensitivity of the hypothalamic-pituitary-thyroid axis and are generally manifested by serum TSH changes and additional findings:

  • Overproduction of T4 by the thyroid or increased peripheral conversion of T4 to T3; FT4 and/or FT3 are elevated in serum (hyperthyroidism)
  • Reduced production of T4 by the thyroid; FT4 and/or FT3 are decreased in serum (hypothyroidism)
  • Insufficient peripheral conversion of T4 to T3 (low T3 syndrome) in severe systemic non-thyroidal illness; serum FT4 normal and FT3 decreased or low normal.

Important exceptions are:

  • Critically ill patients with hyperthyroidism may not show an adequate increase in FT3 despite clear FT4 elevation, due to a low T3 syndrome
  • The conversion of T4 to T3 may be increased while T4 production by the thyroid is normal or decreased; serum FT4 level is low or low normal and FT3 is normal or near the upper reference interval value. This situation can occur temporarily following hyperthyroidism. Due to persistent activation of T4 deiodinase, the increased conversion of T4 to T3 continues (hyperconversion thyrotoxicosis).

Goiter

A goiter is an abnormal enlargement of the thyroid gland. The presence of a goiter does not necessarily mean that the thyroid gland is malfunctioning. A goiter indicates there is a condition present which is causing the thyroid to grow abnormally. Most common factors causing thyroid gland to enlarge are:

  • Iodine deficiency. A lack of dietary iodine is the main cause of goiters in main parts of the world. The initial iodine deficiency may be made even worse by a diet high in hormone-binding foods such as cabbage, broccoli and cauliflower. The urinary iodine excretion is an estimate of the grade of endemic goiter (Tab. 30.1-6 – Severity of endemic goiter).
  • Graves’ disease; autoantibodies directed against thyroid tissue stimulate the production of excess thyroxine
  • Hashimotos thyroiditis; autoimmune disease resulting in underactive thyroid
  • Multinodular goiter; several fluid-filled or solid nodules have developed in both sides of the thyroid
  • Solitary thyroid nodules
  • Thyroid cancer
  • Pregnancy; human chorionic gonadotropin (hCG), may cause the thyroid gland to enlarge slightly
  • Inflammation can cause over- or underproduction of thyroid hormones. The inflamed thyroid causes pain and swelling.

30.1.3 Medication effects on the thyroid

Medications are known to adversely affect thyroid function /11/. Refer to Tab. 30.1-7 – Medication effects on the thyroid.

30.1.4 Diagnosis of thyroid disorders

In the majority of patients, a diagnosis can be made based on a selection of adequate laboratory tests aimed at the detection or exclusion of a thyroid disease or thyroid dysfunction. The selection depends on the clinical concerns. One must differentiate between methods aimed to assess /9/:

  • The function of the thyroid gland
  • The classification of a thyroid disease
  • The hypothalamic-pituitary-thyroid axis.

Laboratory tests for assessing thyroid gland function:

  • Thyroxine (T4); usually determined as free T4 (FT4)
  • Triiodothyronine (T3); determined as total or, more commonly, free T3 (FT3).

Laboratory tests for assessing thyroid disease:

  • Autoantibodies: anti-TPOAb, anti-TgAb, TSH receptor antibodies, inflammation markers (e.g., CRP).

Laboratory tests for assessing hypothalamic-pituitary-thyroid axis:

  • Thyroid stimulating hormone (TSH)
  • TSH releasing hormone test (TRH test).

Recommended diagnostic approaches to the different clinical questions /10/ are shown in:

The recommendation of the thyroid section of the German Endocrine Society are listed in Tab. 30.1-6 – Severity of endemic goiter.

The terms euthyroidism, hyperthyroidism, and hypothyroidism describe the biological effects of thyroid hormones in the peripheral tissues but do not always describe the functional status of the thyroid.

When diagnosing thyroid disorders and dysfunction, it is important to follow the recommendations provided by the relevant national and international societies and organizations.

30.1.4.1 Diagnostic approach using TSH and FT4

Thyroid status can be accurately screened with thyroid-stimulating hormone (TSH), and additional testing such as free thyroxine (FT4) is required only when TSH is abnormal. An inverse relationship usually exists between TSH and FT4, and findings can be interpreted as follows /6711/:

  • An increase in TSH and a decrease in FT4 confirms the diagnosis of overt hypothyroidism caused by thyroid gland failure
  • An increase in TSH together with a normal FT4 indicates latent (subclinical) hypothyroidism
  • A decrease in TSH below 0.1 mIU/L and an increase in FT4 and/or FT3 confirms the diagnosis of overt hyperthyroidism caused by thyroid gland failure
  • A TSH level in the range of 0.5 to 0.1 mIU/L and an increase in FT4 and /or FT3 indicates latent (subclinical) hyperthyroidism
  • A normal or subnormal TSH with a subnormal FT4 suggests that the patient may have hypothyroidism secondary to a decrease in TSH secretion: this combination of results suggests a diagnosis of hypopituitarism and may warrant an examination of hypothalamic-pituitary-thyroid axis.

30.1.4.2 Discordance between TSH and FT4

In hospitalized patients a diagnostic approach that focuses on TSH alone can be misleading, due to the effects of non-thyroidal illness or drugs that affect the metabolism of T4 and T3 and the secretion of TSH. This often may lead to confusing results.

Discordance between the levels of TSH and thyroid hormones can occur:

  • In results indicating latent (subclinical) thyroid disorder, in which the TSH level is abnormal but FT4 is normal
  • If thyroid hormone protein binding is abnormal. In this case, TSH is normal and TT4 (and TT3 less frequently) outside the reference interval; depending on the method used, FT4 and FT3 are usually normal
  • In hypothyroid patients receiving treatment with L-thyroxine, in the initial phase of treatment or following dose adjustment (Tab. 30.1-8 – Initial investigations in suspected thyroid disease). Because it takes 8 weeks to establish a new equilibrium in the hypothalamic-pituitary-thyroid axis, the change in FT4 precedes the change in TSH by several weeks.
  • Following treatment for hyperthyroidism; FT4 returns to normal much more quickly than TSH
  • In patients with unstable thyroid disease. The FT4 level changes much more quickly than that of TSH.
  • In the elderly; compared to younger individuals, they more often have slightly increased or decreased TSH levels with normal FT4 and T3 (FT3)
  • If the conversion of T4 to T3 is disrupted by drugs such as amiodarone, propylthiouracil, dexamethasone, iodine containing contrast medium, or, to a lesser extent, propranolol
  • In heparinized patients; an increase in the release of non esterified free fatty acids results in increased levels of FT4 and FT3.

30.1.4.3 Investigation of the cause thyroid disorder

Investigations for evaluation the cause of thyroid disorders are:

  • Thyroglobulin antibodies (anti-TgAb), thyroid peroxidase antibody (anti-TPOAb), TSH receptor antibodies (TRAb)
  • Thyroglobulin
  • Excretion of iodine in urine
  • Thyroid scan
  • Fine-needle biopsy.

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30.2 Thyroid-stimulating hormone (TSH)

Provided hypothalamic and anterior pituitary function are normal, the serum TSH level reflects the availability of thyroid hormones in the tissues. The TSH secretion changes logarithmically with arithmetic changes in serum concentrations of free T4 (Fig. 30.1-2 – Inverse log-linear relationship between FT4 and TSH). Therefore, alterations in serum FT4 that are within the reference interval will cause increases or decreases in serum TSH that are likely to be outside its reference interval /1/. TSH circulates in the blood as various isoforms that differ from pituitary TSH.

30.2.1 Indication

  • Differentiation of euthyroidism from all forms of hyperthyroidism, provided a high sensitivity immunoassay (detection limit < 0.01 mIU/L) is used /2/
  • Screening for primary hyperthyroidism
  • Detection of subclinical hyperthyroidism or subclinical hypothyroidism (in conjunction with FT4)
  • Monitoring of hyperthyroidism treatment in cases of treatment induced hypothyroidism; in the first few months and always in conjunction with FT4 determination
  • Detection of thyroid hormone resistance (in conjunction with FT4)
  • Screening for neonatal hypothyroidism
  • Screening for primary hypothyroidism
  • Assessment of thyroid function in non-thyroidal illness (in conjunction with FT4).

TRH test

  • Unclear cases where concurrent thyroid disorder is suspected (patients with severe non-thyroidal illness)
  • In individual cases of suspected latent (subclinical) hypothyroidism or hyperthyroidism
  • Hypothalamic or pituitary disorders.

30.2.2 Method of determination

Immunoassay

Two-site (sandwich) immunoassays (enzyme, fluorescence, luminescence, or immunometric) are used /3/. The assays are calibrated against the WHO second IRP 80/558. Four generations of TSH assays with different detection limits exist (Tab. 30.2-1 – Generation nomenclature for TSH assays). Most laboratories use third generation assays.

Neonatal TSH screening

In the first 1–2 days after birth, a capillary blood sample from the heel is collected on filter paper. After the blood has been allowed to dry for one hour, the filter paper is sent to the laboratory for analysis. TSH is determined from 3–8 mm disks of filter paper following elution. Although the TSH concentration in congenital hypothyroidism is usually > 15–20 mIU/L, the assay should have a detection limit of < 5 mIU/L.

TRH test

Three test procedures exist.

Intravenous: administration of 200 μg (400 μg) TRH in adults or 7 μg/kg body weight in children. Blood collection after 30 min. to determine TSH concentration in response to stimulation.

Nasal: nasal application of 2 mg TRH. Blood can be collected for TSH determination for up to two hours following TRH application, after which time plateau levels are reached.

Oral: administration of 40 mg TRH. Blood is collected 3–4 hours after TRH stimulation.

Regardless of which TRH test is used, blood must always be collected before TRH administration to determine the basal TSH concentration. The diagnostic significance of the results of the TRH test is shown in Tab. 30.2-2 – Diagnostic assessment of the TRH test.

30.2.3 Specimen

  • Serum: 1 mL
  • 1 blood spot (neonatal TSH screening)

30.2.4 Reference interval

Refer to Tab. 30.2-3 – Reference intervals for TSH.

30.2.5 Clinical significance

The main clinical scenarios for measurement of serum TSH are screening for thyroid dysfunction. However the TSH concentration is dependent on variables. In iodine sufficient geographical regions the reference interval for serum TSH in adults ages 20–59 years is 0.40–4.0 mIU/L. Age, sex, ethnicity, iodine intake, reproductive status, body mass index, assay specific properties are independent factors that modify TSH level. Because of clinical experiences a TSH level of ≥ 10 mIU/L is generally be considered elevated and may be indicative of subclinical hypothyroidism or overt hypothyroidism and levels below 0.40 mIU/L may be indicative of subclinical hyperthyroidism. Throughout the 24 hour cycle females have significant higher TSH levels than males, the magnitude of these differences did not exceed 0.1 mIU/L /4/.

30.2.5.1 Low TSH

The serum TSH level is a sensitive indicator for assessing the biological effects of T4 and T3. Current recommendations suggest that tests with a detection limit of less than 0.01 mIU/L should be used. Measured values with this limit are highly accurate and can be used to reliably assess the TSH threshold value of 0.10 mIU/L that indicates to the clinician that further investigation of hyperthyroidism is required.

In outpatients without clinical signs of hyperthyroidism or hypothyroidism, no further investigations are necessary if the TSH level is within the reference interval.

Values of 0.01–0.39 mIU/L may indicate subclinical or overt hyperthyroidism /5/ and FT4 and FT3 should also be determined in all cases, because low TSH values are often transitory, especially in the range of 0.1 to 0.4 mIU/L. Some healthy elderly people might have low serum TSH level without identifiable thyroid disease. In these individuals, a low serum TSH might be due to a change in the set point of the hypothalamic-pituitary-thyroid axis with ageing /1/. Values of > 10 mIU/L may indicate subclinical or overt hypothyroidismn /6/.

Causes of low serum TSH levels that are not indicative of subclinical hyperthyroidism are shown in Tab. 30.2-4 – Causes of low TSH not associated with subclinical hypothyroidism.

The behavior of TSH in thyroid disease and non-thyroidal illness is shown in Tab. 30.2-5 – TSH in thyroid disease and non-thyroidal illness. The relationship between TSH and T4 is shown in Fig. 30.2-1 – Relationship between T4 and TSH.

Third generation TSH assays with a detection limit of less than 0.01 mIU/L must be used /7/:

  • To distinguish between subclinical hyperthyroidism (TSH ≥ 0.1 mIU/L) and overt hyperthyroidism (TSH below 0.01 mIU/L)
  • To accurately detect TSH secreting pituitary tumors (TSH ≥ 0.01 mIU/L)
  • To monitor suppression therapy (TSH below 0.01 mIU/L)
  • To differentiate between non-thyroidal illness (TSH ≥ 0.01 to 0.39 mIU/L) and overt hyperthyroidism (TSH below 0.01 mIU/L)
  • During treatment with glucocorticoids, amiodarone, and with dopamine agonists (TSH ≥ 0.01 mIU/L).

30.2.5.2 Elevated TSH

Latent (subclinical) hypothyroidism is defined as TSH level of 4.0–10.0 mIU/L and normal FT4 concentration /6/. The presence of thyroid antibodies predicts the risk of progression from subclinical to overt hypothyroidism. Subclinical and overt hypothyroidism are associated with increased risk of coronary heart disease, particularly with TSH levels of ≥ 10 mIU/L. Overt hypothyroidism is defined as TSH level ≥ 20 mIU/L and decreased FT4 concentration. Hypothyroidism is associated with dyslipidemia, especially increased total cholesterol and LDL cholesterol.

30.2.6 Comments and problems

Quality criteria are required for TSH assays; the most important criteria are listed in Tab. 30.2-6 – Quality criteria for TSH assays.

Reference reagent

The recombinant human TSH reference reagent rTSH 94/674 was created under the auspices of the WHO. It contains 6.70 mIU rTSH per ampoule and is recommended to calibrate TSH assays.

Many manufacturers do not use the WHO reference reagent because there are big changes in the reference range compared to ranges used until now. The IFCC Committee for Standardization of Thyroid Function developed a global harmonization approach for TSH measurements. Reference interval measurements were made using a unique calibrator. The reference intervals showed improved uniformity using the new calibrator /8/.

Method of determination

Heterophilic antibodies reacting with anti-mouse antibodies can interfere in two site (sandwich) immunoassays by forming a linkage between the capture and detection antibody, resulting in a falsely high TSH concentration. Interference by heterophilic antibodies is usually eliminated by adding serum from another animal species. For example, if the antibody is a mouse antibody, bovine serum is added, which contains antibodies that bind to and block the heterophilic mouse antibodies.

An alternative method is to create a chimeric antibody. This is a human antibody with a variable region (Fab) of mouse origin and an Fc fragment of human origin. This prevents the binding of anti-mouse antibody directed against the Fc fragment. The use of these tests leads to fewer false TSH results.

TSH levels may be decreased in two-site immunometric assays using biotin streptavidin technology.

Explanation: in a sandwich assay TSH is sandwiched between two different antibodies: one is labeled with the a signal to be measured (antibody labeled with biotin). The other one, named the capture antibody will allow the separation of the immune complexes on a solid phase mediated by streptavidin. The higher the concentration of TSH, the higher the signal linked to the solid phase will be. At the end of the incubation not solid phase bound reagent is separated from the reaction milieu. Biotin prevents the binding of the immune complex to streptavidin and will result in a false depressed TSH level in the non competitive sandwich assay. This is the case in patients with high dose of biotin therapy (300 mg/day) e.g., in patients with inherited metabolic disease like propionic acidemia, biotinidase deficiency or in patients with parenteral nutrition /9/.

Influence factors

TSH exhibits a peak between 2:00 a.m. and 4:00 a.m. and a nadir between 4:00 p.m. and 8:00 p.m. /10/. The circadian variation in TSH is absent in infants younger than one month old and in patients with Sheehan’s syndrome. Sleep deprivation and short term untreated hypothyroidism affect nocturnal TSH surge. Circadian TSH rhythm is not affected by treatment of hypothyroidism with either thyroxine or T3 /4/.

Stability

In serum at 4 °C or 18–22 °C for four days, longer if deep-frozen /11/.

References

1. Cooper DS, Biondi B. Subclinical thyroid disease. The lancet 2012; 379: 1142–54.

2. Ladenson PW, Singer PA, Ain KB, Bagchi N, Bigos T, Levi EG, et al. American Thyroid Association guidelines for detection of thyroid dysfunction. Arch Int Med 2000; 160: 1573–5.

3. Spencer CA, Takeuchi M, Kazarosyan M, MacKenzie F, Beckett GF, Wilkinson E. Interlaboratory/intermethod differences in functional sensitivity of immunometric assays of thyrotropin (TSH) and impact on reliability of measurement of subnormal concentrations of TSH. Clin Chem 1995; 41: 367–74.

4. Ehrenkranz J, Bach PR, Snow GL, Schneider A, Lee JL, Ilstrup S, et al. Circadian and circannual rhythms in thyroid hormones: determining the TSH and free T4 reference intervals based upon time of day, age, and sex. Thyroid 2015; 25: 954–61.

5. Bondi B, Cooper DS. Subclinical hyperthyroidism. N Engl J Med 2018; 378: 2411–9.

6. Schübel J, Feldkamp J, Bergmann A, Drossard W, Voigt K. Latent hypothyoidism im adults. Dtsch Arztebl 2017; 114: 430–8.

7. Beckett G, MacKenzie F. Thyroid guidelines – are thyroid-stimulating hormone assays fit for purpose? Ann Clin Biochem 2007; 44: 203–8.

8. Thienpoint LM, van Uytfanghe K, De Grande LAC. Reynders D. Das B, Faix JD, Mac Kenzie F, et al. Harmonization of serum thyroid stimulating hormone measurements paves the way for the adoption of a more uniform reference interval. Clin Chem 217; 63: 1248–60.

9. Piketty ML, Polak M, Flechtner I, Gonzales-Briceno L, Souberbielle JC. False biochemical diagnosis of hyperthyroidism in streptavidin-biotin-based immunoassays: the problem of biotin intake and related interferences. Clin Chem Lab Med 2017; 55: 780–8.

10. Russell W, Harrison RF, Smith N, Darzy K, Shalet S, Weetman AP, Ross RJ. Free triiodothyronine has a distinct circadian rhythm that is delayed but parallels thyrotropin levels. J Clin Endocrinol Metab 2008; 93: 2300–6.

11. Koliakos G, Gaitatzi M, Grammaticos P. Stability of serum TSH concentration after non refrigerated storage. Panminerva Medica 1999; 41: 99–101.

12. Referenzbereiche für Kinder und Erwachsene. Elecsys Schilddrüsen-Tests. Edition Roche Diagnostics, 2004.

13. Hollowell JG, Staehling NW, Flanders WD, Hannon WH, Gunter EW, Spencer CA, Braverman LE. Serum TSH, T4 and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab 2002; 87: 489–99.

14. Kratzsch J, Fiedler GM, Leichtle A, Brügel M, Buchbinder S, Otto L, et al. New reference intervals for thyrotropin and thyroid hormones based on National Academy of Clinical Biochemistry criteria and regular ultrasonography of the thyroid. Clin Chem 2005; 51: 1480–6.

15. Chatterjee S, O’Malley BP, Price DE, Fielding AM, Aitken R. Low but detectable serum thyroid-stimulating hormone concentrations in ambulant subjects not receiving thyroxine. Ann Clin Biochem 2003; 40: 639–42.

16. Boelaert K. The association between serum TSH concentration and thyroid cancer. Endocrine-related cancer 2009; 16: 1065–72.

17. Haymart MR, Repplinger DJ, Leverson GE, Elson DF, Sippel SJ, Jaume JC, et al. High serum thyroid stimulating hormone level in thyroid nodule patients is associated with greater risks of differentiated thyroid cancer and advanced tumor stage. J Clin Endocrinol Metab 2008; 93: 809–14.

18. Leger J, Olivieri A, Donaldson M, Torresani T, Krude H, van Vliet G, et al. European Society for Paediatric Endocrinology Consensus Guidelines on Screening, Diagnosis, and Management of Congenital Hypothyroidism. J Clin Endocrinol Metab 2014; 99: 363–84.

19. Marucci G, Faustini-Fustini M, Righi A, Pasquini E, Frank G, Agati R, Foschini MP. Thyrotropin-secreting pituitary tumours: significance of atypical adenomas in a series of 10 patients and association with Hashimoto thyroiditis as a cause of delay in diagnosis. J Clin Pathol 2009; 62: 455–9.

20. Alexander EK, Pearce EN, Brent GA, Brown RS, Chen H, Dosiou C, et al. 2017 Guidelines of the American THyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid 2017; 27: 315–89.

21. Rotondi M, Leporati P, La Manna A, Pirali B, Mondello T, Fonte R, et al. Raised serum TSH levels in patients with morbid obesity: is it enough to diagnose subclinical hypothyroidism? Eur J Endocrinol 2009; 160: 403–8.

22. Heinze HG. Diskrepanz zwischen peripheren Schilddrüsenhormonwerten und TSH-Spiegel. Dtsch Med Wschr 1995; 120: 1639–40.

23. Feldkamp J, Röher HD, Scherbaum WA. Rezidivprophylaxe und medikamentöse Therapiestrategien nach Operation an der Schilddrüse. Dtsch Ärztebl 1998; 95: A-2324–8.

24. National Academy of Clinical Biochemistry 2004. www.nacb.org/Impg/thyroid_Impg_pub.stm

25. Arafah BM. Increased need for thyroxine in women with hypothyroidism during estrogen therapy. N Engl J Med 2001; 344: 1743–9.

26. Bschor T, Bauer M. Schilddrüsenfunktion bei Lithiumbehandlung. Nervenarzt 1998; 60: 189–95.

27. Somwaru LL, Arnold AM, Joshi N, Fried LP, Cappola AR. High frequency of and factors associated with thyroid hormone over-replacement and under-replacement in men and women aged 65 and over. J Clin Endocrinol Metab 2009; 94: 1342–5.

28. Hallengren B, Lantz M, Andreasson B, Grennert L. Pregnant women on thyroxine substitution are often dysregulated in early pregnancy. Thyroid 2009; 19: 391–4.

29. Yandell SD, Harvey WC, Fernandes RJ, Barr PW, Feldman M. Radioiodine studies, low serum thyrotropin, and the influence of statin drugs. Thyroid 2008; 18: 1039–42.

