Renin-angiotensin-aldosterone system (RAS)
The European Society of Cardiology and the European Society of Hypertension retain the threshold value of ≥ 140/90 mmHg for the definition of high blood pressure. At least three measurements should be made on each of several days, with 1–2 minutes between measurements and with a 3–5 minute pause before blood pressure is measured with the patient sitting . Elevated blood pressure may affect as many as 1 billion individuals worldwide. In Germany, approximately 13% of women and 18% of men have uncontrolled high blood pressure . The World Health Organization estimates that 54% of strokes and 47% of cases of ischemic heart disease are the direct consequences of high blood pressure . Suboptimal blood pressure control is considered the number one attributable for death worldwide, and early treatment of blood pressure may reduce the incidence, as well as the long term consequences of hypertension .
Most patients with elevated blood pressure have essential hypertension and there are no tests to investigate the processes involved. Secondary types of hypertension such hyperaldosteronism, low renin hypertension or pheochromocytoma induced increased levels of catecholamines must be considered in the differential diagnosis.
The ESH/ESC guidelines advocate blood pressure treatment of ≥ 140/90 mmH in the age 65-79 years, the ACC/AHA advocates ≥ 130/80 mmHg. The 1999–2000 National Health and Nutrition Examination Survey (NHANES) found that only 31% of hypertensive patients had blood pressure controlled to < 140/90 mmHg . Data from NHANES III suggest that the prevalence of hypertension increases progressively with increasing body mass index (BMI) from about 15% among people with a BMI less than 25 kg/m2 to approximately 40% among those with a BMI of 30 kg/m2 or greater .
Up to 30% of hypertensives have a low or suppressed renin. The phenotype of low renin hypertension may be the manifestation of inherited genetic syndromes, acquired somatic mutations, or environmental exposures. Activation of the mineralocorticoid receptor is a common final mechanism for the development of low renin hypertension .
An early onset of anti-hypertensive treatment provides in addition to clinical benefits (e.g., lower risk of cardiovascular events) psychological benefits to the patient.
6. Malik S, Wong ND, Franklin SS, Kamath TV, L’Italien GJ, Pio JR, et al. Impact on the metabolic syndrome on mortality from coronary heart disease, cardiovascular disease, and all causes in the United States adults. Circulation 2004; 110: 1245–50.
The renin-angiotensin-aldosterone system (RAS) plays a key role in the salt and water retention regulation, the extracellular fluid volume, and the regulation of blood pressure. Refer to:
- Synthesis of prorenin in the afferent arteriole of the glomerulus (e.g., juxtaglomerular apparatus) of the kidney
- Its circulating substrate, synthesized in the liver, is the protein angiotensinogen
- From angiotensinogen the renin generates the decapeptide angiotensin I
- Angiotensin I in turn is converted to the octapeptide angiotensin II by angiotensin converting enzyme (ACE)
- Angiotensin II is the principal effector molecule of the RAS, whose main actions are to stimulate the angiotensin II receptor type 1 (AT1) on arteries and the adrenal cortex to cause vasoconstriction and stimulation of the aldosterone secretion
- The AT1 also facilitates noradrenalin release from sympathetic nerves.
- Vasoconstriction by binding alternatively to G-protein coupled angiotensin II receptors type 1 or angiotensin II receptors type 2. Angiotensin II exerts its classic vasopressive effect through the angiotensin II receptors type 1, and the opposite effect through the angiotensin II receptors type 2.
- Stimulation of aldosterone production and thus an increase in salt and water retention. The adrenal response to angiotensin II takes only minutes, reflecting the fast conversion of aldosterone precursors to aldosterone.
The rate limiting step in the RAS is renin activity, since all other components of the cascade are normally present in excess amounts.
The synthesis and release of renin by the juxtaglomerular apparatus of the kidney depend on:
- The perfusion pressure of the afferent arterioles and thus the blood pressure
- The distal tubular Na+ concentration at the level of the macula densa and thus the sodium chloride supply
- The sympathetic nervous system and thus its activation
- The negative feedback regulation by angiotensin II.
Angiotensin II exerts a negative feedback control on renin secretion. Thus, inhibition of the reaction cascade by ACE inhibitors or AT1 antagonists leads to reduced production of angiotensin II and increased production of renin.
Unadjusted activation or over activation of the RAS, be it systemic or localized, leads to increased production of angiotensin II, which can cause salt and volume retention, an increase in blood pressure and, through activation of transforming growth factor-β, inflammatory vascular hypertrophy and organ fibroses. Increased production of angiotensin II can also be due to mutations in the genes for renin (REN), angiotensinogen (AGT), angiotensin- I-converting enzyme (ACE), angiotensin II receptor type 1 (AT1) and aldosterone synthase (CYP11B2).