30. Mills F, Jeffery J, Mackenzie P, Cranfield A, Ayling RA. An immunoglobulin G complexed form of thyroid-stimulating hormone (macro thyroid-stimulating hormone) is a cause of elevated serum thyroid-stimulating hormone concentration. Ann Clin Biochem 2013; 50: 416–20.

31. Marqusee E, Haden ST, Utinger RD. Subclinical thyrotoxicosis. Endocrin Metab Clin North Am 1998; 27: 37–49

30.3 Thyroid hormones

TSH determination is the primary investigation used to diagnose disorders of thyroid function. The TSH level provides an assessment of the biological effects of triiodothyronine (T3) in the tissues while the concentrations of thyroxine (T4) and T3 are indicators of thyroid secretory function. Hyperthyroidism is characterized by excessive secretion of T4 and/or T3 while hypothyroidism is caused by a deficiency of these hormones. In subclinical hyperthyroidism or hypothyroidism, the TSH level is abnormally low or high, while the concentrations of T4 or T3 are still within the reference interval.

T4 and T3 circulate in the plasma as free (unbound) hormones or bound to one of the following binding proteins: thyroxine binding globulin (TBG), transthyretin (also known as thyroxine binding pre albumin), and albumin. The protein bound and unbound forms are together referred to as “total” hormone (TT4, TT3) whereas the unbound hormone is referred to as “free” hormone (FT4 and FT3). The free hormone is the biologically active form and correlates most closely with thyroid function.

The kinetics of T3 metabolism differ from those of T4 because, compared to T4, it has a 10–15 times lower affinity for TBG. As a result, only 0.02–0.03% of T4 exists in free form while 0.2–0.3% of T3 is unbound. Approximately 75% of T4 is bound to TBG while 10–20% is bound to transthyretin and albumin respectively. T3 shows a similar binding behavior, however, a smaller proportion is bound to transthyretin. Less than 5% of each hormone is bound to lipoproteins. If there is a change in the concentration of TBG, a temporary shift from bound to free hormone occurs. However, because of the fixed set point in the hypothalamic-pituitary-thyroid axis, the concentration of free hormone quickly returns to its previous value. When free hormone is metabolized, bound hormone dissociates immediately from the binding protein because it is bound non covalently. Dissociation also occurs when serum is diluted in vitro, resulting in a decrease in the concentration of free hormone /1/.

T4 can be seen as a pro hormone of T3 because the effects of both thyroid hormones at molecular level in the hypothalamus and peripheral tissues are mediated by nuclear T3 receptors only. All T4, but only 15–20% of T3, is secreted by the thyroid. The remaining T3 is produced from T4 in the liver, kidneys, and other peripheral organs.

The daily production rate of T3 is approximately half that of T4 and the serum level of FT3 is 3–4 times lower than that of FT4 (5 compared to 20 fmol/L). The conversion of T4 to T3 is catalyzed by deiodinase type 1 in the peripheral tissues and deiodinase type 2 in the hypothalamic-pituitary system. Because the Michaelis constant of deiodinase type 2 is 1,000 times lower than that of deiodinase type 1, it can be assumed that more T4 is converted to T3 per cell in the pituitary than in the peripheral tissues. Circulating T4 has a half life of 6.7 days, while the half life of T3 is only 0.75 days. T3 exhibits a circadian rhythm while T4 does not /2/.

Based on the fact that the free hormone fraction is the biologically active fraction, FT4 and FT3 determinations are usually used to assess thyroid function.

30.3.1 Free T4 (FT4), total T4 (TT4)

Approximately 100% of circulating T4 is secreted by the thyroid.

30.3.1.1 Indication

Free thyroxine

  • Further investigation in patients with TSH levels outside the reference interval
  • Suspected thyroid disorder in critically ill patients (in conjunction with TSH determination)
  • Suspected latent (subclinical) hypothyroidism or hyperthyroidism when TSH is elevated or reduced
  • Conditions that suggest deregulation of the hypothalamic-pituitary-thyroid axis
  • Initially after treatment of hyperthyroidism (as the secretion of TSH may be suppressed during weeks to months after treatment)
  • Monitoring of dose titration in L-thyroxine treatment.

Total thyroxine (TT4)

  • Discordance between TSH and FT4 results.

30.3.1.2 Method of determination

Serum T4 and T3 levels reflect not only hormone production, but also the concentrations of thyroid hormone binding proteins. In the presence of elevated levels of binding proteins, TT4 and TT3 are usually increased but FT4 and FT3 levels remain normal.

TT4

Principle: competitive immunoassay with enzyme, luminescent, and fluorescent labels. In the heterogenous competitive assay, the T4 of the sample and labeled T4 (e.g., enzyme labeled T4) compete for the limited quantity of anti-T4 antibodies of the test reagent. The higher the T4 level in the patient sample, the lower the amount of enzyme labeled T4 is antibody bound. Patient T4 antibody complexes and enzyme labeled T4 antibody complexes bind to a second, solid phase bound capture antibody. Substrate is added to measure the activity of the enzyme. The amount of capture antibody bound enzyme labeled T4 is inversely proportional to the concentration of T4 in the sample. The same principle is used for the determination of T3.

FT4

Principle: one step analog immunoassay. In this methodology a chemically modified T4 analog replaces labeled T4 as competitive molecule. The T4 analog binds with a high avidity to the T4 antibody but with a lower avidity as native serum T4 to the binding proteins of the serum /5/.

Equilibrium dialysis method: this is the most reliable method (reference method) for measuring FT4 in serum. The method separates FT4 from protein bound T4 before direct measurement and uses undiluted serum as a retentive.

30.3.1.3 Specimen

Serum: 1 mL

30.3.1.4 Reference interval

Refer to Tab. 30.3-1 – Reference intervals for T4 and FT4.

30.3.1.5 Clinical significance

After diagnosis of TSH, the result of FT4 is the basic parameter used to assess thyroid hormone secretion since it reflects hormone production by the thyroid directly. The clinical assessment of FT4 and TT4 is shown in Tab. 30.3-2 – Assessment of FT4 and TT4 levels.

Because FT4 is not affected by inter individual and intra individual variations in the concentrations of binding proteins, it has a higher diagnostic significance than TT4. FT4 represents the metabolically reactive fraction of serum T4 as well as current hormone production (thyroid secretion, extra thyroidal conversion of T4 to T3) and elimination (extravascular transport, metabolism, renal elimination).

FT4 has better discriminatory power than TT4 at the limits of the reference interval, which is why TT4 determination is only necessary in certain patients, to distinguish between thyroid related and non-thyroidal elevations, for example.

Primary hyperthyroidism must not be diagnosed on the basis of elevated FT4 and/or FT3 unless TSH is also suppressed (< 0.01 mIU/L).

Subclininical hypothyroidism is not diagnosed on the basis of low FT4 unless TSH is significantly elevated. An excessive TSH increase and low FT4 indicates overt hypothyroidism.

If there is discordance between FT4 and TSH values in a patient with non-thyroidal illness (NTI), TT4 should also be determined to establish whether a thyroid disorder is present. A thyroid disorder is more likely if FT4 and TT4 behave similarly. Discordance between FT4 and TT4 is usually caused by NTI /6/.

Tab. 30.3-3 – Diseases and conditions associated with changes in FT4 and/or TT4 lists some conditions and diseases associated with changes in FT4 and T4 levels.

30.3.1.6 Comments and problems

Method of determination

Immunoassays from 9 manufacturers were compared with a candidate reference measurement procedure (cRMP) based on equilibrium dialysis isoptope dilution-mass spectrometry (ED-ID-MS) for determining FT4 and FT3. For FT4 (FT3) the mean bias of 2 (4) assays was up to minus 42% (minus 30%). After recalibration to the cRMP eliminated assay specific biases approximately half of the manufacturers matched the internal quality control targets within about 5%; however, within run instability was observed /7/.

Dilution of serum samples leads in most immunoassays to a reduction in the FT4 concentration compared to equilibrium dialysis. At a dilution factor of 10, measured values are reduced by 20–80% /8/.

Individual variation

Throughout the course of a year, the FT4 concentration can vary by ± 25% and the TSH level can vary by ± 50%. This is due to seasonal variation in the pituitary set point for TRH secretion /9/.

Autoantibodies against T4

Thyroid hormone autoantibodies are directed against T4 and T3 and interfere with the determination of FT4 and FT3 in 1.2% of cases /10/. They interfere with competitive solid phase immunoassays in which an inverse relationship exists between the FT4 concentration of the sample and labeled T4.

In the absence of interfering factors, T4 from the sample and labeled T4 from the test reagent compete for the limited number of T4 specific antibodies bound to the solid phase. In the presence of T4 autoantibodies, however, some of the T4 and labeled T4 may bind to the autoantibody rather than to the solid phase. This results in a falsely low measurement signal that indicates an increased T4 concentration in the sample.

Stability

In serum and plasma at room temperature (22 °C) for 24–48 h; deep-frozen for more than a year.

30.3.2 Total T3 (TT3), free T3 (FT3)

Approximately 80% of circulating T3 is produced by peripheral conversion of T4 and only 20% is secreted directly by the thyroid gland. In plasma, T3 is bound primarily to TBG and, with lower affinity, to transthyretin. FT3 has five times the metabolic activity of FT4, while rT3 has less than 5% of the metabolic activity of FT4.

30.3.2.1 Indication

  • Suspicion of T3 hyperthyroidism in cases with suppressed TSH and normal FT4 concentration
  • Further evaluation of patients with subclinical hyperthyroidism
  • Suspicion of non-thyroidal illness
  • Prognostic assessment of the treatment of patients with Graves’ disease
  • Evaluation of the severity of primary hypothyroidism
  • Monitoring of T3 replacement therapy

30.3.2.2 Method of determination

Refer to T4/FT4.

30.3.2.3 Specimen

Serum: 1 mL

30.3.2.4 Reference interval

Refer to Tab. 30.3-4 – Reference intervals for TT3 and FT3.

30.3.2.5 Clinical significance

Approximately 99.8% of T3 in the serum is bound to proteins and is determined as TT3. Only 0.2–0.3% exists in free, metabolically active form (FT3). Because T3 binds to proteins with 10 times lower affinity than T4, the determination of TT3 and FT3 are of roughly equal diagnostic value /11/.

However, because its protein binding affinity is 10 times lower than that of T4, T3 is affected to a much lesser degree by changes in protein binding capacity. As a result, FT3 does not have the same importance compared to TT3 that FT4 has compared to TT4.

The advantages of determining FT3 rather than TT3 are limited and the secretory performance of the thyroid can demonstrated just as reliably by TT3 as by FT3.

The TT3 and FT3 levels reflect the conversion of T4 to T3 in the tissues, and to a lesser extent, thyroid secretion of T3, which accounts for only 20% in euthyroid individuals. This is not the case in autoimmune hyperthyroidism. In Graves’ disease, regardless of whether thyrotoxicosis is caused by T4 or by T3 or T4-T3, 33 ± 6% of the T3 is secreted by the thyroid /12/.

The conversion rate may be reduced, leading to reduced TT3 and FT3 concentrations:

  • In the critically ill, in non-thyroidal illness, or low T3 syndrome, due to inhibition of deiodinase
  • Due to drugs such as glucocorticoids, propranolol, and amiodarone
  • In the elderly, in whom mild hyperthyroidism can therefore be missed.

In iodine deficiency, a compensatory increase in TT3 and FT3 can occur. TT3 and FT3 are also of limited value in the diagnosis of hypothyroidism, since the levels of both are maintained within the lower reference interval by decreased secretion of T4 with subsequent conversion to T3.

TT3 or FT3 determination is clinically relevant:

  • In patients with subclinical hyperthyroidism who have suppressed TSH level and normal FT4 concentration; these patients are at increased risk for the development of overt hyperthyroidism
  • For the detection of isolated T3 over secretion (T3 thyrotoxicosis), which accounts for up to 10% of all cases of hyperthyroidism.
  • In critical ill patients with non-thyroidal illness TT3 and to a lesser extent FT3 levels are decreased; in critical ill patients with hyperthyroidism therefore TT3 and FT3 concentration may be minimally increased or normal
  • In the early stage of thyroid hyper function, especially in the case of autonomy
  • For diagnosing recurrent hyperthyroidism, since T3 elevation may be an early symptom
  • To exclude factitious hyperthyroidism caused by L-thyroxine treatment.

Progressive central fat accumulation in individuals with a BMI greater than 25 kg/m2 is associated with an increase in FT3. The FT4 to FT3 ratio also correlates with waist circumference, blood pressure, fasting plasma glucose, cholesterol and LDL cholesterol concentration /13/.

The clinical significance of TT3 and FT3 levels is shown in Tab. 30.3-5 – Assessment of TT3 and FT3 levels.

30.3.2.5.1 Low T3 syndrome

The evaluation of thyroid function in critical illness remains complex because the changes occur at all levels of the hypothalamic-pituitary-thyroid axis. During illness, a decrease in T3 and pulsatile TSH release and increase in rT3 occur. This constellation of findings is termed the euthyroid sick syndrome or non-thyroidal illness or the low T3 syndrome /14/. Low T3 syndrome is the most common manifestation in non-thyroidal illness and due to inhibition of 5’deiodinase which catalyzes the conversion of T3 from T4. A study /15/ showed that low T3 syndrome was highly prevalent in chronic kidney disease (CKD). Furthermore, serum T3 levels were associated with the severity of CKD even in the normal TSH level (Tab. 30.3-6 – Prevalence of low T3 in chronic kidney disease).

30.3.2.5.2 T3 and FT3 for treatment monitoring

If thyroid hormone overdosage is suspected during L-thyroxine treatment for goiter or following thyroid surgery, FT3 can be determined in addition to TSH.

During the initial phase of L-thyroxine treatment for hypothyroidism, the FT3 concentration is lower on average than the FT4 concentration. The FT3 level is important in terms of preventing overdosage if TSH declines to the lower reference interval value when the thyroxine dose is increased. The FT3 should remain within the reference interval.

At the start of thyrostatic therapy for hyperthyroidism, the FT3 concentration increases due to a compensatory increase in the production of T3, which contains one less iodine atom per molecule than T4. Blood collection should take place at least 12 hours, but preferably 24 hours, after the last dose.

30.3.2.6 Comments and problems

Reference interval

Depending on the study, FT3 and TT3 levels are 10–50% lower in older than in younger individuals. These lower values, which are thought to result from decreased conversion of T4 to T3, are observed in men from the age of 60 years; in women, levels decline progressively from the age of 70 years.

Method of determination

Refer to TT4 and FT4 and to Ref. /24/.

Circadian rhythm

Like TSH, FT3 exhibits a circadian rhythm. However, the peak and nadir are reached 0.5–2.5 hours later than those of TSH /2/.

30.3.3 Reverse T3 (rT3)

When there is a reduced need for active thyroid hormone, the body can regulate the biotransformation of T4 into T3 by promoting the mono deiodination of T4 to inactive 3,3’,5’-triiodothyronine (rT3) (Fig. 30.6-5 – Pathways of thyroid hormone metabolism).

The plasma rT3 concentration indirectly reflects the conversion of T4 to T3 in the tissues. It is also an indirect measure of the metabolism of thyroid hormones to diiodothyronine.

30.3.3.1 Indication

Determination of the cause of an inexplicably low FT4, T3, or FT3 concentration.

30.3.3.2 Method of determination

Radioimmunoassay

30.3.3.3 Specimen

Serum: 1 mL

30.3.3.4 Reference interval

0.10–0.30 μg/L

0.15–0.50 nmol/L

Conversion: μg/L × 1.54 = nmol/L

30.3.3.5 Clinical significance

In severe non-thyroidal illness, the decline in TT3 or FT3 is associated with an almost identical increase in rT3. Low T3 syndrome occurs during the neonatal period, in seriously ill adults, during fasting and low carbohydrate intake, in liver disease, and as an effect of various drugs such as corticosteroids, antiarrhythmics, and beta blockers. A decline in TT4 is seen only in very severe non-thyroidal illness and FT4 is normal or even increased for a long time. rT3 can therefore be seen as a regulator of T4 deiodination /16/. Refer to Tab. 30.1-2 – Adaptive response to non-thyroidal illness.

The ratio of T3 to rT3 can be of value in identifying clinical deterioration in severe non-thyroidal illness and in monitoring treatment outcome. At present, measurement of the rT3 concentration does not have a role in routine thyroid investigations.

References

1. Roelfsema F, Veldhuis JD. Thyrotropin secretion patterns in health and disease. Endocrine Rev 2013; 34: 619–57.

2. Russell W, Harrison RF, Smith N, Darzy K, Shalet S, Weetman AP, Ross RJ. Free triiodothyronine has a distinct circadian rhythm that is delayed but parallels thyrotropin levels. J Clin Endocrinol Metab 2008; 93: 2300–6.

3. Liappis N, Schlebusch H, von Perjés M, Berg I. Referenzwerte für die Konzentration des freien Thyroxins, des freien Trijodthyronins und des Thyroxin-bindenden Globulins im Blutserum euthyreoter Kinder. Klin Pädiat 1991; 203: 113–5.

4. Referenzbereiche für Kinder und Erwachsene. Elecsys Schilddrüsen-Tests. Edition Roche Diagnostics, 2004.

5. Midgley JEM. Direct and indirect free thyroxine assay methods: theory and practice. Clin Chem 2001; 47: 1353–63.

6. Glinoer D. The regulation of thyroid function during normal pregnancy: importance of the iodine nutrition status. Best Practice & Research Clinical Endocrinol & Metab 2004; 18: 133–52.

7. Thienpont LM, van Uytfanghe K, Beastall G, Faix JD, Ieiri T, Miller WG, et al. Report of the IFCC working group for standardization of thyroid function tests; part 2: free thyroxine and free triiodothyronine. Clin Chem 2010; 56: 912–20.

8. Sapin R. Serum thyroxine binding capacity-dependent bias in five free thyroxine immunoassays: assessment with serum dilution experiments and impact on diagnostic performance. Clin Biochem 2001; 34: 367–71.

9. Andersen S, Pedersen KM, Bruun NH, Lauberg P. Narrow individual variants in serum T4 and T3 in normal subjects: a clue to the understanding of subclinical thyroid disease. J Clin Endocrinol Metab 2002; 87: 1068–72.

10. Gledenning P, Siriwardhana D, Hoad K, Musk A. Thyroxine autoantibody interference is an uncommon cause of inappropriate TSH secretion using the Immulite 2000 assay. Clin Chim Acta 2009; 403: 136–8.

11. Klee GG. Clinical usage recommendations and analytic performance goals for total and free triiodothyronine measurements. Clin Chem 1996; 42: 155–9.

12. Woeber KA. Triiodothyronine production in Grave’s hyperthyroidism. Thyroid 2006; 16: 687–90.

13. De Pergola G, Ciampolillo A, Paolotti S, Trerotoli P, Georgino R. Free triiodothyronine and thyroid stimulating hormone are directly associated with waist circumference, independently of insulin resistance, metabolic parameters and blood pressure in overweight and obese women. Clin Endocrinol 2007; 67: 265–9.

14. The National Academy of Clinical Biochemistry. Laboratory Medicine Practice Guidelines for the Diagnosis and Monitoring of Thyroid Disease. NACB edition.

15. Song SH, Kwak IS, Lee DW, Kang YH, Seong EY, Park JS. The prevalence of low triiodothyronine according to the stage of chronic kidney disease in subjects with a normal thyroid-stimulating hormone. Nephrol Dial Transplant 2009; 24: 1534–8.

16. Vogeser M, Jacob K. Measurement of free triiodothyronine in intensive care patients – comparison of two routine methods. Eur J Clin Chem Clin Biochem 1997; 35: 873–5.

17. Kratzsch J, Fiedler GM, Leichtle A, Brügel M, Buchbinder S, Otto L, et al. New reference intervals for thyrotropin and thyroid hormones based on National Academy of Clinical Biochemistry criteria and regular ultrasonography of the thyroid. Clin Chem 2005; 51: 1480–6.

18. Grüning T, Zöphel K, Wunderlich G, Franke WG. Influence of female sex hormones on thyroid parameters determined in a thyroid screening. Clin Lab 2007; 53: 547–53.

19. Arafah BM. Increased need for thyroxine in women with hypothyroidism during estrogen therapy. N Engl J Med 2001; 344: 1743–9.

20. Kahric-Janicic N, Soldin SJ, Soldin OP, West T, Gu J, Jonklaas J. Tandem mass spectrometry improves the accuracy of free thyroxine measurements during pregnancy. Thyroid 2007; 17: 303–11.

21. Fischer DA. Euthyroid low thyroxine (T4) and triiodothyronine (T3) states in prematures and sick neonates. Pediat Clin North Am 1990; 37: 1297–1312.

22. Centanni M, Gargano L, Canettieri G, Viceconti N, Franchi A, Delle Fave G, et al. Thyroxine in goiter, Helicobacter pylori infection, and chronic gastritis. N Engl. J Med 2006; 354; 1787–95.

23. Kahaly GJ, Dietlein M, Gärtner R, Mann K, Dralle H. Amiodaron und Schilddrüsenfunktion. Dt Ärztebl 2007: 104: A 3550–5.

24. Uy HL, Reasner II CA. Elevated thyroxine levels in a euthyroid patient. Postgrad Med 1994; 96: 195–202.

30.4 Thyroxin-binding globulin (TBG)

TBG is the most important plasma transport protein for thyroid hormones. It is a 54 kDa glycoprotein, consisting of 395 amino acids and four asparagine linked oligosaccharide chains. The carbohydrate component is thought not to influence immunogenicity and T4 binding. The TBG gene is located on the long arm of the X chromosome and is composed of five exons.

With the exception of its functions as a storage and transport protein for T4 and T3 and an inhibitor of their renal elimination, little is known about the physiological role of TBG /1/.

30.4.1 Indication

  • Unexplained discordance between TSH and TT4 and/or FT4 serum levels.
  • Unexplained discordance between TT4 and FT4 serum levels.
  • Markedly increased or decreased serum TT4.
  • Suspected congenital TBG deficiency.

30.4.2 Method of determination

Radioimmunoassay or enzyme immunoassay.

30.4.3 Specimen

Serum: 1 mL

30.4.4 Reference interval

13–30 mg/L (220–510 nmol/L)

Conversion: mg/L × 17 = nmol/L

30.4.5 Clinical significance

TBG determination may be necessary to assess thyroid function if there is discordance between the levels of TSH and TT4 or FT4.

The assessment criterion used in this case is the ratio of T4 to TBG (Tab. 30.4-1 – Assessment of thyroid function using the T4/TBG ratio).

Hepatic synthesis of TBG is increased during pregnancy or by the use of estrogen containing oral contraceptives.

Congenital TBG disorders include partial or complete TBG deficiency and TBG excess. Complete deficiency, caused by a nucleotide deletion at codon 352 has been described in a number of Japanese families. The TBG molecule is truncated and retained within the endoplasmic reticulum. The incidence of TBG deficiency in the Japanese population is estimated to be 0.09% /2/.

References

1. Horn K, Kubiczek T, Pickardt CR, Scriba PC. Thyroxine-binding globulin (TBG): preparation, radioimmunoassay and clinical significance. Klin Wschr 1977; 55: 881–94.

2. Noguchi T, Miki T, Takamatsu J, Nakajima T, Kumahara Y. Mass screening for complete deficiency of thyroxine-binding globulin in adult Japanese by comprehensive health examination. Internal Medicine 1996; 35: 266–9.

30.5 Autoimmune thyroid diseases

Autoimmune endocrine disease of the thyroid can result in either underactivity or over activity of the gland and may be associated with autoimmune disease of other endocrine glands such as the adrenals, pancreatic islet cells, and ovaries. The most common autoimmune endocrine diseases are type 1 diabetes (T1D) and autoimmune thyroid diseases (AITD). The characteristic features of both disorders are T cell infiltration of target organs and autoantibody production, leading to organ dysfunction and destruction. Often, a patient may have more than one autoimmune endocrine disease at the same time. This is known as polyglandular autoimmune syndrome. T1D and AITD are the most commonly associated diseases. Up to 20% of patients with T1D have thyroid autoantibodies and half of these will develop AITD in addition. Conversely, 2.3% of children with AITD have islet cell autoantibodies /1/.

T1D and AITD share specific HLA associations:

  • A positive association of Graves’ disease with DR3, but a negative association with DR5
  • A positive association of Graves’ disease with DQA1*0501 in linkage disequilibrium with DQB1*0201 in DR3 haplotypes
  • A positive association of Hashimoto’s thyroiditis with HLA DQB1*301.

Autoimmune thyroid diseases, including Graves’ disease and Hashimotos’ thyroiditis, are archetypal organ-specific autoimmune diseases. Graves’ disease is the common cause of hyperthyroidism and is characterized by the presence of anti-thyrotropin receptor antibody (TRAb), Hashimotos thyroiditis is the most common cause of hypothyroidism and is characterized by the presence of anti-thyroglobulin antibody and/or anti-thyroid peroxidase antibody and thyroid tissue destruction by infiltrating lymphocytes. Thyroid hyper function or hypofunction is the result of hereditary factors, inadequate iodine intake, pregnancy, radiotherapy, viral infection, surgery, invasive diseases, or autoimmunity. The term AITD refers to both autoimmune hyperthyroidism (Graves’ disease) and autoimmune hypothyroidism (Hashimotos thyroiditis).

According to a systematic review of the literature, the incidence of AITD per 100,000 individuals varied /2/:

  • The reported incidence of autoimmune hypothyroidism varied between 2.2/100,000/year (males) and 498.4/100,000/year (females).
  • The reported incidence for autoimmune hyperthyroidism ranged from 0.70/100,000/year (black males) to 99/100,000/year (Caucasian females).

The prevalence of hypothyroidism has been found to be between 0 and 7.8/1,000 men and between 2.0 and 19.4/1,000 women /2/.

The three most important antigens involved in thyroid autoimmunity are:

  • Thyroglobulin (Tg), against which thyroglobulin antibodies (anti-TgAb) are produced
  • Thyroid peroxidase (TPO), against which thyroid peroxidase antibodies (anti-TPOAb) are produced
  • The TSH receptor (TR), against which TSH receptor antibodies (TRAb) are produced.

Thyroid autoantibodies of any kind inform the physician concerning the etiology of the thyroid disease, but these measurements do not substitute for a thorough history and physical examination and measurements of TSH, FT4 and FT3. Anti-TPOAb and/or anti-TgAb are common in patients with Hashimotos’ thyroiditis and in patients with Graves’ disease, but are not diagnostic of Graves’ disease. If the diagnosis of Graves’ disease is questionable in the setting of hyperthyroidism, a measurement of TRAb can be helpful /3/.

The prevalence of autoantibodies in the healthy population and in patients with autoimmune thyroid disease shows geographical variation. Some of this variation is explained by differences in detection limit and cutoff value (30–100 kIU/L) used when determining anti-TgAb and anti-TPOAb. Thus, according to a clinical study /4/, the prevalence of autoimmune thyroid disease was 0.2% in men and 2% in women, while the prevalence of subclinical autoimmune thyroid disease determined on the basis of anti-TgAb and anti-TPOAb was 10 times higher.

Other investigators have found that although 98% of serum samples that are positive for anti-TgAb are also positive for anti-TPOAb, only 65% of anti-TPOAb-positive sera also show an increased anti-TgAb concentration /5/. Therefore, samples should only be screened for anti-TgAb if Tg determination is undertaken in patients with differentiated thyroid cancer (Section 28.22 – Thyroglobulin).

Low concentrations of anti-TgAb and anti-TPOAb are present in the healthy population. However, these autoantibodies belong to the group of natural antibodies that have low affinity for a broad spectrum of antigens.

Diagnostic algorithms are shown in:

30.5.1 Thyroglobulin antibodies (anti-TgAb)

Thyroglobulin (Tg) is a water soluble glycoprotein that consists of two subunits, each with a molecular weight of 300 kDa (Section 28.22 – Thyroglobulin)). It is the most important precursor in the biosynthesis of thyroid hormone. Of the 134 tyrosine residues in the Tg molecule, less than one-fifth are iodized. Following its synthesis, Tg is stored in the thyroid follicles. TSH stimulates the release of Tg into the circulation, where it has a half life of 3–65 hours and from where it is subsequently eliminated /4/.

Each Tg subunit has only two auto antigenic epitopes. The autoantibodies produced consist mainly of IgG type. In Hashimotos thyroiditis, the predominant autoantibody is IgG2, whereas in Graves’ disease, thyroid cancer, and non-thyrotoxic goiter, the predominant autoantibody type is IgG4.

30.5.1.1 Indication

  • Screening of serum from patients with differentiated thyroid cancer in which Tg determination is undertaken
  • Monitoring of differentiated thyroid cancer after therapeutic intervention
  • Suspected autoimmune thyroiditis with negative anti-TPOAb.

30.5.1.2 Method of determination

Immunoassay

Enzyme immunoassay (EIA), immunochemiluminescence assay (ICMA), or immunometric assay (IMA), calibrated against the MRC (Medical Research Council) reference preparation 65/93 with 1,000 MRC units per ampoule.

30.5.1.3 Specimen

Serum: 1 mL

30.5.1.4 Reference interval

Up to 60 kIU/L /5/ or 100 kIU/L /6/, depending on the manufacturer of the assay.

30.5.1.5 Clinical significance

In a study of the Danish population /5/ using a cutoff value of 60 kIU/L, the prevalence of anti-TgAb was 7.7% in male aged 60–65 years. In female aged 18–22 years the prevalence was 9.1% and increased to 20% in the age of 60–65 years. In regions with adequate iodine intake, normal anti-TPOAb with increased anti-TgAb rarely indicated a functional thyroid disorder, unless thyroid cancer was present.

30.5.1.5.1 Differentiated thyroid cancer

Anti-TgAb have been found to be prognostic in patients with thyroid differentiated cancer, with failing antibody levels associated with low risk of recurrent disease, whereas rising or continously positive antibodies indicated risk for recurrent cancer /7/. They also have been found to predict risk of cancer in thyroid nodules.

The Tg concentration is used to monitor differentiated thyroid cancer after treatment /5/. However, no or only low thyroglobulin (Tg) is measured due to elevated anti-TgAb levels or due to undetected anti-TgAb (i.e., lack of analytical sensitivity of the assay) in 15–30% of patients /8/. Using immunometric assays for the determination of Tg the anti-TgAb interfere with the Tg assay by measuring a falsely low Tg concentration (Section 28.22 – Thyroglobulin). Increased Tg levels due to residual metastatic disease may be overlooked in these patients.

30.5.1.5.2 Autoimmune thyroiditis

Increased anti-TgAb concentrations are measured in 12–30% of patients with autoimmune hyperthyroidism and up to 60–80% of patients with autoimmune thyroiditis (Tab. 30.5-1 – Thyroglobulin antibodies in different disorders/9/. Because their incidence is much lower than that of anti-TPOAb, anti-TgAb determination has a minor role in the diagnosis and monitoring of autoimmune thyroiditis. However, since the prevalence of isolated anti-TgAb positivity in autoimmune thyroiditis is thought to be approximately 6%, some investigators caution against omitting anti-TgAb determination /10/.

30.5.1.6 Comments and problems

Guidelines recommend that anti-TgAb should be measured concurrently with thyroglobulin when monitoring thyroidectomized thyroid cancer patients to alert the laboratory to the possibility of assay interference /11/.

Method of determination

Despite the availability of an international reference preparation, current anti-TgAb assays show unacceptable variance. Concordance between the assays was only 74% /12/.

30.5.2 Thyroid peroxidase antibodies (anti-TPOAb)

Thyroid peroxidase (TPO) is a membrane bound heme protein with a molecular weight of approximately 100 kDa that catalyzes thyroid hormone synthesis on the apical membrane of follicular cells, where it exists as a dimeric enzyme linked by disulfide bonds. In thyroid hormone synthesis, TPO is involved in the iodination of tyrosine residues and the oxidative coupling of two tyrosine residues to thyroglobulin. It requires iodide and H2O2 in order to initiate hormone synthesis. The function of TPO is described in detail in Ref. /13/. TPO can be purified following trypsin treatment of thyroid microsomes. In the past, microsomes were used as antigens to detect TPOAb, which is why the term “microsomal antibodies” is used in older literature.

30.5.2.1 Indication

Recommended indications are /5/:

  • TSH increase of unknown etiology
  • Goiter of unknown etiology
  • Evaluation of suspected polyglandular autoimmune disease
  • Family evaluation in established cases of autoimmune thyroid disease
  • Risk assessment for the development of hypothyroidism during treatment with drugs affecting thyroid (e.g., lithium salts, amiodarone, interferon-alpha, and interleukin 2)
  • Risk assessment for the development of hypothyroidism in Down’s syndrome
  • Risk assessment for the development of a functional thyroid disorder during pregnancy and postpartum thyroiditis
  • Screening for latent (subclinical) hypothyroidism prior to in vitro fertilization
  • Prior to treatment of subclinical hypothyroidism.

30.5.2.2 Method of determination

Immunoassays

Enzyme immunoassay (EIA), immunochemiluminescence assay (ICMA), or immunometric assay (IMA), calibrated against the MRC (Medical Research Council) reference preparation 65/93 with 1,000 MRC units per ampoule.

30.5.2.3 Specimen

Serum: 1 mL

30.5.2.4 Reference interval

Up to 60 kIU/L /5/ or 100 kIU/L /6/, depending on the manufacturer.

30.5.2.5 Clinical significance

Using a cutoff value of 60 kIU/L, the prevalence of anti-TPOAb in the Danish population was 11.3% in men aged 60–65 years and ranged from 12.3% in women aged 18–22 years to 29.7% in women aged 60–65 years /5/. The prevalence was 5.8% in euthyroid blood donors and greater than 98% in autoimmune thyroiditis /5/. TPO is a primary antigen in the initiation of an immune response in the thyroid. Patients with increased anti-TPOAb levels have a significantly increased risk of developing latent or overt hypothyroidism.

High concentrations of anti-TPOAb (> 2,000 kIU/L) were exclusively found in persons with HLA-DR3 or HLA-DR5, the same haplotypes as those associated with thyroid autoimmune diseases /14/.

Tab. 30.5-2 – TPOAb in thyroid diseases describes the behavior of anti-TPOAb in thyroid disorders.

30.5.2.6 Comments and problems

Method of determination

A comparative study of eight kits for anti-TPOAb determination with isotopic, enzymatic and luminescent tracers and with recombinant highly purified TPO showed approximately 6% positive results in control sera. The majority of assays demonstrated a good diagnostic performance /15/.

In autoimmune thyroid disease, anti-TPOAb react mainly with two immunodominant regions (IDR) of TPO, A and B (IDR-A and IDR-B). Some antibodies, however, bind to antigenic determinants outside these regions. Around half of anti-TPOAb bind to IDR-B while the rest react with IDR-A or antigens outside the IDRs. The corresponding distribution in healthy individuals is unknown. It is thought that IDR-A and IDR-B are associated with forbidden detrimental reactions while the non-A and non-B regions are part of the natural antigen reservoir.

30.5.3 TSH receptor antibodies (TRAb)

There are two types of TRAb; thyroid stimulating antibodies (TSAb) and TSH-stimulation blocking antibodies (TSBAb) /26/. TSAb stimulate thyroid gland and cause Graves’ hyperthyroidism. TSBAb block TSH stimulation of thyroid gland and cause hypothyroidism. Both TSAb and TSBAb block TSH binding to thyroid cells as TSH receptor antibodies (TRAb), which have been measured as TSH binding inhibitory immunoglobulin (TBII). TRAb has been measured by different assay methods and given various names. Among them, TBII and TSAb have been measured as TRAb to diagnose Graves’ disease. TBII is measured as a receptor assay. TSAb is measured as a stimulator assay, and TSBAb as a TSH stimulation blocking assay. The former TRAb is a stimulating antibody (TSAb) and the latter TRAb is a blocking antibody.

It has been generally believed that Graves’ patients have TSAb but do not have TSBAb, and that blocking antibody (TSBAb) positive patients with hypothyroidism have TSBAb but do not have TSAb. But TSBAb positive patients with hypothyroidism and Graves’ patients with hyperthyroidism may have both TSBAb and TSAb /26/.

30.5.3.1 Indication

TRAb measurements as either TSAb or TBII have the following clinical applications /3/:

  • Diagnosis of Graves’ disease in the clinical setting of thyrotoxicosis, diffuse goiter, opthalmopathy (exophthalmos) and dermatopathy (pretibial myxedema)
  • Differentiating of Graves’ disease from other forms of hyperthyroidism
  • Follow-up of treated Graves’ disease
  • In early pregnancy to differentiate gestational thyrotoxicosis from Graves’ disease
  • Differentiation the transient hyperthyroid phase of postpartum thyroidism from Graves’ disease
  • In cases where the cause of thyrotoxicosis is not clearly apparent.

30.5.3.2 Method of determination

Two different principles are used to assess the level of autoantibodies directed against the TSH receptor (TSHR):

  • Classical receptor assay e.g., TSHR binding inhibitory immunoglobulin (TBII) assay. Autoantibodies compete with labelled TSH or labeled M22 antibody for the binding sites of the TSHR
  • Bioassays e.g., TSHR stimulating immunoglobulin (TSI) bioassay. This method is based on the ability of autoantibodies similar to TSH to induce the second messenger cAMP in cell lines.

Classical receptor assay for determination of TRAb

The competitive immunoassay measures the inhibition of the binding of labeled TSH by antibodies in patients sera. These methods use porcine or recombinant TSHR. The test cannot distinguish whether the autoantibodies have blocking or stimulating capabilities. Assays are calibrated against NIBSC standard MRC 90/672. A new NIBSC standard (08/204) has been evaluated /27/. Second generation assays /28/ using recombinant human TSHR and third generation assays using the monoclonal TSHR stimulating antibody M22 are commercially available.

An automated third generation assay is performed as follows /29/: the reagents used are porcine TSHR, mouse monoclonal capture antibody, and a ruthenium-labeled monoclonal signal antibody (M22) directed against TSHR. The capture antibody binds to the C-terminal end of the TSHR in the test preparation. Following the addition of streptavidin coated micro particles and ruthenium labeled M22, the autoantibodies in the patient sample are detected based on their ability to inhibit M22 binding. The entire complex is bound to the solid phase of the micro particle by the interaction of biotin with streptavidin.

TSH receptor stimulating immunoglobulin (TSI) bioassay for determination of TRAb

The TSI assay measures the ability of autoantibodies similar to TSH to induce the second messenger cAMP in cell lines. These bioassays as they called, are able to distinguish between stimulating (TSAb) or blocking (TSBAb) autoantibodies, based on their biological activity to either enhance or inhibit the cAMP production. Refer to Ref. /3031/.

30.5.3.3 Specimen

Serum: 1 mL

30.5.3.4 Reference interval

Classic receptor assay in dependence of the manufacturer, cutoff values are 1.5, 1.75, 1.8, or 2.0 IU/L

30.5.3.5 Clinical significance

TRAb are autoantibodies against membrane receptors and are much more prevalent in hyperthyreosis than other thyroid autoantibodies (anti-TPOAb, anti-TgAb). TRAb are stimulatory and the determination has few potential clinical applications. They inform the clinician concerning the etiology of hyperthyroid disease and differentiate Graves’ disease from other forms of hyperthyroidism, even though the diagnosis of Graves’ disease is easily confirmed in most patients with measurements low TSH level, an elevated FT4 concentration and the clinical setting of diffuse goiter, exophthalmos and dermatopathy. In early Graves’disease, if FT4 levels are normal, FT3 is useful /31/.

Detection of TRAbs may have the most value in complex clinical settings such as /3/:

  • Differentiating Graves’ disease from non autoimmune cause (e.g., toxic multinodular goiter) of hyperthyroidism
  • Predicting the risk of recurrent Graves’ disease following therapy with antithyroid drugs
  • In pregnancy including of postpartum period, and the risk neonatal thyroid dysfunction /24/.

Refer to Tab. 30.5-3– Thyroglobin receptor antibodies in thyroid diseases.

For the detection of autoimmune thyroid disease, either Hashimoto thyroiditis or Graves’ disease, based on the robustness of the current assays and their availability, anti-TPOAb and anti-TgAb can be considered as the first-line tests, especially since this strategy may also be more cost-effective than measuring TRAb /3/.

In patients with transient thyrotoxicosis (TSH level below 0.1 mIU/L, and without clinical symptoms) TRAb measured as thyroid stimulating immunoglobulin (TSI) can be twice the upper reference value. Spontaneous resolution occurred at a mean of 5.5 weeks /35/.

30.5.3.6 Comments and problems

Third generation TRAb assays have no advantages over second generation assays in the diagnosis or follow-up of patients with Graves’ disease. Both second and third generation assays exhibit high inter method variability, which may lead to misinterpretation of results, especially in the follow-up of pregnant women with recent onset of Graves’ disease. For this group, use of the second-generation assay is recommended until the outcome of calibration against the NIBSC standard is known /27/.

30.5.3.7 T4 and T3 antibodies

The presence of T4 or T3 autoantibodies should be considered if there is discordance between the TSH level and the concentrations of FT4 or FT3, and the clinical symptoms do not correspond with the thyroid hormone concentrations. The prevalence is dependent on the immunoassay used and was 0.05% in the samples used in a study /36/.

References

1. Huber A, Menconi F, Corathers S, Jacobson EM, Tomer Y. Joint genetic susceptibility to type 1 diabetes and autoimmune thyroiditis: from epidemiology to mechanisms. Endocrine Reviews 2008; 29: 697–725.

2. McGrogan A, Seaman HE, Wright JW, de Vries CS. The incidence of autoimmune thyroid disease: a systematic review of the literature. Clin Endocrinol 2008; 69: 687–96.

3. Winter WE, Jialal I, Devaraj S. Thyrotropin receptor antibody assays. Am J Clin Pathol 2013; 139: 140–2.

4. Schott M, Seissler J, Scherbaum WA. Diagnostik bei autoimmunen Schilddrüsenerkrankungen. J Lab Med 2006; 34: 254–7.

5. Pedersen IB, Knudsen N, Jorgensen T, Perrild H, Ovensen L, Laurberg P. Thyroid peroxidase and thyroglobulin autoantibodies in a large survey of populations with mild and moderate iodine deficiency. Clin Endocrinol 2003; 58: 36–42.

6. Okosieme OE, Evans C, Moss L, Parkes AB, Kuvera LD, Premawardhana E, et al. Thyroglobulin antibodies in serum of patients with differentiated thyroid cancer: relationship between epitope specificities and thyroglobulin recovery. Clin Chem 2005; 51: 729–34.

7. Kim ES, Lim DJ, Baek KH, Lee JM, Kim MK, Kwon HS, et al. Thyroglobulin antibody is associated with increased cancer risk in thyroid nodules. Thyroid 2010; 20: 885–91.

8. Dufour DR. Thyroglobulin antibodies – failing the test. J Endocrinol Metab 2011; 96: 1276–8.

9. Saravanan P, Dayan CM. Thyroid autoantibodies. Endocrin Metab Clin North Am 2001; 30: 315–37.

10. Tozzoli R, Villalta T, Kodermaz G, Bagnasco M, Tonutti E, Bizzaro N. Autoantibody profiling of patients with autoimmune thyroid disease using a new multiplexed immunosaasay. Clin Chem Lab Med 2006; 44: 837–42.

11. Perros PE, ed. British Thyroid Association, Royal College of Physicians. Guidelines for the management of thyroid cancer. 2 nd edn. Royal College of Physicians, 2007.

12. Taylor KP, Parkington D, Bradbury S, Simpson HL, Jefferies SJ, Halsall D. Concordance between thyroglobulin antibody assays. Ann Clin Biochem 2011; 48: 367–9.

13. Ohtaki S, Nakagawa H, Nakamura M, Kotani T. Thyroid peroxidase: experimental and clinical integration. Endocrine J 1996; 43: 1–14.

14. Feldt Rasmussen U. Analytical and clinical performance goals for testing autoantibodies to thyroperoxidase, thyroglobulin, and thyrotropin receptor. Clin Chem 1996; 42: 160–3.

15. D’Herbimez M, Sapin R, Gasser F, Schlienger JL, Wemeau JL. Concordance of eight kits for antithyroid peroxidase autoantibodies determination. Clin Chem Lab Med 2000; 38: 561–6.

16. Cooper DS. Subclinical hypothyroidism. N Engl J Med 2001; 345: 260–5.

17. Targher G, Chonchol M, Zoppini G, Salvagno G, Pichiri I, Franchini M, et al. Prevalence of thyroid autoimmunity and subclinical hypothyroidism in persons with chronic kidney disease not requiring chronic dialysis. Clin Chem Lab Med 2009; 47: 167–71.