Aldosterone is produced in the zona glomerulosa of the adrenal cortex and regulates the body’s electrolyte and volume balance via its mineralocorticoid effect . The regulators of adrenal aldosterone biosynthesis are the angiotensin II produced by the RAS, the extracellular concentration of K+, and ACTH. The effect of each agonist is modified by prevailing Na+ and K+ status. Aldosterone biosynthesis is acutely sensitive to small changes in serum K+ concentration. Increased K+ levels, increase aldosterone secretion, thereby restoring the K+ homeostasis. The effects of extracellular K+ concentration and angiotensin II are synergistic. Aldosterone secretion is also influenced by factors, such as atrial natriuretic peptide, serotonin, and adrenomodullin. ACTH stimulates renal blood flow and contributes moderately to the synthesis of aldosterone by interacting directly with G-protein coupled receptors in the zona glomerulosa of the adrenal cortex.
Aldosterone is synthesized from cholesterol via a series of hydroxylations and oxidations. The final steps of this pathway, the conversion of 11-deoxycorticosterone to aldosterone, require conversion via the intermediates 18 hydroxydeoxycorticosterone or corticosterone and 18-hydroxycorticosterone. The enzymes involved in these reactions are mostly members of the cytochrome P450 super family.
The effect of aldosterone is mediated by the cytoplasmic mineralocorticoid receptor (MR), particularly in cells of the renal collecting duct. The receptor belongs to the nuclear receptor super family of proteins and consists of an N-terminal domain, a DNA-binding domain and a C-terminal ligand binding domain. The binding of aldosterone to this domain induces a conformational change in the MR. As a result, the MR dissociates from heat shock proteins, dimerizes and trans locates to the nucleus where it binds to the hormone responsive element of genes responsible for aldosterone and activates gene transcription.
In the epithelia, aldosterone regulates the reabsorption of Na+, which also effects the transport of water, K+ and H+ across the cell membrane. An electrochemical gradient permits the passage of Na+ from the lumen into the epithelial cell via the amiloride sensitive epithelial Na+ channel (ENaC). From the cytoplasm, Na+ is actively transported by the Na+-K+-ATPase across the basolateral membrane from the epithelial cell into the bloodstream, while simultaneously excreting K+; water follows the movement of the Na+ .
The effect of aldosterone on Na+ reabsorption consists in modulating the activity of ENaC by inducing the α-, β- and γ-subunit expression although a major effect is achieved by increasing the number of channels in the plasma membrane.
The RAS has a major role in the control of extracellular volume in Na+ and K+ homeostasis, and in the regulation of blood pressure. Disorders of the RAS are divided into those with mineralocorticoid excess and those with mineralocorticoid deficiency. The former are generally associated with hyperaldosteronism and the latter with hypoaldosteronism.
Disorders of mineralocorticoid excess are differentiated into:
- Primary disorders (primary aldosteronism), which are due to an adrenal adenoma or adrenal hyperplasia with autonomous aldosterone production
- Secondary disorders of the RAS (secondary hyperaldosteronism), which are due to renin producing tumors or systemic diseases such as renal artery stenosis, arteriosclerosis of both renal arteries, or renal and cardiac diseases.
Mineralocorticoid deficiency is due to lack of aldosterone synthesis or action. While normally 99.5% of the filtered Na+ is reabsorbed by the kidneys with the aid of mineralocorticoids, only 98.5% is reabsorbed if there is a lack of aldosterone effect. This causes a daily loss of approximately 1 mol of Na+, corresponding to about 28 g of sodium chloride. Chronic Na+ loss leads to reduction of the extracellular volume and hyponatremia. Plasma osmolality decreases, the cell water content increases, and there is hypotonic dehydration in the extracellular space. Since the aldosterone dependent transport of Na+ is coupled with the exchange of K+ , the renal excretion of K+ is reduced, resulting in hyperkalemia. Since both K+ and H+ excretion is insufficient, hyperkalemic acidosis develops. As in other electrolyte shifts, the Mg++ behave analogously to the K+.
Mineralocorticoid deficiency can be primary and is due to reduced aldosterone synthesis in:
- Adrenal insufficiency (Addison’s disease)
- Central disorder (ACTH deficiency)
- Enzyme deficiencies of aldosterone biosynthesis, such as 18-hydroxylase deficiency or aldosterone synthase (P450aldo, CYP11B2) deficiency. Refer to ).
A more common form of mineralocorticoid deficiency is secondary deficiency, which is due to reduced target organ responsiveness to aldosterone, such as in pseudo hypoaldosteronism.