18. Meloni A, Mandas C, Jores RD, Congia M. Prevalence of autoimmune thyroiditis in children with celiac disease and effect of gluten withdrawal. Pediatrics 2009; 155: 51–5.

19. Schmidt M, Voell M, Rahlff I, Dietlein M, Kobe C, Faust M, et al. Long-term follow-up of antithyroid peroxidase antibodies in patients with chronic autoimmune thyroiditis (Hashimoto’s thyroiditis) treated with levothyroxine. Thyroid 2008; 18: 755–60.

20. Vestgaard M, Ringholm Nielsen L. Rasmussen AK, Damm P, Mathiesen ER. Thyroid peroxidase antibodies in pregnant women with type 1 diabetes: impact on thyroid function, metabolic control and pregnancy outcome. Acta Obstet et Gynecol 2008; 87: 1336–42.

21. Stagnaro-Green, Abalovich M, Alexander E, Azizi F, Mestman J, Negro R, Nixon A, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid 2011; 21: 1081–1121.

22. Wesseloo R, Kamperman AM, Bergink V, Pop VJM. Thyroid peroxidase antibodies during early gestation and the subsequent risk for first-onst postpartum depression: a prospective study. J Affect Disord 2018; 225: 399–403.

23. Bahn RS, Burch HB, Cooper DS, Garber JR, Greenlee MC, Klein I, et al. Hyperthyroidism and other causes of thyrotoxicosis: management guidelines of the American Thyroid Association and American Association of Clinical Endocrinologists. Thyroid 2011; 21: 593–641.

24. Alexander EK, Pearce EN, Brent GA, Brown RS, Chen H, Dosiou C, et al. 2017 Guidelines of the American THyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid 2017; 27: 315–89.

25. National Academy of Clinical Biochemistry 2004. www.nacb.org/Impg/thyroid_Impg_pub.stm

26. Takasu N, Yamashiro K, Ochi Y, Sato Y, Nagata A, Komiya I, Yoshimura H. TSBAb (TSH-stimulation blocking antibody) and TSAb (thyroid stimulating antibody) in TSBAb positive patients with hypothyroidism and Graves patients with hyperthyroidism. Horm Metab Res 2001; 33: 232–7.

27. Massart C, d’Herbomez M. Thyroid-stimulating hormone receptor antibody assays: Recommendation for correct interpretation of results in Graves disease. Clin Chem 2013; 59: 855.

28. Chauvet J, Leven C, Thuillier P, Capaldo C, Moineau MP, Plee-Gautier E, et al. Comparison of the new, rapid, and fully automized Kryptor TSH receptor antibodies assay with the radioimmunological assay. Clin Lab 2019; 65: 1993–8.

29. Schott M, Hermsen D, Broeker-Preuss M, Casati M, Mas JC, Eckstein A, et al. Clinical value of the first automated TSH receptor autoantibody assay for the diagnosis of Graves’ disease (GD): an international multicenter trial. Clin Endocrinol 2009; 71: 566–73.

30. Schott M, Feldkamp J, Bathan C, Fritzen R, Scherbaum WA, Seissler J. Detecting TSH receptor antibodies with the recombinant TBII assay: technical and clinical evaluation. Horm Metab Res 2000; 32: 429–35.

31. Leschik JC, Diana T, Olivo PD, König J, Kran U, Li Y, et al. Analytical performance and clinical utiliy of a bioassay for thyroid-stimulating immunoglobulins. Am J Clin Pathol 2013; 139: 192–200.

32. Eckstein AK, Plicht M, Lax H, Neuhäuser M, Mann K, Lederbogen S, et al. Thyrotropin receptor autoantibodies are independent risk factors for Graves’ ophthalmopathy and help to predict severity and outcome of the disease. J Clin Endocrinol Metab 2006; 91: 3464–70.

33. Bülow Pedersen I, Knudsen N, Perrild H, Ovesen L, Laurberg P. TSH-receptor antibody measurement for differentiation of hyperthyroidism into Graves’ disease and multinodular toxic goiter: a comparison of two binding assays: Clin Endocrinol 2001; 55: 381–90.

34. Shyamasunder H, Abraham P. Measuring TSH receptor antibody to influence treatment choices in Graves disease. Clin Endocrinol (Oxf.) 2017; 86: 652–7.

35. Angell TE, Van Benschoten O, Cohen DA, Haas AV, Alexander AK, Marqusee E. Positive thyrotropin receptor antibodies in patients with transient thyrotoxicosis. Endocr Pract 2018; 24: 512–6.

36. Fielding AM. Prevalence of serum autoantibody binding of Amerlex thyroxine analog. Clin Chem 1984; 30: 501–2.

30.6 Biochemistry and physiology of thyroid function

30.6.1 Thyroid hormone synthesis

The thyroid gland produces T4 and T3 using iodide obtained from the diet or from the metabolism of thyroid hormones /1/. At least 100 μg of iodide is required per day. The iodide ions are transported via the blood to the thyroid, where thyroid hormones are synthesized in the following steps:

  • Active uptake of iodide ions into the thyroid follicular cells across the basolateral cell membrane by the sodium-iodide symporter. The iodide ion is concentrated by a factor of 30–40 in the cytoplasm of the thyroid follicular cell and then released across the apical cell membrane by an ion transporter into the colloid of the thyroid follicle.
  • Oxidation of the iodide ions to iodine and iodination of tyrosine residues in immature thyroglobulin molecules, which contain 134 tyrosine residues (iodization). This takes place extra cellularly, with the involvement of a H2O2 generating system in a reaction catalyzed by TPO. The iodine then undergoes a series of biochemical reactions that yield the intermediate products 3-monoiodotyrosine (MJT) and diiodotyrosine (DJT).
  • T4 is produced in the thyroglobulin molecule by the coupling of two DIT molecules, while T3 is produced by the coupling of MIT with DIT or the 5’-monodeiodination of T4 catalyzed by type 1,5’-deiodinase, which is found in the thyroid, pituitary, liver, and kidney. T4 and T3 produced in the thyroid remain attached to mature thyroglobulin molecules, which are packaged into vesicles and transported to the colloid of the thyroid follicle. T3 and T4 is stored in the thyroid follicles in form of sterically levorotatory, metabolically active substances (LT3 and LT4).

Refer to Fig. 30.6-1 – Thyroidal iodine metabolism and hormone synthesis.

T4 and T3 (plus a small amount of thyroglobulin) are released into the circulation by reverse endocytosis into capillaries surrounding the thyroid follicular cells. Approximately 10 μg of T3 and 100 μg of T4 are released into the circulation daily. T3 has a biological half life of 19 hours whereas T4 has a half life of 190 hours. Approximately 25 μg of T3 is produced daily by the conversion of T4 to T3. Metabolically inactive reverse T3 (rT3) is also produced in the peripheral tissues when metabolically active LT4 is converted to LT3.

99.5% of circulating thyroid hormone is bound to transport proteins. T3 and T4 binding has the following advantages:

  • Only minimal loss of thyroid hormone through the kidneys
  • Presence of a large thyroid hormone pool with maintenance of a constant hormone concentration
  • Even supply to all tissues.

Drugs and other hormones also affect thyroid hormone synthesis and binding affinity. Changes in the total thyroid hormone concentration in the blood (especially T4) can therefore occur in the absence of thyroid dysfunction. Approximately 0.3% of T3 occurs as FT3 and 0.03% of T4 occurs as FT4.

30.6.2 Thyroid hormone action in the tissues

In order to exert their effects on the tissues, thyroid hormones first have to bind to cellular hormone receptors /2/. These receptors are located on the cell membrane, in the mitochondria, in the nucleus, and in the form of cytoplasmic binding proteins. Approximately 15% of the T3 in hepatocytes is bound to the cell membrane, 10–15% to the mitochondria, and 50% to cytoplasmic binding proteins. These proteins are thought to act as a reserve for T4 and T3, controlling the amount of hormone available for the nucleus and mitochondria.

Refer to Fig. 30.6-2 – Cellular binding sites of T3 and T4.

For thyroid hormones to exert their biological functions at nuclear receptors, T4 must first be converted to T3. Conversion of T4 to T3 takes place /3/:

  • In the peripheral tissues, catalyzed by 5’-deiodinase type 1 (D1). In autoimmune hyperthyroidism, the relative proportion of T3 to T4 is increased due to TSH receptor stimulation or thyroid stimulating immunoglobulin. This is partly due to the promotion of the iodotyrosine coupling reaction that produces T3 and partly due to the activation of D1 in the thyroid, which leads to increased intrathyroidal conversion of T4 to T3. In non-thyroidal illness, D1 activity is reduced, which results in low T3 (low T3 syndrome).
  • In the brain, catalyzed by 5’-deiodinase type 2 (D2). The function of D2 is to maintain a constant T3 concentration in special tissues such as the central nervous system, even in the face of iodine deficiency or hypothyroidism.
  • In the skin and placenta, catalyzed by 5’-deiodinase type 3 (D3). This deiodinase transforms T4 into inactive reverse T3 (rT3). During pregnancy, thyroid hormone metabolism is altered by increased degradation of T4 to rT3.

The actions of thyroid hormones on a target cell are mediated by the transportation of T4 and T3 into the cytoplasm and the subsequent conversion of T4 to T3, catalyzed by 5’-deiodinase. T3 enters the nucleus and binds to the thyroid hormone receptor. Thyroid hormone receptors are proteins that act as nuclear transcription factors. Following T3 binding, they regulate gene transcription in the nucleus, thereby mediating actions of T3 such as the synthesis of mitochondrial enzymes.

Refer to Fig. 30.6-3 – Regulation of mitochondrial enzyme induction by T3 via the nuclear receptor.

Thyroid hormone receptors have a central DNA binding domain as well as a T3 binding site and bind to thyroid response elements (TRE) of target genes /4/. TREs are located in the regulatory region of genes and control the transcription of genes that are regulated by T3. The increase in alkaline phosphatase that occurs in autoimmune hyperthyroidism is a clinical marker of the effect of T3 that reflects transcriptional regulation: it results from the actions of T3 on liver and bone.

Refer to Fig. 30.6-4 – Effect of T3 on gene transcription.

The thyroid hormone receptor is regulated by the genes, TR-α and TR-β, which are located on chromosomes 3 and 17. More than 30 mutant forms of TR-β are known. These mutations are responsible for thyroid hormone resistance syndrome, in which TR is unable to bind T3 effectively. This results in reduced transcription of genes that are activated physiologically by T3.

30.6.3 Thyroid hormone metabolism

Quantitatively T4 is the main synthetic product of the thyroid follicular cells and T3 is the metabolically active form of thyroid hormone in the peripheral tissues /3/. The most important reactions in metabolic activation and inactivation are ring deiodination reactions. Metabolically active T3, for example, is produced mainly by outer ring deiodination of T4 in peripheral tissues. T3 and T4 are inactivated by inner ring deiodination.

Refer to Fig. 30.6-5 – Pathways of thyroid hormone metabolism.

Thyroid hormone is inactivated in the following reactions:

  • Inner ring deiodination e.g., of T3 to 3,3’-diiodothyronine by various deiodinases, which are integral membrane proteins that require thiols as co factors and contain selenocysteine residues
  • Esterification of the phenolic OH group with sulfate by sulfotransferases. Sulfation (e.g., conjugation of a sulfo group to the phenolic OH group of thyroid hormones), accelerates the deiodination of various iodothyronines by deiodinases and causes irreversible hormone inactivation.
  • Etherification of the phenolic OH group with glucuronic acid by UDP-glucuronyltransferases. Glucuronidized forms of thyroid hormones are excreted in the bile or feces and are intermediate products of enterohepatic circulation.

30.6.4 Hypothalamic-pituitary-thyroid system

The hypothalamus and anterior pituitary control the concentration of FT4 and FT3 in the circulation /6/. When FT3 or FT4 contacts the para ventricular nucleus (FT4 is converted to FT3 by a 5’-deiodinase type 2), the intracellular concentration of FT3 increases, which leads to a decline in the secretion of TRH into the hypothalamic-pituitary-thyroid axis. TRH is a tripeptide amide (pGlu-His-ProNH2) that regulates the synthesis, release and biological activity of TSH.

Hypophysiotropic TRH neurons secrete TRH into the pericapillary space of the external zone of the median eminence for conveyance to anterior pituitary thyrotrophs. Under basal conditions, the activity of the hypophysiotropic TRH neurons is regulated by the negative feedback effects of TSH, to ensure stable circulating thyroid concentrations. However, the set point for negative feedback regulation by TSH is influenced by other hormonal and neuronal inputs on the hypophysiotropic TRH neurons /5/.

TSH is released from anterior pituitary thyrotrophs following TRH binding to its receptors. An influx of Ca2+ into the cell occurs, which leads to activation of the calcium-phosphatidyl cascade. As a result, the α- and β-subunits of TSH are synthesized and glycosylated and TSH is secreted. Glycosylation is very important for the biological activity of TSH.

TSH released into the circulation binds to the TSH receptors of the thyroid follicle. The actions of TSH at its receptor in the thyroid follicle are mediated by TSH receptor (TSHR)-G protein coupled synthesis of intracellular cyclic adenosine mono phosphate.

Refer to Fig. 6.2-4 – Gs-protein-mediated signal transmission of the parathyroid hormone sensing receptor.

TSH binding to its receptors initiates the following:

  • The cAMP regulatory cascade, which has a positive effect on the growth of thyroid follicular cells and thyroid hormone secretion
  • The phospholipase C-diacylglycerol regulatory cascade, which requires a TSH concentration that is 5–10 times higher for activation. This pathway, which generates the intracellular signals myo-inositol 1,4,5 triphosphate (1,4,5 PIP3) and diacylglycerol, is important for iodination and hormone synthesis.

30.6.5 Biochemistry of TSH

The approximate molecular weight of TSH is 28 kDa, but its circulating forms are heterogeneous and have a higher molecular weight due to varying degrees of post translational glycosylation and sialylation of the α- and β-chains, which in turn determine the half life and biological activity /7/. Sialylation of the α-chain increases the biological activity of TSH, while sialylation of the β- chain reduces it. Lowest TSH levels are measured in the late afternoon; levels increase throughout the evening and reach a maximum during the late evening and night. The TSH concentration declines again during sleep. TSH secretion is partially basal (non pulsatile) and partially pulsatile /8/. In relation to the distribution volume, the daily basal secretion is 12.5 ± 1.1 mU/L and the daily pulsatile secretion is 18.1 ± 1.7 mU/L.

Subclinical hypothyroidism

Subclinical hypothyroidism in younger population is a cardiovascular risk factor such as elevated LDL cholesterol and hypertension /9/. In individuals above the age of 65 in whom the serum TSH levels are elevated but who do not manifest other clinical signs and symptoms of disease the question arises of whether TSH elevations have the same clinical and metabolic implication as in younger patients. One possibility is that a decrease in deiodinase type 2 activity give rise to a higher TSH level at any serum level of FT4. Because it is the deiodinase type 2 that gives rise to most serum FT3, this would also explain the age related increase in TSH and decline in serum FT3.

30.6.6 Biochemistry of the TSH receptor

The TSH receptor has an important role in controlling the growth of the thyroid and regulating hormone synthesis /7/. Mutations in the TSH receptor gene can lead to increased or decreased receptor activity. The P162A and I167N mutations are associated with loss of function; the receptor remains deactivated. Mutations that lead to continuous activation of the receptor, on the other hand, are associated with increased function. This is the case for gain-of-function mutations of the TSH receptor that produce the toxic adenoma or multinodular goiter phenotype. Germ line loss-of-function mutations of the TSH receptor are associated with hypothyroidism and TSH resistance.

References

1. Larsen RP, Ingbar SH. The thyroid gland. In: Wilson JD, Foster DW (eds). Williams Textbook of Endocrinology. Philadelphia: Saunders, 1992; 357–487.

2. McNabb FMA. Thyroid hormones, their activation, degradation and effects on metabolism. J Nutr 1995; 125: S1773–S1776.

3. Motomura K, Brent GA. Mechanisms of thyroid hormone action. Endocrin Metab Clin North Am 1998; 27: 1–23.

4. Graves PN, Davies TF. New insights into the thyroid-stimulating hormone receptor. Endocrin Metab Clin North Am 2000; 29: 267–286.

5. Burch HB. Drug effects on the thyroid. New Engl J Med 2019; 381: 749–61.

6. Winter WE, Signorio MR. Review: Molecular thyroidology. Ann Clin Lab Sci 2001; 31: 221–44.

7. Roelfsema F, Veldhuis JD. Thyrotropin secretion patterns in health and disease. Endocrine Rev 2013; 34: 619–57.

8. Soboll S. Thyroid hormone action on mitochondrial energy transfer. Biochim Biophys Acta 1993; 1144: 1–16.

9. Klein I. Subclinical hypothyroidism – just a high serum thyrotropin (TSH) concentration or something else? J Clin Endocrinol Metab 2013; 98: 508–10.

Table 30.1-1 Thyroid function and physiological states

Clinical and laboratory findings

Newborn

TSH, T4, and T3 are detectable in the fetus from the 12th week of gestation. With increasing gestational age, the concentration of T4 and T3 (and therefore also FT4 and FT3) increases continuously. By gestational week 26–28, the response to exogenous TSH is similar to that seen in adults. Due to very low T4 deiodinase activity, the conversion rate of T4 to T3 is low and the T3 level in cord blood is extremely low. From the 30th week of gestation to the first month of life, deiodinase activity increases by a factor of 10 /12/. T4 levels reach a peak 24 hours after birth.

TSH increases abruptly to > 40 mIU/L within 24 hours of birth in full-term infants and then declines continuously. By postnatal day 3, only 10% of newborns still have levels > 5 mIU/L. The incidence of TSH levels > 40 mIU/L on postnatal day 3 is 0.027% in full-term infants, 0.10% in low birth weight infants, and 0.38% in infants with a birth weight of < 1500 g /12/. The WHO criteria for adequate iodine supply in the newborn are a TSH level of < 5 mIU/L and urinary iodine of ≥ 100 μg/L /13/.

It takes 4–8 weeks for pre term infants to achieve the thyroid hormone status of full-term infants. A transient decrease in T4 occurs in up to 85% of cases. Transient hypothyroxinemia (T4 in cord blood ≤ 10th percentile corrected for gestational age) is the most common thyroid dysfunction of pre term infants and is characterized by TSH levels between 8 and 20 mIU/L /14/. Hypothyroidism in infants of less than 30 weeks gestation is associated with neurodevelopmental abnormalities, low intelligence quotient (IQ), and increased incidence of seizures. Risk factors for hypothyroxinemia include bacteremia, endotracheal bacterial cultures, persistent ductus arteriosus, hypoxia, and the use of aminophylline, dexamethasone, caffeine, diamorphine, and dopamine /15/. T4 levels in full-term newborns are around twice as high as those seen in adults. The T4 level is dependent on the birth weight and in pre term infants is around half as high as the level seen in full-term infants /12/.

Children

The daily T4 turnover in early childhood (5–6 μg/kg body weight) is significantly higher than the turnover in older children. The T4 turnover in adults is 1.5 μg/kg. Serum thyroglobulin concentration reaches adult level by the age of 6 months. The weight of the thyroid gland increases by 1 g per year and by the age of 15 years has reached the adult weight of 15–20 g. The FT3 concentration declines continuously at a rate of approximately 0.1 pmol/L per year, from a mean level of 6.3 ng/L (8.3 pmol/L) at birth to 5.3 ng/L (6.3 pmol/L) by the age of 20 years /16/.

Athletes

The concentrations of TSH, FT4, and FT3 can change during exercise, particularly during prolonged and strenuous activity. The observed variations are mainly linked to increased metabolic demand. Increases of up to 60% in TSH levels and 10–20% in FT4 and FT3 levels compared to baseline values have been found in ice skaters /17/.

The elderly

Disorders of thyroid function are more common in individuals > 65 years of age /18/. In the USA, 11% of over-65s have hypothyroidism and 2.5% have hyperthyroidism. In Continental Europe, hyperthyroidism is more common in the elderly than hypothyroidism. These differences are due to differences in the iodine supply. Hyperthyroidism and decreased TSH are more common in iodine-deficient regions while hypothyroidism and elevated TSH are more common in iodine replete regions. Thyroid dysfunction in the elderly is thought to be due to the following /19/:

  • TRH synthesis and release are reduced as a result of age induced changes in neuroregulation
  • TSH levels are in the low normal range; TSH bioactivity is thought to be decreased
  • The feedback inhibition mechanism becomes more sensitive to circulating T4 and T3 levels.

Overall, TSH and FT3 are in the low normal range, FT4 is normal, and rT3 is elevated.

Pregnancy /820/

Pregnancy has a profound impact on thyroid function and thyroid gland. In iodine replete countries the gland increases 10% in size and by 20–40% in areas of iodine deficiency. The daily iodine requirement and the production of T4 and T3 increase by 50%. The glomerular filtration rate of iodide increases early in pregnancy, resulting in increased renal iodide clearance and increased iodine requirements. If sufficient iodine is not available to meet these increased requirements, the resulting relative iodine deficiency leads to increased TSH secretion, increased thyroglobulin concentration, an increase in the T3/T4 ratio, and finally to maternal and fetal goiter.

The glycoprotein hCG has an α-subunit identical to that of TSH. hCG increases continuously throughout the first trimester. Desialylated or deglycosylated forms of hCG have weak TSH activity and act as agonists at the thyroid follicle. When hCG production reaches its peak at around the 10th week of gestation, TSH begins to decline and T4 begins to increase. The increased estrogen concentration causes thyroxine-binding globulin (TBG) levels to double in weeks 16–20. This increase in TBG causes an increase in T4, a less marked increase in T3, and a decrease in FT4. Due to the effects of hCG, the decrease in FT4 is accompanied by a decrease in TSH rather than an increase. Transient hyperthyroid states in the first trimester are usually due to inappropriately high hCG secretion. In the second and third trimesters, FT4 and FT3 decline continuously (FT3 by about 20% and FT4 by 40%). However, levels usually remain within the reference interval (although they may be borderline).

Thyroid function should be monitored during pregnancy using a combination of TSH and FT4. Because the TSH level declines by an average of 1–1.5 mIU/L, FT4 is often helpful in determining whether a suspected pituitary dysfunction is present.

  • Cigarette smoking during pregnancy was associated with the following changes /21/: in smokers compared with non smokers, median serum TSH was lower (first-trimester cohort: 1.02 compared to 1.17 mIU/L; third-semester cohort 1.72 compared to 1.90 mIU/L) and FT3 was higher (first-trimester cohort: 5.1 compared to 4.9 pmol/L; third-trimester cohort: 4.4 compared to 4.1 pmol/L). In both cohorts, serum FT4 levels in smokers and non smokers were similar.
  • Subclinical hypothyroidism and thyroid autoantibodies are associated with pre term delivery /22/.
  • The concentrations of thyroid hormone and TSH from a Swiss study /23/ and an U.S. study /24/ are shown in Tab. 30.1-5 – Reference intervals for TSH (mIU/L) and thyroid hormones in pregnant women. The data of a review are published in Ref. /25/.

The diagnostic assessment of thyroid disorders in pregnancy and the postpartum period are discussed in detail in the American Thyroid Association guidelines /826/.

Table 30.1-2 Adaptive response to non-thyroidal illness /27/

Clinical and laboratory findings

Non-thyroidal illness (NTI), low T3 syndrome

During acute and particularly severe illness profound changes may occur in the hypothalamic-pituitary-thyroid axis. The most consistent change is a decrease in serum T3 and FT3 concentration and a rise in rT3 level. In severe non-thyroidal illness (NTI) T4 and FT4 may also decrease. The persistence of a normal or even decreased TSH level in the face of decreased thyroid hormone concentrations implies a major change in the hypothalamic-pituitary-thyroid axis set point regulation. Since the abnormalities of thyroid hormone concentration occur without evidence of thyroid disease and disappear with recovery they have been referred to as NTI or low-T3 syndrome /27/.

Laboratory findings: in critically ill patients, a decline in T4 to < 40 μg/L (51 nmol/L) is associated with a mortality risk of up to 50% and a decline to < 20 μg/L (26 nmol/L) is associated with a poor prognosis. Although the TSH level does not usually decrease to < 0.05 mIU/L, it is inadequately low in relation to T3 and T4 /29/. In the case of low TSH, using a third generation assay, it is possible to distinguish NTI from hyperthyroidism which typically has TSH levels of < 0.01 mIU/L. A high TSH level raises the possibility of hypothyroidism but this may also occur in the framework of NTI, especially upon recovery. A high ratio of T3 to T4, a low thyroid hormone binding ratio and a low concentration of rT3 favour the presence of hypothyroidism over NTI and vice versa /28/.

Liver disease /30/

Acute hepatitis: T4, FT4, and TBG concentrations are elevated. At the onset of illness, T3 and FT3 are decreased and rT3 is elevated.

Liver cirrhosis: TBG is elevated in compensated cirrhosis and declines as decompensation occurs. As decompensation progresses, TBG synthesis decreases further and T4 and T3 levels are reduced. FT4 levels are usually normal while FT3 levels are low.

Renal failure /31/

Hypothyroidism is associated with a reduced glomerular filtration rate (GFR), reduced excretion of free water, and a low serum Na+ concentration. Increased concentrations of thyroid hormone lead to increased renal plasma flow and increased GFR.

  • Acute renal failure: thyroid hormone results in these patients are comparable to those seen in patients with NTI; however, the decrease in T3 is not accompanied by an increase in rT3.
  • Chronic kidney disease (CKD): patients with CKD have larger thyroid glands and an increased prevalence of goiter, compared to the normal population. TSH levels are usually normal or mildly elevated but the response of TSH to TRH is attenuated. A decrease in FT3 is the most common thyroid function abnormality seen in CKD. This occurs due to impaired peripheral conversion of T4 to T3, possibly as a result of chronic metabolic acidosis. FT4 concentrations may be increased during hemodialysis due to heparin induced inhibition of T4 protein binding. Patients with CKD often have subclinical hypothyroidism. In a study /32/, the prevalence of hypothyroidism in patients with a GFR > 90 [mL × min–1 × (1.73 m2)–1] was 7%, while in patients with a GFR < 60 [mL × min–1 × (1.73 m2)–1], the prevalence was 17.9%. The prevalence of hypothyroidism is higher in women than in men and in those who have elevated thyroid antibody titers. The prevalence of hyperthyroidism in CKD is the same as in the general population (about 1%).

Diabetes mellitus

TSH, FT3, and FT4 levels are normal in well controlled diabetes. Low T3 syndrome can develop in diabetics with severe ketoacidosis.

Acromegaly /27/

Normal/diminished TSH levels but attenuated TSH response to TRH. T4 normal, T3 mildly elevated and rT3 decreased as a result of growth hormone (GH) induced increase in D2 deiodinase activity in the liver under GH excess.

Cushing’s disease /27/

In patients with hypercortisolism and in healthy individuals after glucocorticoid administration a decreased efficacy of TRH in inducing TSH release has been documented. Diminished TSH levels were shown in patients with pituitary dependent hypercortisolism and those with cortisol secreting adrenal adenoma. T3 is decreased and T4 is slightly decreased while FT4 levels are normal. Possible explanations for the relatively normal T4 levels during hypercortisolism are that T4 to T3 conversion is inhibited by glucocorticoids and that the biological activity of TSH is increased by altered post translational processing of its oligosaccharide chains.

Addison’s disease /27/

Withholding hydrocortisone substitution for 2 days elevates TSH levels.

Sheehan’s syndrome /22/

Untreated patients have diminished FT4 and increased TSH concentrations. The paradoxical elevation in TSH is due to reduced TSH bioactivity, caused by increased sialylation.

Psychiatric illness /27/

TSH is mildly elevated in major depression /33/ or response to TRH is diminished. T4 may be elevated in acutely ill psychiatric patients upon admission to hospital. Patients with posttraumatic stress may have elevated T3 and FT3 due to changes in T4 metabolism.

Neurological disorder /27/

The rationale for investigating thyroid function in some neurological diseases is possible involvement of the thalamus and hypothalamus. Narcolepsy is associated with a reduction in TSH within the reference interval and normal levels of T3 and T4. In Huntington’s disease, TSH, T3, and T4 are increased within the reference interval while in Parkinson’s disease, levels of these hormones are normal.

Central hypothyroidism /34/

Central hypothyroidism may be secondary or tertiary. Secondary hypothyroidism is caused by a pituitary disorder while tertiary hypothyroidism is caused by a hypothalamic disorder. Causes include trauma, tumors, radiotherapy, infiltrative disorders such as sarcoidosis and amyloidosis, and inflammation of the pituitary. More than 50% of cases are due to tumors. Other causes include:

  • Inactivating mutations in the TRH receptor gene or TSH gene
  • Inhibition of TSH secretion by drugs such as bexarotene (used in the treatment of lymphoma).

Laboratory findings: FT4 decreased, TSH level varies from below normal to mildly elevated. In a study /33/, TSH was decreased in 35%, normal in 41%, and elevated in 25% of cases. Normal or mildly elevated TSH levels occurred as a result of reduced TSH bioactivity caused by increased sialylation of the β chain. The abnormal TSH shows normal immunoreactivity in immunoassays. It may only be possible to recognize mild central hypothyroidism based on the absence of the nocturnal decline in TSH. Central hypothyroidism can be distinguished from primary hypothyroidism (which is usually autoimmune) (Tab. 30.1-3 – Primary thyroid disorders associated with hyper-, hypo- and euthyroidism) by determining thyroid peroxidase antibodies (anti-TPOAb), which are always positive in autoimmune hypothyroidism, or by performing the TRH test, in which abnormally low TSH stimulation indicates hypopituitarism.

Pituitary adenoma (thyrotropinoma)

Approximately 72% of these tumors produce TSH only, 16% also produce GH, and 11% also produce prolactin. Approximately 90% of adenomas are macro prolactinomas and 71% show suprasellar extension or penetration into surrounding tissue /35/. The prevalence of macro adenomas and tumor invasion is higher in patients with a history of radioiodine therapy or thyroidectomy.

Clinical presentation: this depends on the size of the adenoma, whether it secretes hormones, and which hormones are secreted. If the tumor secretes excess TSH only, the main clinical features are hyperthyroidism and diffuse goiter. If GH is also secreted, the clinical picture is dominated by acromegaly. If prolactin is also secreted, the main symptoms are amenorrhea in women and impotence in men. It is thought that goiter formation is due to the production of TSH molecules with higher than normal biological activity.

Laboratory findings: basal TSH, FT4, and FT3 are elevated. Absent TSH response in TRH test. It is important to rule out the presence of T4 autoantibodies, which, depending on the immunoassay used, may mimic an increase in T4 as well as heterophilic antibodies and anti-mouse antibodies, which can also interfere with TSH determination, depending on the immunoassay used.

Resistance to thyroid hormone (RTH) /36/

Thyroid hormone resistance is a rare autosomal inherited syndrome of reduced end-organ responsiveness to thyroid hormone. Mutations in the thyroid hormone receptor (TR) β gene are responsible for RTH. Approximately 122 different mutations in 300 families have been identified. RTH has an incidence of 1 in 40,000 live births. Mutations are located in the functionally relevant domain of the T3 binding site. As a result of these mutations, T3 fails to suppress TSH secretion and peripheral tissues are resistant to the metabolic effects of T3. The phenotype of this autosomal recessive disorder exhibits intrafamilial and inter familial variation. A distinction is made between generalized RTH (GRTH) and partial RTH (PRTH), which does not represent a separate entity. GRTH affects all thyroid sensitive tissues while PRTH is characterized by selective pituitary resistance and partial peripheral resistance to T3. Clinical symptoms include neurological symptoms (hyperactivity, attention deficits), goiter, tachycardia, and delayed bone maturation. Mutations in the TRH receptor α-gene have also been described.

FT3 and FT4 are transported actively through the cell membrane by mono carboxylate (MCT) transporters. Mutations in the MCT8 (SLC16A2) gene in particular can lead to a syndrome characterized by cognitive impairment and limited speech in boys.

Laboratory findings: in RTH, FT4 and FT3 are elevated while TSH levels are normal or mildly elevated (usually < 10 mIU/L); there is a marked TSH response in the TRH test. With mutations in the MCT8 gene, TSH is normal or mildly elevated, FT4 is below normal, FT3 is significantly elevated, and reverse T3 is decreased.

Heparin therapy

Heparin inhibits the binding of T4 to its binding proteins. Therefore, FT4 may be elevated in patients with severe non-thyroidal illness receiving heparin therapy without accurately reflecting the metabolic situation. In this case, further investigations should be performed based on the clinical symptoms (e.g., T4, TSH, the TRH test, or TBG). Blood should be collected for thyroid hormone determination at least 24 hours after the last heparin dose.

Table 30.1-3 Primary thyroid disorders associated with hyper-, hypo- and euthyroidism

Clinical and laboratory findings

Primary hyperthyroidism /37/

Hyperthyroidism can be divided into 12 nosological subtypes with different etiology, clinical presentation, prognosis, and outcome of therapy. According to a Danish study /38/ the overall standardized incidence rate per 100,000 person years was 81.6. Nosological types of hyperthyroidism were: multi nodular toxic goiter 44.1%, Graves’ disease 37.6%, solitary toxic adenoma 5.5% mixed type hyperthyroidism 5.4% subacute thyroiditis 2.3%, postpartum thyroid dysfunction 2.2%, amiodarone associated hyperthyroidism 0.8%, hyperthyroidism after thyroid radiation 0.7%, and lithium associated hyperthyroidism 0.8%. Overt hyperthyroidism is a risk factor for osteoporosis and fractures.

Hyperthyroidism is characterized by a hyper metabolic state with increased synthesis and secretion of thyroid hormones. Hyperthyroidism can result from:

  • Activation of the synthesis and autonomous secretion of T4 and T3 by the thyroid
  • The release of excessive quantities of stored T4 and T3 as a result of autoimmune, infectious, mechanical, or chemical injury to the thyroid
  • Extra thyroidal production of T4 and T3 (struma ovarii, metastatic differentiated thyroid cancer) or administration of thyroid hormone (factitious thyrotoxicosis).

The main subtypes of hyperthyroidism are autoimmune hyperthyroidism (Graves’ disease), toxic multinodular goiter, and subclinical hyperthyroidism. Overall, hyperthyroidism is more prevalent in iodine deficient areas than in areas with an adequate supply of iodine.

The signs and symptoms of overt and subclinical thyrotoxicosis are similar, but differ in magnitude:

  • Subclinical hyperthyroidism: patients are asymptomatic; low TSH, FT4 and FT3 within the reference interval
  • Overt hyperthyroidism: patients exhibit typical symptoms of hyperthyroidism; low TSH, elevated FT3 or FT4, or both
  • Thyrotoxic crisis: pronounced symptoms of hyperthyroidism; low TSH markedly elevated FT3, and FT4.

Laboratory findings: the determination of TSH is the most sensitive test; in the absence of a pituitary adenoma, a normal TSH level rules out hyperthyroidism. In clinical hyperthyroidism, FT3 and FT4 are usually elevated and TSH is below 0.01 mIU/L or undetectable. In mild hyperthyroidism, TSH is below 0.01 mIU/L, FT4 is normal, but FT3 is elevated.

Autoimmune hyperthyroidism (Graves’ disease)

Autoimmune hyperthyroidism is a T cell mediated immune response of unknown origin that is characterized by the production of antibodies directed against the TSH receptor (TR). This results in stimulation of thyroid follicular cells with increased secretion of thyroid hormones and thyroid enlargement. The predisposition to developing autoimmune hyperthyroidism is thought to be 79% genetic and 21% environmental. A1, B8, and DR3 are frequently seen haplotypes /39/. Gender, age, cigarette smoking, iodine intake, and drugs also play a role. Autoimmune hyperthyroidism that is accompanied by endocrine orbitopathy is known as Graves’ disease.

Depending on the region, the incidence of autoimmune hyperthyroidism is 15–100 per 100,000 in the general population and it is 5–10 times more common in females than in males /36/. The incidence increases with age, is low in childhood, and displays a peak during adolescence. In adults, Graves’ disease occurs most frequently between the ages of 20 and 50 years. Graves’ disease accounts for 37.6% of all cases of hyperthyroidism.

Clinical presentation: the severity of the clinical symptoms is determined by the amount of thyroid hormone excess and the associated metabolic status. Two additional factors that determine the clinical symptoms are age and concurrent medication use. The hyper metabolic status is much less pronounced in older than younger individuals. α-adrenergic drugs and tranquilizers may mask the symptoms of hypermetabolism. Typical symptoms include tachycardia, excessive sweating, nervousness, diarrhea, diffuse thyroid enlargement, weight loss, oligomenorrhea and anovulation, myopathy, ophthalmopathy, and episodic anxiety.

Laboratory findings: TSH less than 0.01 mIU/L, elevated FT3 and/or FT4, TRAb above the reference interval in > 95% of cases, elevated anti-TgAb and anti-TPOAb in most patients.

Toxic multinodular goiter (TMNG), Plummers’s disease /40/

The prevalence of goiter is variable depending on the degree of iodine sufficiency in the geographic are, the genetic predisposition of the population, and the definition of goiter. TMNG is invariably one of the top two listed causes of thyrotoxicosis in different parts of the world. TMNG is the final phase in the evolution of goiter over time and develops slowly in a thyroid whose nodules gain autonomy. The prevalence of TMNG is higher in iodine deficient areas and in older individuals. The clinical symptoms are typically milder than those of autoimmune hyperthyroidism. TMNG accounts for 44.1% of all cases of hyperthyroidism.

Development: TMNG is caused by iodine deficiency in > 90% of cases. More than 15% of the world’s population live in iodine deficient areas and 4–5% have iodine-deficiency goiter /41/. The severity of endemic goiter can be classified into three grades based on the urinary iodine excretion (Tab. 30.1-6 – Severity of endemic goiter). Iodine deficiency and goitrogens cause a shift in the normal set point of hypothalamic-pituitary-thyroid axis and stimulate thyroid enlargement. This enlargement results mainly from an increase in the size rather than the number of thyroid cells. Toxic nodules result from monoclonal proliferation of thyroid follicular cells. These cells have a higher capacity for iodine uptake and hormone production that is independent of TSH. Autonomic hormone production is caused by somatic activating mutations of genes that regulate follicular cell activity, such as mutations of G-protein coupled receptors. For example, a mutation in the Gs alpha protein gene leads to chronic activation of this protein (Fig. 6.2-4 – Gs-protein-mediated signal transmission of the parathyroid hormone sensing receptor (PTHSR) or of the calcium-sensitive receptor (CaSR)), which results in continuous activation of the cAMP pathway and TSH independent follicle growth with hyper function. Subclinical hyperthyroidism may develop into overt hyperthyroidism over a number of years due to a progressive increase in thyroid hormone production.

Laboratory findings: the diagnosis of TMNG is largely clinical and supported by laboratory findings. Often patients are asymptomatic. TSH is decreased, FT3 and FT4 are normal.

Mixed hyperthyroidism

Mixed hyperthyroidism refers to a combination of scintigraphically confirmed TMNG and elevated TSH receptor antibodies (TRAb). Mixed hyperthyroidism accounts for 5.4% of all cases of hyperthyroidism.

Solitary toxic adenoma (STA)

STA is a toxic nodule that exhibits increased TcO4 uptake combined with low or absent uptake in the surrounding tissue. Scintigraphic examination shows hyper function in approximately 5% of solitary nodules; these nodules are referred to as autonomous toxic adenomas. Only 25% of these patients are hyperthyroid and young. Patients with a node measuring less than 2.5 cm in diameter, are nearly always euthyroid. Solitary nodules produce thyroid hormone in the absence of TSH and TRAb. Activating gene mutations of the TRH receptor, α subunit of the stimulating G protein (Gsα), or both, cause uncontrolled signal transmission via the cyclic AMP pathway. This results in proliferation and growth advantage for the stimulated cells as well as increased production of thyroid hormones. STA accounts for 5.7% of all cases of hyperthyroidism.

Subacute thyroiditis (SAT)

SAT (also known as de Quervain’s thyroiditis) is the most common cause of a painful thyroid gland. It occurs most frequently between the ages of 40 and 50 years and is four times more common in women than in men. It is characterized by a soft goiter, which is accompanied in around half of patients by transient hyperthyroidism due to the release of thyroid hormone from damaged follicles. There is typically no history of excessive iodine uptake (amiodarone, contrast medium) or the use of potentially implicated drugs (amiodarone, lithium, cytokines). It is the result of a viral infection or post viral process, often due to Coxsackievirus infection. SAT is frequently preceded by a viral infection of the upper respiratory tract which presents with pain that starts in one part of the thyroid and then spreads to the rest of the organ over the next few days. Histologically, it is characterized by an infiltrate of multinucleated giant cells, granulocytes, and lymphocytes. Symptoms of hyperthyroidism are transient. At least two of the three criteria for SAT must be fulfilled: low or absent TcO4 uptake, absent thyroid nodules, increased erythrocyte sedimentation rate (ESR), or neck pain. SAT accounts for 2.3% of all cases of hyperthyroidism.

Laboratory findings: ESR above 50 mm/h (often above 100 mm/h), mild leukocytosis and anemia, association with HLA-B35, FT4 no higher than twice the upper reference interval value, low TSH. Possible slight increase in anti-TPOAb and anti-TgAb.

Postpartum hyperthyroidism /8/

Postpartum hyperthyroidism refers to clinical hyperthyroidism that occurs within a year of childbirth. Cases in which TSH receptor antibodies (TRAb) are negative or were not measured are, by definition, postpartum hyperthyroidism. If TRAb are positive or were positive prior to pregnancy, a diagnosis of autoimmune hyperthyroidism is made. Postpartum hyperthyroidism accounts for 2.2% of all cases of hyperthyroidism. Refer also to the sections about hyperthyroidism in pregnancy and postpartum thyroiditis.

Amiodarone associated hyperthyroidism

Amiodarone associated hyperthyroidism occurs within a year of starting treatment. It accounts for 0.8% of all cases of hyperthyroidism.

Radioiodine associated hyperthyroidism

Transient hyperthyroidism that occurs within a month of radio iodine therapy for euthyroid goiter. It accounts for 0.7% of all cases of hyperthyroidism.

Lithium associated hyperthyroidism

Clinical hyperthyroidism that develops soon after or within 12 months of treatment. It accounts for 0.7% of all cases of hyperthyroidism.

Manipulation thyroiditis

Transient hyperthyroidism that develops shortly after thyroid manipulation (thyroid or parathyroid surgery). It accounts for 0.7% of all cases of hyperthyroidism.

Thyroid storm /42/

Thyrotoxicosis refers to any cause of excessive thyroid hormone concentration. Thyroid storm represents the extreme manifestation of thyrotoxicosis. The most underlying cause of thyrotoxicosis in cases of thyroid storm is Graves’ disease but can also occur in cases with solitary toxic adenoma or toxic multinodular goiter. Thyrotoxicosis and thyroid storm occur most frequently in young women. Rare causes of thyrotoxicosis leading to thyroid storm include hyper secretory thyroid carcinoma, TSH secreting pituitary adenoma, struma ovarii/teratoma, and hCG secreting hydatiform mole. The characteristic clinical symptoms are tachycardia, hyperthermia and heat intolerance, restlessness, tremor, and weight loss.

Laboratory findings: TSH less than 0.01 mIU/L; a normal TSH level rules out thyroid storm. T3, FT3 and T4, FT4 concentrations are elevated. Additional findings include mild leukocytosis, CRP elevation, and mild hyperglycemia; the aminotransferases, CK and ALP are elevated in over 50% of cases.

Thyroid disease during pregnancy and postpartum /8363744/

In the USA about 4% of women coming for prenatal care have previously been diagnosed with hypothyroidism and 0.4% with clinical and 0.6% with subclinical hyperthyroidism /43/.

– Hypothyroidism

Primary maternal hypothyroidism is defined as the presence of an elevated TSH concentration during gestation. Levels > 2.5 mIU/L in the first trimester and > 3.0 mIU/L in the second trimester are considered abnormal. FT4 should be determined if TSH is > 2.5 mIU/L to classify the patient status as either subclinical or overt hypothyroidism. Thyroid autoantibodies are present in about 50% of pregnant women with subclinical and 80% with overt hypothyroidism. Untreated hypothyroidism in pregnancy is associated with preeclampsia, preterm delivery, and miscarriage. An observational study found a 7 point lower mean IQ at age 8 years among offspring of women with autoimmune thyroiditis and undiagnosed overt hypothyroidism during pregnancy; the rate of offspring IQ scores below 85 was also increased 4-fold /44/.

– Hyperthyroidism

Gestational hyperthyroidism: the most frequent cause of thyrotoxicosis in pregnancy is the syndrome of gestational hyperthyroidism, defined as transient hyperthyroidism. It is diagnosed in 1–3% of pregnancies depending on the geographic area and is secondary to elevated hCG concentrations. Transient hyperthyroidism is limited to the first trimester of pregnancy and may be associated with hyperemesis gravidarum which occurs in 0.5–10 per 1,000 pregnancies. Laboratory findings are elevated FT4 and suppressed TSH.

Graves’ disease: it is the most common cause of autoimmune hyperthyroidism in pregnancy, occurring in 0.1–1% of all pregnancies. It may present at the first time or as a recurrent episode in a women with past history of hyperthyroidism. However, first time autoimmune hyperthyroidism is rare during pregnancy. Regardless of whether hyperthyroidism in pregnancy is of new onset or is an existing autoimmune hyperthyroidism that has been treated with surgery, drugs, or radiotherapy, thyroid receptor antibodies (TRAb) are always present. These cross the placenta and put the fetus at risk of a transient hyperthyroidism that can persist during the first six months of life until maternal antibodies have been eliminated. TRAb do not become important until gestational weeks 10–12, when the fetal thyroid starts to produce hormones. The thyroid starts to grow gradually in response to stimulation and the fetus eventually becomes hyperthyroid. High maternal TRAb in the third trimester indicates neonatal hyperthyroidism. TSH secretion is suppressed during the neonatal period and, as the elimination of TRAb increases, the neonatal hyperthyroidism gives way to secondary hypothyroidism until TSH secretion returns to normal.

Non autoimmune causes of thyrotoxicosis: included are toxic multinodular goiter, toxic adenoma, and factitious thyrotoxicosis.

Laboratory findings: in the presence of suppressed TSH in the first trimester (TSH below 0.1 mIU/L) a history and physical examination are important. FT4, and possibly FT3 should be measured in all patients. Reference values are shown in (Tab. 30.1-4 – Latent hypothyroidism). TRAb may be helpful in the diagnosis of Graves’disease. In the presence of nodular goiter, a T3 determination is helpful in assessing the possibility of the T3 toxicosis syndrome.

Overt hypothyroidism

Primary hypothyroidism accounts for approximately 99% of all cases and may or may not be associated with goiter. Most cases of hypothyroidism are due to iodine deficiency, autoimmune, iatrogenic or congenital in origin. A decrease in the amount of active thyroid tissue may be due to inflammation, surgery, radiotherapy, or treatment with goitrogens. Worldwide, iodine deficiency is the most common cause of hypothyroidism. In iodine replete areas, the most common cause is autoimmune thyroiditis. The prevalence of hypothyroidism increases with age and in Germany is approximately 1% in individuals over the age of 60 years.

Congenital hypothyroidism /44/

The incidence of neonatal hypothyroidism is 1 in 4,000 newborns, with a female to male ratio of 2 : 1. If untreated, it can lead to mental retardation and abnormal growth. Because treatment with thyroid hormone needs to be started immediately, the condition must be diagnosed in the first few days of life. Approximately 85% of cases of congenital hypothyroidism are caused by thyroid dysgenesis and 15% have a central cause (hypothalamic-pituitary). In most cases of thyroid dysgenesis (45–60%), the thyroid is ectopic, while in 15–35% of cases, it is completely absent. The aim of neonatal screening for hypothyroidism is to diagnose mild, moderate, and severe congenital hypothyroidism at an early stage. TSH determination is the most sensitive test. Pre term and low birth weight infants, newborns who are ill or have been admitted to the intensive care unit, and twins should all be screened.

Approximately 90% of cases detected by neonatal screening go on to develop normally. There are no differences in the subsequent development of moderate and severe cases, provided treatment is started in the first two weeks of life with L-thyroxine replacement therapy at a dose of 19–15 μg/kg/day. Reasons for a poor outcome despite treatment include a delay in starting treatment, an inadequate initial dose, poor compliance, and adverse socioeconomic conditions.

Thyroiditis /4445/

Thyroiditis is an inflammation of the thyroid gland that has various etiologies and can be associated with normal, depressed, or elevated thyroid function. The differentiation is based on the clinical setting, rapidity of symptom onset, family history, and presence or absence of prodromal symptoms and neck pain. The clinical classification of thyroiditis is based on the onset and duration of symptoms and includes acute, subacute, chronic, and postpartum forms. Hyperthyreosis is seen in a proportion of patients when activated cytotoxic T cells damage the thyroid follicular cells, resulting in the unregulated release of T3 and T4 into the circulation This process is transient, lasting 3–6 weeks and ceasing when the thyroid hormone stores are exhausted. A triphasic sequence is observed: in the early phase (3 to 6 weeks) of thyroiditis FT4 is elevated and TSH suppressed, followed by a phase (lasts up to 6 months) of hypothyroidism with low FT4 and elevated TSH and return to euthyroidisms after 1 year. In 10–15% of patients hypothyroidism persists.

Suppurative thyroiditis

Suppurative thyroiditis is a rare form of thyroiditis caused by acute infection, most often bacterial. In young people, it is associated with a fistula of the fourth branchial pouch that connects the oropharynx to the thyroid. Common pathogens include staphylococci, streptococci, and pneumococci. Infection may also be transmitted to the thyroid via the bloodstream or lymphatic vessels. Suppurative thyroiditis mainly affects patients with pre-existing thyroid disease and immunocompromised patients. Patients usually present with acute neck pain (unilateral, anterior), fever, dysphagia, and dysphonia.

Laboratory findings: increased erythrocyte sedimentation rate, leukocytosis with left shift. The inflammatory response may cause hyperthyroidism by allowing stored thyroid hormone to enter the circulation.

Subacute lymphocytic thyroiditis

This type of thyroiditis belongs to the spectrum of autoimmune thyroid diseases and accounts for 1–25% of all cases of hyperthyroidism. Lymphocytic thyroiditis has a similar pathology to that of Hashimoto’s thyroiditis but has much fewer lymphocytic germinal centers and little or no fibrosis. Approximately 50–60% of patients have a small goiter and some have mild hyperthyroidism that usually lasts for only a few months.

Laboratory findings: mild elevation of FT4 or FT3; 60% of patients have low antibody titers against thyroid peroxidase (TPO) and thyroglobulin (Tg); association with HLA-DR3 and HLA-DR5.

Hashimotos thyroiditis /46/

Hashimotos thyroiditis, the prototype of chronic lymphocytic thyroiditis is an inflammatory disorder of the thyroid gland characterized by goiter, lymphocytic infiltration of the gland, and various degrees of thyroid hypofunction. Hashimotos thyroiditis is an autoimmune disease characterized by a cellular and humoral immune response directed against TPO and Tg with increased titers of circulating antibodies against TPO (anti-TPOAb) and antibodies against Tg (anti-TgAb).

Classically the Hashimotos thyroiditis phenotype is a firm rubbery goiter with a high titer of anti-TPOAb and anti-TgAb. Many variants of this prototypical presentation exist:

  • The goitrous Hashimotos thyroiditis is the hypertrophic form and is commonly diagnosed in children and young people. The goitrous type is characterized not only by lymphocytic infiltration but also by the regrowth of thyroid follicles that were destroyed by cytotoxic T cells.
  • In some patients, the thyroid is atrophic as an effect of a distinctly destructive disease process with profound hypothyroidism. This is associated with the replacement of the thyroid tissue by connective tissue, a hard rubbery goiter, and high anti-TPOAb and anti-TgAb concentrations.
  • In a subset of hypothyroid patients, the thyroid dysfunction is caused by antibodies against the TSH receptor, blocking the action of TSH, rather than by thyroid tissue destruction.

The early stage of the disease is characterized by mild lymphocytic infiltration of the thyroid that is frequently missed. Patients are euthyroid at this stage. This stage is followed, especially in women above the age of 40 years, by a dense lymphoid infiltrate and increasingly destructive thyroiditis. The final phase is characterized by fibrosis and plasma cell infiltration. The clinical picture in the advanced stage is characterized by hypothyroidism.

Laboratory findings: concentrations of TSH, FT4, and FT3 depend on the stage. Clinical symptoms are usually associated with elevated TSH and FT4 and FT3 levels within the lower reference interval or slightly decreased. The concentrations of anti-TPOAb and anti-TgAb (Tab. 30.5-2 – anti-TPOAb in thyroid diseases) are increased; there is an association with HLA-DR3.

Postpartum thyroiditis /844/

Postpartum thyroiditis (PPT) is the occurrence of thyroid dysfunction, excluding Graves´ disease, in the first postpartum year in women who were euthyroid prior to pregnancy. PPT is an autoimmune disorder associated with the presence of anti-TPOAb and anti-TgAb, lymphocyte abnormalities, complement activation, increased levels of IgG1, increased natural killer cell activity, and specific HLA haplotypes (DR3, DR4, DR5) /44/. The prevalence of PPT is approximately 5% and increases to 33–50% in women who present with increased anti-TPOAb and anti-TgAb in the first trimester of pregnancy. Women with other autoimmune disorders have an increased risk of PPT. Specifically, the prevalence is 3–4 times higher in women with diabetes type 1 compared to unselected populations. The frequency is 25% in women with chronic viral hepatitis, 14% in women with systemic lupus erythematosus, 44% in women with primary history of gestational diabetes. Individuals who recover fully from PPT have a 70% chance of developing PPT in each subsequent pregnancy. The clinical course of PPT varies, with approximately one quarter of patients presenting with the classical form, one quarter with isolated thyrotoxicosis, and one half presenting with isolated hypothyroidism.

The thyrotoxic phase of PPT typically occurs between 2 and 6 months post partum, but episodes have been reported as late as 1 year following delivery. All episodes of thyrotoxicosis resolve spontaneously.

The hypothyroid phase of PPT occurs from 3 to 12 months postpartum with 10–20% of cases resulting in permanent hypothyroidism. It should be noted, however, that 50% of women with PPT remained hypothyroid at the end of the first post partum year /8/.

PPT is painless and most women are asymptomatic. The hypothyroid phase of PTT is more frequently symptomatic. Symptoms include cold intolerance, dry skin, fatique, impaired concentration, and paresthesias.

Laboratory findings: the most efficient screening is TSH measurement. If TSH is abnormal FT4 testing should be performed to indicate the degree of hyper- or hypothyroidism. In women with decreased TSH an assay for TSH receptor antibody should be performed to rule out new onset Graves’ disease /47/. Women with postpartum depression should have TSH, FT4 and anti-TPOAb tests performed /44/. Refer to Fig. 30.1-3 – Typical patterns of T4 and T3 in postpartum thyroiditis.

Thyroid cancer

Thyroid cancer accounts for 1.6% of newly diagnosed cancer cases (3.6% in individuals below the age of 20 years) Refer also to Section 28.22 – Thyroglobulin (Tg). The female to male ratio is 3 : 1.

The classification is based on the cellular origin of the cancer:

  • Differentiated thyroid cancer, arising from thyroid follicular epithelial cells, accounts for the vast majority of thyroid cancers. Papillary cancer comprises about 85% of cases compared to 10% with follicular histology. The cells of these tumors are iodine avid, secrete thyroglobulin (Tg), and are responsive to TSH /48/. Tg levels above 75 μg/L are a specific finding for these tumors. Approximately 20–30% of patients with papillary thyroid cancer experience a recurrence and those with a BRAFV600E mutation have a more aggressive phenotype.
  • Undifferentiated cancers (medullary and anaplastic) /49/. Medullary thyroid cancer (MTC), also known as C-cell cancer, accounts for 3–10% of all malignant thyroid tumors. Approximately 75% of cases are sporadic, and 25% are familial, occurring as part of multiple endocrine neoplasia type A (MEN 2A; MTC, hyperparathyroidism, pheochromocytoma), MEN 2B (MTC, pheochromocytoma, mucosal neuromas, ganglioneuromas, marfanoid habitus), or as familial MTC without further organ involvement. The C-cells in MTC produce calcitonin and the transition from C-cell hyperplasia to a micro carcinoma can be recognized in some cases by an increase in serum calcitonin in the pentagastrin stimulation test. Refer also to Section 28.12 – Calcitonin (CT).

Table 30.1-4 Subclinical thyroid disease

Clinical and laboratory findings

Latent (subclinical) hyperthyroidism /6/

In subclinical hyperthyroidism TSH levels are subnormal (≤ 0.4 mIU/L) and serum thyroid hormone levels (FT3, FT4) are within the middle to upper reference range. Between 65–75% of individuals with subclinical hyperthyroidism have TSH concentrations of 0.1 to 0.4 mIU/L (mild or subclinical hyperthyroidism). Progression to overt hyperthyroidism may occur, especially when TSH level is less than 0.1 mIU/L /45/. Some healthy elderly people may have TSH levels below 0.1 mIU/L without thyroid disease due to a change in the set point of the hypothalamic-pituitary-thyroid axis.

Even without progression to overt hyperthyroidism, subclinical hyperthyroidism can be associated with adverse outcomes. Old persons with subclinical hyperthyroidism are usually asymptomatic but younger individuals may have mild adrenergic symptoms. In the absence of overt disease the physician should order further TSH investigations, because subnormal TSH concentrations are transient in up to 50% of individuals. Subclinical hyperthyroidism in patients with solitary autonomous nodules and multinodular goiters is likely to persist or progress while patients with Graves’ disease are more likely to return to normal.

Adverse outcomes in subclinical hyperthyroidism can be comparable to overt hyperthyroidism including cardiovascular disease (e.g., atrial fibrillation, heart failure and coronary heart disease), bone loss, fractures, cognitive impairment or dementia, particularly in persons > 65 years of age with severe disease /46/.

Latent (subclinical) hypothyroidism /447/

Subclinical hypothyroidism is defined as a state of increased serum TSH level with circulating FT4 concentration within the reference interval. The incidence and prevalence of subclinical hypothyroidism varies between 4% and 10%, being more frequent in women and increases with advancing age. Subclinical hypothyroidism is more frequent in iodine sufficient regions and in individuals of white Caucasian origin. Elevations of TSH levels are found in up to 15% of the U.S. population over age 70 years. Subclinical hypothyroidism is modestly associated with goiter in adults, but the association is much stronger in children.

According to serum TSH level subclinical hypothyroidism is generally classified in two categories:

  • Mildly increased TSH levels (4.0–10.0 mIU/L); constitute approximately 90% of subclinical hypothyroidism cases on a population level
  • More severely increased TSH levels (> 10.0 mIU/L).

Measurement of FT4 is necessary to rule out overt hypothyroidism.

To establish a firm diagnosis repeated measurements, preferably after a 2 to 3-month interval, are required because transient elevations of TSH may occur in numerous circumstances:

  • Circadian variation; the degree of variation in TSH is lower in subclinical hypothyroidism than in euthyroid controls
  • TSH level may be elevated: in individuals with irregular sleep patterns or following vigorous exercise, in depression, and in night shift workers (the nocturnal peak of TSH may be delayed)
  • TSH may be increased in painless thyroiditis, following withdrawal of L-thyroxine or during recovery from a significant non-thyroidal illness, and during treatment with various drugs such as lithium or amiodarone.

Etiologically, persistent subclinical hypothyroidism in most cases is due to autoimmune thyroiditis or results in a small proportion of cases from germ line loss of function mutations in the TSH receptor. Since chronic autoimmune thyroiditis is the most common cause of subclinical hypothyroidism the determination of thyroid peroxidase antibodies (anti-TPOAb) and/or thyroglobulin antibodies (anti-TgAb) will allow a firm etiological diagnosis.

Patients with subclinical hypothyroidism, represented by TSH levels of ≥ 10 mIU/L have an increased risk of coronary heart disease events /48/. The measurement of thyroid antibodies helps predict the progression to overt hypothyroidism, but it is unclear whether thyroid autoimmunity independently affects coronary heart disease risk /49/. Even in the absence of symptoms, replacement therapy with L-thyroxine is recommended for younger patients (< 65–70 years) with TSH > 10 mIU/L. In younger subclinical hypothyroidism patients (TSH < 10 mIU/L) with symptoms suggestive of hypothyroidism, a trial of L-thyroxine replacement therapy should be considered. The TSH level should be re-checked 2 months after starting L-thyroxine therapy, and dosage adjustments made in cases where TSH is not in the range of 0.4–2.5 mIU/L. Individuals (> 80–85 years) with elevated TSH levels ≤ 10 mIU/L should be carefully followed with a wait-and-see strategy /47/.

Table 30.1-5 Reference intervals for TSH and thyroid hormones in pregnant women

Parameter

1st trimester

2nd trimester

3rd trimester

TSH /18/

0.088–2.83

0.200–2.79

0.307–2.90

FT4 /18/

8.18–14.20
(10.53–14.20)

7.40–12.18
(9.53–15.68)

6.70–10.57
(8.63–13.61)

FT3 /18/

2.29–4.04
(3.52–6.22)

2.21–3.75
(3.41–5.78)

2.16–3.63
(3.33–5.59)

TSH /19/

0.13–4.15

0.36–3.77

FT4 /19/

7.9–13.8
(10.16–17.76)

7.1–12.6
(9.13–16.21)

TSH levels expressed in mIU/L, FT4 and FT3 in ng/L (pmol/L). Values are 2.5th and 97.5th percentiles in Ref. /18/ and 5th and 98th percentiles in Ref. /19/. An overview of reference ranges worldwide is given in Ref. /8/.

Table 30.1-6 Severity of endemic goiter /50/

Grade

Endemic goiter population

I

24 h urinary iodine excretion > 44.6 nmol/mmol creatinine; normal mental development

II

24 h urinary iodine excretion 44.6–22.3 nmol/mmol creatinine; risk of hypothyroidism, no risk of cretinism

III

24 h urinary iodine excretion < 22.3 nmol/mmol creatinine; significant risk of endemic cretinism

Table 30.1-7 Medication effects on the thyroid /11/

Clinical and laboratory findings

Drugs affecting hypothalamic-pituitary control of the thyroid

  • Synthetic retinoid hexarotene induces rapid thyrotropin suppression.
  • Checkpoint inhibitors like CTLA-4 (cytotoxic T-lymphocyte antigen 4) cause hypophysitis and the anti-CTLA agent ipilimumab is linked to destructive hypophysitis with varying degrees of central hypothyroidism, adrenal insufficiency, and hypogonadism occurring in 3–10% of patients treated for melanoma. PD-1 (programmed cell death inhibitor) causes primary thyroid dysfunction.
  • Glucocorticoids, dopamine agonists, somatostatin analogues, and metformin exert suppressive effects on thyrotropin release without affecting the concentration of T4 in serum.

Drugs affecting thyroid hormone synthesis or release

  • Lithium causes goiter and hypothyroidism by decreasing thyroid hormone release through the inhibition of colloid pinocytosis.
  • Drugs with iodine in excess like iodinated contrast agents used for computed tomography and cholecystography or drugs such as amiodarone undergo partial deiodination and cause hypothyroidism in some patients. Excess intra thyroidal iodine inhibits thyroid hormone synthesis, resulting in the Wolff-Chaikoff effect (a reduction in thyroid hormone level caused by ingestion of a large amount of iodine).

Drugs using direct thyroid autoimmunity

  • The use of nonspecific immunostimulatory cytokines such as interleukin-2 or interferon alpha in patients with carcinoma results in thyroid dysfunction in 15 to 50% of patients.
  • Alemtuzumab a monoclonal antibody against the cell-surface antigen CD52 leads to thyroid autoimmunity because of a depletion of circulating T cells and B cells.

Drugs causing direct thyroid damage

  • Amiodarone exerts direct cytotoxic effects on thyrocytes and causes destructive thyroiditis.
  • Targeted cancer therapy using tyrosine kinase inhibitors is associated with an increased risk of thyroiditis.

Drugs affecting protein binding of thyroid hormone

  • Oral estrogens lead to increases in thyroxine binding globulin.
  • Selective estrogen-receptor modulators (methadone, heroin, mitotane, fluorouracil) cause increases in thyroxine binding globulin.

Drugs affecting thyroid hormone activation, metabolism and excretion

  • Conversion of T4 to T3 is inhibited by drugs like amiodarone, propranolol dexamethasone and other glucocorticoids.

Drugs affecting absorption of thyroid hormone preparations

  • Ferrous sulfate, calcium carbonate, aluminum hydroxide, sucralfate, bile acid sequestrates, and raloxifene interfere with gastrointestinal absorption of thyroid hormone. Taking thyroid hormone 4 hours before ingesting any of these medications or moving the levothyroxine dose to the bedtime is recommended.

24 h urinary iodine excretion < 22.3 nmol/mmol creatinine; significant risk of endemic cretinism

Table 30.1-8 Initial investigations in suspected thyroid disease /51/

Disease

Clinical and laboratory findings

Euthyroid goiter

Thyroid ultrasound

Confirm euthyroid status by determining TSH and possibly FT4 and T3 (FT3).

Thyroid autonomy

Check metabolic status by determining TSH and possibly FT4 and T3 (FT3).

If TSH levels are ≤ 0.4 mIU/L, always determine FT4 and T3 (FT3) to confirm or rule out overt hyperthyroidism.

Thyroid ultrasound, thyroid scintigraphy with TcO4.

Autoimmune
hyperthyroidism

Confirm hyperthyroidism by determining FT4 and T3 (FT3).

Thyroid ultrasound

Determine GGT, ALT, and complete blood count before starting antithyroid therapy.

Hypothyroidism and
autoimmune thyroiditis

Determine TSH and possibly FT4.

If TSH levels are > 4 mIU/L, always determine FT4 to confirm or rule out overt hypothyroidism

Thyroid ultrasound.

Table 30.2-1 Generation nomenclature for TSH assays

Generation

Detection limit

I

< 0.5

II

< 0.1

III

< 0.01

IV

< 0.001

Data are expressed in mIU/L.

Table 30.2-2 Diagnostic assessment of the TRH test

TSH response

Diagnostic assessment

Rise of < 2 mIU/L

An absent TSH response in combination:

  • With normal FT4 and FT3 levels as well as clinical euthyroidism with a peripherally balanced metabolic state may indicate a number of different thyroid disorders e.g., a disorder of the pituitary thyroid feedback loop associated with the onset of autonomous hormone production in the thyroid gland, early form of Graves’ disease, and therapy with thyroid hormone
  • With elevated FT4 and FT3 levels may indicate overt hyperthyroidism or adequate therapy with L-thyroxine
  • With decreased FT4 and FT3 levels may indicate secondary hypothyroidism.

Rise of 2–25 mIU/L (up to 30 mIU/L after oral TRH)

Appropriate TSH rise. If FT4 and FT3 levels are within the reference interval, a functional disorder of the pituitary thyroid feedback loop can be ruled out.

Increase more than 25 mIU/L (up to 30 mIU/L after oral TRH)

An excessive TSH response in combination with:

  • Normal FT4 and FT3 levels indicates latent hypothyroidism, abnormal iodine utilization, extreme nutritional iodine deficiency, and the early stage of chronic thyroiditis
  • With decreased FT4 (and FT3) levels indicates overt hypothyroidism.

Table 30.2-3 Reference intervals for TSH

Fetuses

Gestational week

19–27

4.1 ± 1.4

28–38

6.9 ± 2.4

36–42

4.2 ± 1.5

Data expressed in mIU/L; values expressed as x ± s

Children /12/

1–3 days

5.2–14.6

1–4 weeks

0.4–16.1

2–12 months

0.6–8.1

2–6 years

0.5–4.5

7–11 years

0.7–4.1

12–19 years

0.5–3.6

Data expressed in mIU/L; values expressed as 2.5th and 97.5th percentiles

Adults

0.40–4.2 /13/

0.30–3.6 /14/

Data expressed in mIU/L; values expressed as 2.5th and 97.5th percentiles

Table 30.2-4 Causes of low TSH not associated with subclinical hypothyroidism /1/

  • At the end of the first trimester of pregnancy
  • In severe non-thyroidal illness and with treatment with high dose glucocorticoids or dopamine
  • In some elderly individuals without apparent thyroid disease
  • In some black individuals as a consequence of racial differences in the distribution of TSH levels in the general population
  • In some smokers
  • Serum TSH below the reference interval but at a normal concentration for that individual because the reference interval only encompasses 95 or 97.5% of the general population.

Table 30.2-5 TSH in thyroid disease and non-thyroidal illness

Clinical and laboratory findings

Hyperthyroidism

TSH concentrations of less than 0.01 mIU/L indicate hyperthyroidism, which may be due to autoimmune hyperthyroidism, toxic multi nodular goiter, toxic adenoma, or postpartum thyroiditis.

Nodular thyroid disease

Nodular thyroid disease is a major cause of TSH suppression with concomitant normal FT4 and FT3 levels. The prevalence of thyroid nodules increases with age and, depending on the region, ranges from 10–50% if ultrasound is used. Approximately 10% of patients have hot nodules and 10% have warm nodules; both are hyper functional. According to a study /15/, most patients with measurable TSH (> 0.01 to 0.39 mIU/L) and normal FT4 and FT3 levels have nodular thyroid disease with hot nodules. The serum TSH level is a predictor of thyroid malignancy in patients with thyroid nodules. At a TSH concentration of ≤ 0.4 mIU/L, the risk of thyroid cancer is moderate. The risk begins to increase once the level reaches 0.9 mIU/L, and is at its highest at TSH levels above 5.5 mIU/L /16/. The likelihood of malignancy was 16% for TSH levels below 0.06 mIU/L, 25% for levels in the range 0.40–1.39 mIU/L, 35% for levels in the range 1.40–4.99 mIU/L, and 52% for TSH levels ≥ 5.0 mIU/L /11/.

Hypothyroidism

TSH levels above 20 mIU/L are a clear indicator of hypothyroidism, even in the absence of clinical signs. FT4 determination is the next step if the reason for TSH elevation needs to be clarified. In the case of borderline or mildly reduced TSH, it is important to determine anti-TPOAb, anti-TgAb, and lipids since these are frequently elevated in autoimmune hypothyroidism.

Latent (subclinical) hypothyroidism /6/

Subclinical hypothyroidism is defined as a state of increased serum TSH level with FT4 concentration within the reference interval (Tab. 30.1-4 – Subclinical thyroid disease).

Congenital hypothyroidism /18/

The biochemical criterion for initiating treatment is a TSH level ≥ 40 mIU/L in capillary blood collected for neonatal screening. A control sample of venous blood is also tested. If the result in the neonatal screening test is < 40 mIU/L, treatment should not be initiated until the TSH level in the venous blood sample is known. Treatment with T4 is initiated if:

  • The FT4 level is below the age adjusted reference interval value for venous blood
  • The TSH level is > 20 mIU/L, even if the FT4 level is within the reference interval.

If the TSH level in the venous blood is ≥ 6–20 mIU/L after day 21, the baby is well, and the FT4 level is within the reference interval, the following is recommended:

  • An (imaging) investigation that will enable a definitive diagnosis
  • A wait-and-see approach, discussion with the family, possibly repeated investigation.

Initial monitoring is carried out 1–2 weeks after the start of treatment and consists of FT4 or T4 and TSH determination four hours after the last dose of lT4. The TSH level should be within the age adjusted reference interval and the FT4 and T4 should be in the upper half of the reference interval. The lT4 dose should not be reduced on the basis of a single result showing an increase in FT4.

Follow-up tests: these should be performed every two weeks until the TSH is normal and then every 1–3 months until the age of 12 months and every 2–4 months thereafter until growth is complete.

Pituitary adenoma

TSH secreting adenomas account for 1.1% of all pituitary adenomas. High or normal TSH with elevated FT4 and FT3 are important diagnostic criteria but they often occur at a relatively late stage. Usually, a macro adenoma is already present by the time these findings become evident /19/.

Pregnancy /20/

Normal pregnancy is associated with an increase in renal iodine excretion, an increase in thyroxine binding proteins, an increase in thyroid hormone production, and thyroid stimulatory effects of hCG. All these factors influence thyroid function tests in the pregnant patient. Following conception circulating thyroxine binding globulin (TBG) and total T4 concentrations increase by week 7 of gestation and reach a peak by approximately week 16 of gestation. These concentrations then remain high until delivery. In the first trimester maternal hCG directly stimulates the TSH receptor, increasing thyroid hormone production and resulting in a subsequent reduction in TSH concentration. Therefore, during pregnancy, women have lower TSH concentrations than before pregnancy and a TSH below non pregnant lower limit of 0.40 mIU/L is observed in as many as 15% of healthy women during first trimester of pregnancy.

Recommendations in suspicion of hypothyroidism /20/:

  • Evaluation of TSH concentration is recommended for all women seeking care for infertility
  • In hypothyroid women treated with L-thyroxine who are planning pregnancy, TSH should be evaluated preconception, and L-thyroxine dose adjusted to achieve a TSH value between the lower reference limit and 2.5 mIU/L
  • Subclinically hypothyroid women undergoing IVF or intracytoplasmic sperm injection (ICSI) should be treated with L-thyroxine. The goal of treatment is to achieve a TSH level below 2.5 mIU/L
  • In the setting of pregnancy, maternal hypothyroidism is defined as a TSH concentration elevated beyond upper limit of pregnancy-specific reference range. An upper reference limit of approximately 4.0 mIU/l may be used. For most assays, this limit presents a reduction in the nonpregnant TSH upper reference limit of approximately 0.5 mIU/L.
  • Women with overt and subclinical hypothyroidism (treated or untreated) or those at risk for hypothyroidism (e.g., patients who are euthyroid but anti-TPOAb or anti-TgAb positive, post-hemithyroidectomy, or treated with radioactive iodine) should be monitored with TSH measurement approximately every 4 weeks until midgestation and at least once near 30 weeks gestation.

Serum TSH may decrease in the first trimester of normal pregnancy as a physiological response to the stimulating effect of hCG upon the TSH receptor. A peak hCG level typically occurs between 7–16 weeks gestation. In particular, a TSH below 0.1 mIU/L may be present in approximately 5% of women by week 11 of pregnancy. The most common cause of hyperthyroidism in women of childbearing age is autoimmune hyperthyroidism occurring before pregnancy in 0.4–1.0% of women and in approximately 0.2% during pregnancy.

Recommendations in suspicion of hyperthyroidism /20/:

  • Thyrotoxic women should be rendered stable euthyroid before attempting pregnancy.
  • When a suppressed TSH is detected in the first trimester, a medical history, physical examination and maternal serum FT4 and total T4 (TT4) should be performed. Measurement of TSH receptor antibodies and TT3 may be helpful in clarifying the etiology of thyrotoxicosis.
  • For antithyroid medication refer to Ref. /20/
  • Fetal surveillance should be performed in women who have uncontrolled hyperthyroidism in the second half of pregnancy. In women with high TSH receptor antibodies (greater than three times the upper limit of normal) surveillance should be performed at any time during pregnancy.

Refer to Tab. 30.5-2 – Anti-TPOAb in thyroid disease.

Morbid obesity

The level of obesity in developed countries has reached epidemic proportions. In the United States, for example, 23.9% of the population have a BMI ≥ 30 kg/m2 and, of these, 5% have morbid obesity (BMI ≥ 40 kg/m2). A considerable proportion of individuals with morbid obesity have subclinical hypothyroidism. Compared to normal weight controls, who had TSH levels of 1.2 ± 0.46 mIU/L and FT3 levels of 3.41 ± 0.54 ng/L, morbidly obese individuals had TSH levels of 1.8 ± 0.83 mIU/L and FT3 levels of 3.08 ± 0.47 ng/L /21/.

TSH in therapeutic settings

Normalization of TSH level after six months of radioiodine therapy indicates a successful outcome /22/. Following surgery for benign thyroid disease (not including endemic euthyroid goiter) with unifocal and multi focal autonomy, the goal of l-thyroxine therapy should be to achieve TSH levels of 0.5–2.0 mIU/L in Graves’ disease and levels within the reference interval following thyroiditis /23/.

The National Academy of Clinical Biochemistry has specified an optimal TSH level of 0.5–2.0 mIU/L as the therapeutic goal of l-thyroxine therapy in hypothyroidism /24/. In women undergoing l-thyroxine therapy for hypothyroidism, estrogen therapy causes a mean increase in the TSH level from 0.9 ± 1.1 to 3.2 ± 3.1 mIU/L and a decrease in the FT4 level. These alterations are due to an estrogen induced increase in thyroid-binding globulin /25/.

Pregnant women with overt or subclinical hypothyroidism should receive an l-thyroxine dose that maintains their TSH level within the range 0.5–2.5 mIU/L.

Lithium is known to have thyrostatic effect and lithium therapy is associated with an increase in TSH and a goiter prevalence of 40–50% /26/.

Thyroid hormone over medication and under medication

The prevalence of thyroid hormone over medication and under medication among patients ≥ 65 years is considerable. When a TSH value of < 0.45 mIU/L was classified as low (indicating over replacement) and a level above 4.5 mIU/L was classified as high (indicating under replacement), 43% of 339 patients were in the euthyroid range, 41% had low TSH, and 16% had high TSH /27/. Low body weight was significantly associated with over replacement (low TSH). For every 10 kg lower weight, the odds ratio increased by a factor of 1.65. Patients with renal failure were less likely to have low TSH levels and diabetes mellitus was associated with both low and high TSH levels.

In a study of pregnant women in the first trimester with subclinical or overt hypothyroidism, 51% were in the euthyroid range, 30% had TSH levels of less than 0.40 mIU/L (indicating over replacement), and 19% had TSH levels of greater than 4.0 mIU/L (indicating under replacement) /28/.

Statin therapy

Statins are used to lower LDL cholesterol but can also lower TSH levels in individuals with normal thyroid function and morphology. The odds ratio of pseudo hypothyroidism in statin users is 3.6 /29/.

Macro TSH /30/

TSH is bound to IgG in 0.6% of samples in which TSH is > 10 mIU/L but FT4 is within the reference interval.

Table 30.2-6 Quality criteria for TSH assays, modified from Ref. /3/

  • Functional sensitivity (lowest concentration that measures a TSH concentration of ≤ 0.01 mIU/L with a coefficient of variation of ≤ 20%)
  • Cross reactivity of < 0.01% (if possible, < 0.001%) with glycoproteins such as hCG, FSH, and LH
  • Parallelism between dilution curves of patient sera and the standard curve (± 10% deviation)
  • Precise measurement of standard material added to patient serum (recovery ± 10%)
  • Measurement of WHO or MRC reference materials with ± 5% expected value
  • No high dose hook effect at concentrations up to 300 mIU/L
  • No interference by heterophile antibodies.

Table 30.3-1 Reference intervals for TT4 and FT4

Adults /17/

TT4

56–123 μg/L
(72–158 nmol/L)

FT4

9.9–16.2 ng/L
(12.7–20.8 pmol/L)

* Values are 2.5th and 97.5 th percentiles

Children

Age

TT4* /3/

FT4** /4/

μg/L
(nmol/L)

(ng/L)
(pmol/L)

Umbilical cord blood

60–131
(77–167)

6.6–27
(8.5–35)

1–2 days

107–258
(138–332)

8.3–31
(10.7–40)

3–30 days

78–197
(100–254)

4.8–23
(6.2–30)

2–12 mos

54–138
(69–178)

8.5–18
(10.9–23)

2–6 yrs

53–123
(68–158)

9–17
(11.6–22)

7–11 yrs

60–111
(77–143)

9–17
(11.6–22)

12–19 yrs

49–107
(63–138)

9–16
(11.6–21)

* Values are 5th and 95 th percentiles;

** Values are 2.5th and 97.5 th percentiles. Conversion: ng/L × 1.287 = pmol/L

μg/L × 1.287 = nmol/L

Table 30.3-2 Assessment of FT4 and TT4 levels

FT4 or T4

Diagnostic interpretation

Reference
interval

  • Healthy thyroid gland
  • Endemic iodine deficiency goiter with peripheral euthyroidism
  • Thyroid suppression therapy (upper reference interval)
  • Thyroid hormone replacement therapy
  • Subclinical hyperthyroidism (e.g., early stage of focal or disseminated autonomy, occasionally autoimmune hyperthyroidism)
  • Isolated T3 hyperthyroidism (e.g., early stage of thyroid autonomy in autoimmune hyperthyroidism)
  • Latent (subclinical) hypothyroidism.

Elevated

Hyperthyroidism

  • Autonomous adenoma, toxic multinodular goiter
  • Graves’ disease
  • Possible early stage of subacute thyroiditis or Hashimotos thyroiditis
  • Factitious hyperthyroidism
  • Thyroid suppression therapy (blood collection within 24 hours of last thyroxine dose)
  • Possible iodine premedication (e.g., with iodine containing contrast media or drugs)
  • Rarely, pituitary tumor.

Low

Hypothyroidism

  • Primary (thyrogenic) hypothyroidism (e.g., chronic thyroiditis, iatrogenic hypothyroidism following goiter resection or radioiodine therapy)
  • Treatment with thyrostatic drugs
  • Congenital hypothyroidism
  • Severe iodine deficiency
  • Secondary (central) hypothyroidism.

Table 30.3-3 Diseases and conditions associated with changes in the levels of FT4 and/or TT4

Clinical and laboratory findings

Use of hormonal contraceptives /18/

Women with normal thyroid morphology using hormonal contraceptives have higher TT4 and TT3, and lower FT3, than non users. FT4 and TSH remain unchanged. This is thought to be due to increased hepatic synthesis of TBG.

Postmenopausal estrogen therapy /19/

In women with normal thyroid function, serum TT4 level increased from 80 ± 9 μg/L (103 ± 12 nmol/L) to 104 ± 15 μg/L (134 ± 19 nmol/L) within 12 weeks of starting estrogen therapy /19/. The TBG concentration increased from 20.3 ± 3.5 mg/L to 31.3 ± 3.2 mg/L. Women with hypothyroidism had similar increases in TT4 and TBG, which suggests that in women with hypothyroidism treated with L-thyroxine, estrogen therapy may increase the need for L-thyroxine.

Pregnancy /20/

FT4 levels decrease as pregnancy progresses. When an isotope dilution LC-MS/MS assay was used, x ± s was 9.3 ± 2.5 ng/L in nonpregnant women, 11.3 ± 2.3 ng/L in the first trimester, 9.2 ± 3.0 ng/L in the second trimester, and 8.6 ± 2.1 ng/L in the third trimester. When an immunoassay was used, the levels from the same samples were 10.5 ± 2.2 ng/L in the first trimester, 8.8 ± 1.7 ng/L in the second trimester, and 8.9 ± 1.7 ng/L in the third trimester.

Transient hypothyroxinemia in pre term infants /21/

Transient hypothyroxinemia is common in pre term infants. The incidence of TT4 levels below 65 μg/L (84 nmol/L) is almost 50% in infants born before the 30th week of gestation and 25% in all other pre term infants. FT4 is also decreased, but not to the same extent as TT4. FT4 levels are comparable to those of healthy adults and significantly higher than those observed in congenital hypothyroidism. Nevertheless, the mean FT4 concentration in pre term infants is only half that of full term infants. Transient hypothyroxinemia resolves spontaneously within 4–8 weeks.

Transient hypothyroidism in pre term infants /21/

Transient hypothyroidism characterized by low FT4 and high TSH can occur in pre term neonates as a result of iodine deficiency. The incidence is approximately 20% in Belgium but only about 1 in 50,000 in the USA and Japan. Although their cord blood concentrations may be normal, these infants develop hypothyroidism with elevated TSH in the first two weeks after birth. Urinary iodine excretions and thyroid iodine stores are low. Hypothyroidism responds to iodine replacement and usually resolves within 2–3 months. Transient hypothyroidism can also occur in full term infants in iodine deficient regions following exposure to excessive amounts of iodine.

Monitoring of L-thyroxine replacement therapy

During L-thyroxine replacement therapy, the serum FT4 concentration is higher than would be expected based on the TSH level. This is thought to be due to a lack of T3 secretion by the thyroid.

The TT4 and FT4 levels are highly dependent on the length of time between medication intake and blood collection. In athyroid patients who take 150–200 mg of L-thyroxine daily, TT4 and FT4 concentrations increase by 20% within 1–4 hours and return to their initial values after 9 hours. Serum TSH and FT3 concentrations remain unchanged /6/. Blood collection should take place at least 12 hours, but preferably 24 hours, after the last dose.

In a study, serum FT4 levels in patients with multi nodular goiter who were receiving replacement therapy were approximately 15 ng/L (19.3 pmol/L). Patients who also had Helicobacter pylori gastritis required a 20–30% higher daily dose of thyroxine to achieve the same FT4 concentration /22/.

Acute hepatitis

An increase in serum FT4 can occur due to a decrease in the peripheral conversion of T4 to T3.

Drug related effects

Heparin, phenytoin, phenobarbital, and carbamazepine can elevate FT4 levels by displacing T4 from binding proteins.

Ketoacidosis

FT4 levels are increased in diabetic ketoacidosis and fasting due to the displacement of T4 from its binding proteins by ketone bodies.

Amiodarone

Amiodarone is an anti-arrhythmic drug that contains 370 g of iodine per kg. At the start of amiodarone therapy, there is a decline in FT4 and a transient increase in TSH due to the inhibitory effect of iodine on the thyroid. During treatment, FT4 is elevated, FT3 is reduced, reverse T3 is elevated, and TSH can increase to as much as 20 mIU/L due to the inhibitory effect of amiodarone on deiodinase activity and thyroid TSH receptors. During long-term therapy, TSH returns to normal or is mildly suppressed. Typical laboratory findings in this situation are: normal or mildly suppressed TSH, FT4 20–30% above the upper reference interval value, FT3 slightly decreased or in the lower range of the reference interval /23/.

FT4 elevation without TSH suppression

This constellation can occur in critically ill patients, TSH secreting tumors, thyroid hormone resistance, or due to immunoassay interference caused by the presence of T4 antibodies.

Familial dysalbuminemic hyperthyroxinemia (FDH)

FDH is an autosomal dominant disorder characterized by the presence of serum albumin with increased affinity for T4. Albumin has low affinity and high capacity for carrying T4. In FDH, 30–40% of the serum T4 is bound to albumin. TT4 and FT4 levels are elevated if a one step immunoassay is used; if a two step immunoassay is used, FT4 and FT3 are normal /24/.

Table 30.3-4 Reference intervals for TT3 and FT3

Adults /17/

TT3

0.78–1.82 μg/L (1.2–2.8 nmol/L)*

FT3

2.5–4.4 ng/L (3.9–6.7 pmol/L)*

* Values are 2.5th and 97.5th percentiles; determined using a manufacturer’s test kit.

Children /4/

Age

TT3

FT3

(μg/L)

(nmol/L)

(ng/L)

(pmol/L)

0–3 days

1.0–2.9

1.5–4.5

1.9–7.9

3.0–12.1

4–30 days

0.6–2.4

0.9–3.7

1.9–5.3

3.0–8.1

2–12 mo.

0.8–2.8

1.2–4.3

1.6–6.4

2.4–9.8

2–6 yrs

0.8–2.5

1.3–3.9

1.9–5.9

3.0–9.1

7–11 yrs

0.9–2.2

1.4–3.4

2.7–5.1

4.1–7.9

12–19 yrs

0.8–2.1

1.3–3.3

2.3–5.6

3.5–5.0

* Values are 2.5th and 97.5th percentiles; determined using a manufacturer’s test kit. Conversion: ng/L × 1.54 = pmol/L; μg/L × 1.54 = nmol/L

Table 30.3-5 Assessment of TT3 and FT3 levels /14/

TT3 or FT3

Diagnostic interpretation

Within the
reference
interval

Euthyroidism

Subclinical hyperthyroidism

Hypothyroidism due to compensatory increase in thyroidal and extra thyroidal conversion of T4 to T3

Compliance with L-T3 suppression therapy

In iodine deficiency, FT4 is near the lower reference interval value or is decreased and FT3 (or TT3) is near the upper reference interval or is elevated

Elevated

Hyperthyroidism (common) with disproportionate increase in FT3 or TT3 in relation to TT4

In 5–10% of hyperthyroidism, only TT3 and FT3 are elevated

Disorder of hormone binding capacity (only TT3 elevated, FT3 normal)

Administration of T3 containing hormone preparations

Elevated or rising FT3 or TT3 may be an early sign of relapse in hyperthyroidism

A decline in FT3 or TT3 is an early effect of treatment for hyperthyroidism

FT3 or TT3 elevation accompanied by TSH suppression in suspected non-thyroidal illness indicates that hyperthyroidism is more likely

Inappropriately high FT3 or TT3 during amiodarone therapy indicates amiodarone induced hyperthyroidism

In congenital goiter, FT3 or TT3 elevation may occur as a result of defective thyroglobulin synthesis

In TSH secreting pituitary tumors, elevated FT3 or TT3 is a more common finding

Multinodular goiter and iodine induced hyperthyroidism are often preceded by elevated FT3 or TT3.

FT3 or T3 elevation is often seen in thyroid hormone resistance in the absence of clinical signs of hyperthyroidism

FT4 may be reduced during thyroid suppression therapy even though there is an excess of T3

Decreased

Pronounced hypothyroidism; in latent hypothyroidism compensatory elevation of FT3 or TT3

Long-term thyrostatic therapy

Chronic severe illness and elderly individuals with a reduced conversion of T4 to T3; so called low T3 syndrome or non-thyroidal illness. Reverse T3 is usually elevated in these cases.

Table 30.3-6 Prevalence of low T3 in chronic kidney disease /15/

eGFR [mL × min–1 × (1.73 m2)–1]

Prevalence (%)

≥ 90

8.2

≥ 60 but less than 90

10.9

Less than 60 but ≥ 30

20.8

Less than 30 but ≥ 15

60.6

Less than 15

78.6

Table 30.4-1 Assessment of thyroid function using the T4/TBG ratio /2/

T4/TBG ratio

Functional status

4.3 ± 1.2

Euthyroidism

1.1 ± 0.9

Hypothyroidism

11.2 ± 3.6

Hyperthyroidism

Values expressed as x ± 1 s

Table 30.5-1 Anti-TgAb in different disorders* /9/

Disease

Prevalence (%)

Hashimotos thyroiditis

60–80

Autoimmune hyperthyroidism

30

Subacute thyroiditis

10–20

Endemic goiter

7–14

Differentiated thyroid cancer

2–45

Idiopathic Addison’s disease

87

Diabetes mellitus type 1 (Caucasians)

50

Pulmonary sarcoidosis

27

Vitiligo

1.5

Women with repeated miscarriages

23

Elderly women

32

* Determined using RIA or ELISA

Table 30.5-2 Anti-TPOAb in thyroid diseases

Clinical and laboratory findings

Graves’ disease

Depending on the literature, 45–80% of patients have increased anti-TPOAb levels on initial examination.

Latent (subclinical) hypothyroidism

Subclinical hypothyroidism has a prevalence of 4–10% in the adult population. According to the NHANES III study, 4.3% of the U.S. population had subclinical hypothyroidism (TSH reference interval 0.39–4.9 mIU/L). In the total population, positive anti-TPOAb were detected in 18.5% of women and 8.6% of men. The prevalence of increased anti-TPOAb was significantly associated with reduced thyroid function, increased with age, was higher in females than males, and higher in whites than blacks. The prevalence of increased anti-TPOAb was greater than the prevalence of elevated TSH /16/.

The thyroid undergoes anatomical changes with increasing age. The weight of the gland, follicular size, and amount of colloid all decrease. There is also increased fibrosis, accompanied by a lymphocytic infiltrate. The level of FT4 is not reduced by these changes but its half life increases from 6.7 to 9.3 days in the seventh decade of life. The prevalence of anti-TPOAb also increases with age but the majority of older patients who have increased anti-TPOAb do not have elevated TSH levels. On the other hand, 40–70% of older individuals with elevated TSH also have increased anti-TPOAb levels. The age related increase in the prevalence of anti-TPOAb is thought to be due to illness rather than age per se. After the age of 80 years, the prevalence of anti-TPOAb positivity declines again.

Patients with renal failure have a higher prevalence of subclinical hypothyroidism and increased anti-TPOAb than individuals with healthy kidneys. Thus, patients with a GFR below 60 [mL × min–1 × (1.73 m2)–1] had a anti-TPOAb level of 510 ± 2741 kIU/L while those with a GFR above 90 [mL × min–1 × (1.73 m2)–1] had an anti-TPOAb level of only 96 ± 402 kIU/L /17/.

Overt hypothyroidism

Hypothyroidism is a relatively common condition that affects 3–10% of women over the age of 18 years. Risk factors for hypothyroidism include female gender, the presence of anti-TPOAb, and chronic hepatitis C infection (the prevalence of hypothyroidism during treatment with interferon-α is 7–39%) /16/.

Autoimmune thyroiditis (Hashimotos thyroiditis) is more common in women than men (ratio 9 : 1), occurs mainly in the 4th or 5th decade of life, and is associated with HLA-DR3, HLA-DR4, and HLA-DR5. TSH is increased, while FT4 and FT3 are within the reference interval. More than 70% of cases have increased anti-TPOAb and 40–70% increased anti-TgAb. In general, the higher the initial anti-TPOAb and TSH levels in autoimmune thyroiditis, the more rapid the progression. More than 90% of patients with autoimmune hypothyroidism have elevated anti-TPOAb levels. The recommended approach to patients with suspected autoimmune thyroiditis is as follows /9/:

  • Anti-TPOAb levels should be checked in patients with euthyroid goiter. High TPOAb concentrations (≥ 2,000 kIU/L) indicate the presence of chronic immune thyroiditis.
  • Anti-TPOAb levels should be checked in patients with subclinical thyroiditis to identify those cases who are likely to progress to hypothyroidism with low FT4. Progression is indicated by a rise in anti-TPOAb during annual checks.
  • If FT4 levels are decreased, the cause of the goiter should be investigated. One possible cause is iodine deficiency. However, in patients living in iodine replete regions, primary hypothyroidism or inactivating TSH receptor mutations that only become apparent in adulthood must be considered. In these cases, anti-TPOAb levels are normal.

Hashimotos thyroiditis is the most common thyroid disorder in children and adolescents and is often associated with a polyglandular syndrome or celiac disease. In a study /18/, the prevalence of autoimmune thyroiditis in Sardinian children with celiac disease was 10.5%, with a mean age of onset of 10 years. In children who had autoimmune thyroiditis at the onset of celiac disease, anti-TPOAb elevation persisted for 2–9 years despite a gluten free diet.

Serum anti-TPOAb levels decline in most patients with Hashimotos thyroiditis who receive treatment. In patients with a mean pre-treatment level of 4,779 ± 4,099 kIU/L, the mean decrease after three months was 8%, after one year was 45%, and after five years was 70%. However, normalization of anti-TPOAb levels (to below 100 kIU/L) occurred after five years in only 20% of patients /19/.

Pregnancy

Thyroid function markers were measured in 12,000 women in Maine, USA who were 17 weeks pregnant /19/. 2.3% of the women had TSH levels of greater than 6 mIU/L and 70% of the women with elevated TSH levels also had elevated levels of anti-TPOAb, compared to 11% of the controls. In a review of 14 studies investigating thyroid function in 14,148 pregnant women, the prevalence of elevated anti-TPOAb or anti-TgAb was 10.8%. In a Danish study, anti-TPOAb was detected in 32% of pregnant women with type 1 diabetes /20/. The relationship between anti-TPOAb, anti-TgAb, and male and female fertility is complex and not yet fully understood. Refer also to Tab. 30.2-5 – TSH in thyroid disease and non-thyroidal illness.

Subclinical hypothyroidism in pregnancy

Subclinical hypothyroidism increases the risk of pregnancy complications in anti-TPOAb positive women. The reported miscarriage rate in women with anti-TPOAb and TSH levels of 2.5–5.0 mIU/L is 6.1%, compared to 3.6% in those with TSH levels below 2.5 mIU/l. L-thyroxine treatment in early pregnancy is thought to reduce the rate of miscarriage and pre term delivery /21/.

Spontaneous abortion and thyroid autoantibodies

Spontaneous pregnancy loss and miscarriage, has been reported to occur in 17–31% of all pregnancies. Prospective studies have shown that women who are positive for anti-TPOAb or anti-TgAb have a two-fold (17% vs. 8.4%) or 4-fold (13.3% vs. 3.3%) increase in the risk of a pregnancy loss /21/.

Preterm delivery and thyroid autoantibodies

Prospective studies have shown that patients who are positive for anti-TPOAb or anti-TgAb have a two-fold (16% vs. 8%) or 3-fold (26.8% vs. 8.0%) increase in the risk of pre term delivery /21/.

Risk of first-onset postpartum depression

Women with increased anti-TPOAb titer during early gestation are at increased risk for self-reported first-onset depression. The longitudinal pattern of self-reported postpartum depression in the anti-TPOAb positive group was similar to the typical course of postpartum anti-TPOAb titer changes. This suggests overlap in the etiology of first-onset postpartum depression and autoimmune thyroid dysfunction /22/.

Postpartum thyroiditis

The prevalence of postpartum thyroiditis is approximately 10%. It is an autoimmune disorder that occurs following pregnancy in predisposed women. TSH, FT4, and anti-TPOAb determinations should be used to diagnose and monitor this disorder /212324/. Postpartum thyroiditis is associated with the presence of anti-TPOAb and anti-TgAb, lymphocyte abnormalities, complement activation, increased levels of IgG1, increased natural killer cell activity, and specific HLA haplotypes. The risk of postpartum thyroiditis is 27 times higher in women who are positive for anti-TPOAb in the first trimester than in women who are anti-TPOAb negative.

Autoimmune hyperthyroidism must be ruled out in anti-TPOAb positive women with postpartum thyroiditis. Goiter, ophthalmopathy, and elevated TSH receptor antibodies indicate that autoimmune hyperthyroidism is more likely than postpartum thyroiditis.

Amiodarone therapy

Hypothyroidism or hyperthyroidism develops in 14–18% of patients treated with amiodarone. This can occur either in patients with apparently normal thyroid function or in patients with pre-existing thyroid disease. The presence of anti-TPOAb prior to treatment indicates an increased risk of developing amiodarone induced thyroid dysfunction /25/.

Table 30.5-3 Thyroglobulin receptor antibodies (TRAb) in thyroid diseases

Clinical and laboratory findings

Graves´ disease (GD)

GD is characterized by the production of autoantibodies to thyroid-associated antigens such as the thyrotropin receptor (TSHR). GD results from the presence of thyroid stimulating antibodies (TSAb) also termed thyroid stimulating immunoglobulins (TSI) that activate the TSHR localized on the cell membrane of follicular cells. Some TRAb are functional antibodies and mimic TSH by stimulating cyclic adenosine mono phosphate (cAMP) dependent signal transduction. Other TRAb antagonize the TSHR by either blocking TSH binding (TSBAb) or interacting with TSHR epitopes that inhibit cAMP production /31/.

If the diagnosis of GD is questionable in a patient with hyperthyroidism the determination of TRAb is critical /3/.

Differentiating GD from other forms of hyperthyroidism is important for the proper management of patients, especially in the presence of other autoimmunne diseases or autoimmune polyglandular syndrome type 1 /3/.

Higher levels of TRAb identify patients who are likely to enter into a sustained remission following medical therapy /3/.

Monitoring therapy with antithyroid drug (ATD) after 12 weeks follow-up a decrease in TRAb was observed in responders immediately following the initiation of ATD therapy in contrast to the titers of non responders /31/.

TRAb levels decline with ATD therapy. Level > 12 IU/L at diagnosis of GD is associated with 60% risk of relapse at 2 years and 84% at 4 years. The prediction of risk of relapse improves further to over 90% with TRAb > 7.5 IU/L at 12 months or > 3.85 IU/L at cessation of ATD therapy /34/.

TRAb are independent risk factors for Graves’ ophthalmopathy and help to predict the severity and outcome of the disease. Thus, in one study /32/ of patients being treated for Graves’ ophthalmopathy, TRAb were determined every three months for 12–24 months. If TRAb antibody levels at consecutive time points were less than 5.7, 2.6, 1.5, 1.5, 1.5, and 1.5 IU/L, the patients had a 2.6–15.6 fold higher chance of a mild course. If levels at consecutive time points were above 8.8, 5.1, 4.8, 2.8, and 2.8 IU/L, the patients had an 8.7–31.1 fold higher risk of a severe course.

Toxic multinodular goiter

Classical toxic multinodular goiter is a complication of multinodular goiter in which increased levels of thyroid hormone are secreted by autonomous nodules. TRAb assays are generally negative in toxic multinodular goiter. A proportion of these patients also develop autoimmune hyperthyroidism (mixed hyperthyroidism), which can be recognized based on the presence of TRAb. In a study /33/, TRAb were detectable in 17% of patients with toxic multinodular goiter.

Pregnancy

Around 0.1% of pregnant women are, or have been treated for, autoimmune hyperthyroidism and of these, 2–10% have TRAb, which can cross the placenta and cause fetal or neonatal hyperthyroidism. If TRAb are detectable in a pregnant woman, they can also be detected in the fetus or neonate /21/.

In all pregnant women who have recently undergone radioiodine therapy or surgery for autoimmune hyperthyroidism, TRAb should be measured in gestational week 24–28. Values that are more than three times the upper reference interval value indicate a need for close fetal monitoring and close cooperation with a perinatologist. Signs of potential hyperthyroidism include tachycardia with a heart rate of more than 170/min. for more than 10 minutes, intrauterine growth retardation, fetal goiter, accelerated bone maturation, congestive heart failure, and hydrops fetalis.

Use of ATDs in early pregnancy is associated with increased risk of congenital anomalies; early ablative treatment (radioactive therapy/surgery) should be considered in women of childbearing age at higher risk of relapse of GD. TRAb ≥ 5 IU/L in pregnant women with current or previously treated GD is associated with increased risk of fetal and neonatal thyrotoxicosis, and hence needs close monitoring /34/. For further recommendations concerning thyroid association with pregnancy refer to Ref. /24/.

Figure 30.1-1 Hypothalamic-pituitary-thyroid axis. TRH produced in the para ventricular nucleus of the hypothalamus stimulates the pituitary to produce and secrete TSH. The release of TSH is influenced by biological factors such as dopamine, dopamine antagonists, α-adrenergic substances, glucocorticoids, estrogens, growth hormone, and neurotensin. Circulating thyroid hormone exerts feedback inhibition on the hypothalamus and pituitary. NT, neurotransmitter.

Tissue Brain Hypothalamus Pituitary Thyroid Liver Kidney Biologicalfactors Inhibition NT TRH TSH T3, T4 T4 T4 T3 rT3 T4 T3

Figure 30.1-2 Inverse log-linear relationship between FT4 and TSH. Values are the 2.5th and 97.5th percentiles. TSH levels < 0.005 mIU/L are in the horizontal bar. Modified from Ref. /52/.

5001005010510.50.10.050.010.005 TSH (mIU/L) 0 50 100 150 200 250 300 500 FT4 (nmol/L) 650 < Detection limit

Figure 30.1-3 Typical patterns of T4 and T3 (broken lines) and TSH (solid line) in postpartum thyroiditis. With kind permission from Ref. /51/; modified.

Referenceinterval Hyperthyroidphase1–6 months Hyperthyroidphase2–8 months Recovery

Figure 30.1-4 Laboratory diagnosis for suspected hyperthyroidism. With kind permission from Ref. /53/.

Hyperthyroidism symptoms TSH Normal FT4 Normal High Low FT4 High Normal FT3 Euthyroidism TSH-secr. Tumor,T3-, T4-Resistance,Medicines,Auto-Ab against T4 High FT4 High Normal/low Destructive thyreoiditisHashimotos thyreoiditisHyperthyroidism factitia Euthyroidism Primary hyperthyroidism Intake of T4? Eigure 30-5 Clarification of etiology High Normal

Figure 30.1-5 Etiological clarification of primary hyperthyroidism. With kind permission from Ref. /53/.

Etiology clarification of hyperthyroidism Recent pregnancy? No Low High Hyperthyreosisfactitia Destructivethyroiditis,ectopic thyroid Painful struma? Type of struma Nodular Diffuse Non No Yes Postpartumthyroiditis TPO-Ak, Tg-Ak, control FT4, T3 oder FT3 Single M. Basedow TPO-Ab, Tg-Ab Multi Toxicadenoma No toxicsigns Multinodularhyperthyroidism Low High Thyroglobulin Immune hyperthyroidism(non palpable struma; men) Positive Negative Immune hyperthyroidism Confirmation by thyroid Scan Radioactive iodine uptakethyroid Scan Yes Subacutethyroiditis

Figure 30.1-6 Laboratory diagnosis for suspected hypothyroidism. With kind permission from Ref. /53/.

Hypothyroidism symptoms TSH Normal Euthyroid Normal Low FT4 Hypopituitarismhypothalamic diseasephenytoin treatment non esterified fatty acids,eg. heparin treatment Low High FT4 Low Normal Increased Normal High Small Primaryhypothyroidism Subclinicalhypothyroidism Risc for thyroid insufficiency ClarificationFig. 30.1-7 TPO-AkTg-Ak

Figure 30.1-7 Etiological clarification of primary hypothyroidism. With kind permission from Ref. /53/.

Etiology clarification of hypothyroidism Recent treatment of a hyperthyroidism? Yes No No Postpartumthyroiditis Painfulstruma No Yes TPO-Ab,Tg-Ab Subacutethyroiditis Yes No Hashimoto's thyroiditis Primary myxedema Yes No Drug relatedhypothyroidism Antibody negativeautoimmune thyroiditis Increased Normal Struma? Drug medication? Yes Recent pregnancy? Treatment relatedhypothyroidism

Figure 30.2-1 Relationship between T4 and TSH in healthy individuals (circle) and in patients with primary and subclinical hyperthyroidism, central hypothyroidism, and non-thyroidal illness (NTI). Due to overlap, it is not possible to differentiate these groups clearly using T4, TSH, or a combination of both. Only primary hyperthyroidism is clearly defined. Modified with kind permission from Ref. /31/.

4.00.40.01 Serum-TSH (mIU/L) Central hypothyroidism Non thyroidal illness (NTI) Serum-T4 (mg/L) Subclinicalhyperthyroidism Primarilyhyperthyroidism 110

Figure 30.5-1 Diagnosis of autoimmune thyroiditis, with kind permission from Ref. /4/. The upper reference interval value for TSH can vary between 3.6 and 4.5 mIU/L, depending on the laboratory.

Clinic: euthyroid or hypothyroidism Sonography: mostly struma, diffuse hypoechogenicity ~ 70–90% positiveTPO-Ak TSH increased (> 4,5 mIU/L)(possibly diagnosis of T3 and fT4) Autoimmune thyroiditis(Type hashimoto or possibly atrophic course)

Figure 30.5-2 Diagnosis of Graves’ disease. With kind permission from Ref. /5/. * Indicates autoimmune hyperthyroidism.

Clinic: hyperthyroidism(possibly endocrine orbitopathy*) Sonography: mostly struma, diffuse hypoechogenicity, elevated circulation Ab against human TSH receptor(TR-Ab in nearly 100% positive) Low TSH (< 0.01 mIU/l)(possibly determination of T3 and fT4) Immune hyperthyroidism (Morbus Basedow)

Figure 30.6-1 Thyroidal iodine metabolism and hormone synthesis. I is oxidized to I2 and bound to tyrosine to produce 3-monoiodotyrosine (MJT) and 3,5-diiodotyrosine (DJT). 3,5,3’-triiodothyronine (T3) and 3,5,3’,5’-tetraiodothyronine (T4, thyroxine) are then produced from these hormone precursors.

Jodination OH CO OH CH 2 2 J CH NH 2 + JJ J OH CO OH CH 2 CH NH 2 OH CO OH CH 2 CH NH 2 Artery DJT J JJ JJ JJ OH OH CH 2 CH 2 CO OH NH 2 O O CH NH 2 CH T3 T4 Inorganic iodine pool Periphery Diet Excretion Deiodination Vein Tyrosine MJT J 2 150–200 μgiodide/day CO OH 100–200 μgT3 + T4/day Jodisation

Figure 30.6-2 Cellular binding sites of T3 and T4 on the cell membrane, mitochondria (mito), cytoplasmic binding protein (CBP), and nuclear receptor (TR); modified with kind permission from Ref. /8/.

T3 T3 T3 T4 T4 T4 Cell Blood vessel TR DNA Nucleus mRNA Protein ZBP Mito

Figure 30.6-3 Regulation of mitochondrial enzyme induction by T3 via the nuclear receptor, modified from Ref. /8/. T3 binding to the nuclear receptor leads to:

– Protein synthesis, directly via mRNA

– Protein synthesis, indirectly via the regulation of transcription or translation

– Stimulation of mitochondrial DNA (mt-DNA) by MRP RNA. The RNase MRP RNA is the RNA subunit of the RNase mitochondrial RNA processing (MRP) enzyme complex that is involved in multiple cellular RNA processing events.

T3- Receptor Proteins m-RNA m-RNA nDNA mt DNA Nucleus (n) Mitochondrion (mt) MRP-RNA Respiratoryenzymes, ATPsynthase Stimulation ofmt DNAReplication Regulation of Transcrip-tion or translation

Figure 30.6-4 Effect of T3 on gene transcription; modified with kind permission from Ref. /6/. Thyroid hormone receptors (TRs) that form heterodimers do so with TR accessory (auxiliary) proteins (TRAPs). In the normal state, when the TR-TRAP heterodimer is bound to the thyroid hormone responsive element (TRE) in the absence of T3, gene transcription is inactivated as a co-repressor is bound to the TR-TRAP complex and the basal transcription machinery (BTM). The BTM is bound to the TATA box. Lower left and upper right: upon T3 binding, the co-repressor leaves the TR-TRAP complex and a co-activator associates with the TR-TRAP complex. Lower right: the co-activator can subsequently interact with the basal transcriptional machinery to activate gene transcription.

Co repressor Basal status without T3 TRA PTR BTM Inhibitedtranscription TRE TATA T3 BTM Co activator Stimulatedtranscription TRE TATA Co repressor T3 TRAP BTM TRE TATA T3 BTM TRE TATA

Figure 30.6-5 Pathways of thyroid hormone metabolism /3/. G, glucuronide; S, sulfate; TA4, acetic acid metabolite of T4; DIT, diiodothyronine; rT3, reverse T3.

Ether bond cleavage (DIT ) Glucuronidation (T4G ) Oxidative deamination (TA4) Sulfation (T4S) Deiodination (T3) Deiodination (rT3) II O HO CH 2 – CH – COOH NH 2 I I
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