Disorders of mineralocorticoid deficiency can be due to:
- A defect in the hypothalamic-pituitary-adrenal axis
- Isolated hypoaldosteronism
- Reduced target organ response to aldosterone, as is the case in pseudo hypoaldosteronism.
Biomarkers for diagnosing and differentiating disorders of the RAS include Na+, K+, renin, aldosterone, aldosterone/renin ratio (ARR), 18-hydroxycorticosterone, and 18-hydroxy cortisol.
The ARR is the most reliable means for screening of hyperaldosteronism. The main indications are:
- Hypertension and suspected primary aldosteronism; the ARR is increased
- Hypertension and suspected secondary aldosteronism; the ARR is normal
- A low arterial blood volume or chronic renal insufficiency; renin and aldosterone are increased.
A high ARR should be confirmed by functional tests.
The determination of steroid hormones of the mineral corticoid pathway with negligible mineral corticoid activity such as plasma 18-hydroxycorticosterone (18-OHB) and urinary 18-hydroxy cortisol (18-OHF) excretion can be markedly elevated in primary aldosteronism.
The RAS plays an important role in maintaining blood pressure and the balance of water, Na+ and K+ metabolism (). The release of renin from the juxtaglomerular apparatus of the kidney is stimulated by a decrease in blood volume, blood pressure, and renal perfusion. Renin converts angiotensinogen, which is produced in the liver, into angiotensin I. Pulmonary angiotensin converting enzyme (ACE) cleaves angiotensin I to produce angiotensin II, which leads to a rise in blood pressure through vasoconstriction and stimulates the adrenal cortex to produce aldosterone. The rise in aldosterone causes Na+ and water retention with an increase in extracellular fluid volume. Thus, there is no further stimulus for excessive renin secretion and normal renin secretion is resumed; negative feedback mechanism.
Aldosterone secretion may also be stimulated by decreased Na+ or elevated K+ plasma levels as well as by ACTH.
Aldosterone is metabolized rapidly, having a biological half life of only 31 minutes. Its metabolism occurs primarily in the liver, where it is converted to tetrahydroaldosterone (approximately 40%), and secondarily in the kidney, where it is converted to aldosterone 18-glucuronide (approximately 10%). About 0.2–0.5% is excreted into the urine as free aldosterone.
Aldosterone binds to the mineralocorticoid receptor (MR) which belongs to the super family of steroid receptors. In the tissues, the MR is coexpressed with the enzyme 11β-hydoxysteroid dehydrogenase 2 (11β-HSD2), which metabolizes cortisol to cortisone. Cortisol is another potential ligand of the MR and competes with aldosterone for binding to the receptor. Although both have approximately the same receptor affinity, cortisol inhibits the binding of aldosterone, since its tissue concentration is at least 10-fold higher and its plasma concentration 100-fold higher than that of aldosterone. The MR is thus protected from binding by aldosterone. However, when cortisol is metabolized to inactive cortisone by 11β-HSD2, the receptor is free for aldosterone. Inactivating mutations of 11β-HSD2 result in apparent mineralocorticoid excess syndrome.
The precursor of renin, prorenin, circulates in plasma at a concentration 10-fold higher than the renin level. While several tissues are capable of producing prorenin, only the juxtaglomerular cells of the kidneys are capable of the controlled release of prorenin from the storage vesicles. Prorenin and renin have high binding affinity for a receptor known as prorenin receptor. Binding of renin to the receptor increases its activity by a factor of 5. Free prorenin, which normally does not exhibit enzymatic activity, is activated by binding to the receptor, and as a result has the same activity as free renin.
Mutations in the proteins of the RAS are playing an increasingly important role in the search for causes of aldosterone producing adenoma. Approximately one third of patients with adenoma are found to have mutations in the KCNJ5 gene, which encodes the cell-specific K+ channel. Mutations are also found in ATPase genes, which encode the α-subunit of the Na+-K+-ATPase in aldosterone producing adenoma .
Renin and sodium are the two main factors in blood pressure control and renin levels vary conversely with sodium load. Renin maintains blood pressure through vasoconstriction when there is adequate sodium to maintain volume. Blood pressure control requires a combination of natriuresis and blocking the consequential increase in renin activity .
Patients with hypertension and suspicion of low-renin hypertension.
Renin is synthesized as the inactive zymogen, prorenin. Prorenin contains a pro segment that masks the active site, thereby preventing access by angiotensinogen the renin substrate. Cleavage of the pro segment converts prorenin to renin. The plasma concentration of prorenin is approximately 10-fold higher than for renin. Prorenin exists in two different conformations. More than 98% of prorenin is in a closed conformation in which the pro segment masks the binding site for angiotensinogen; the molecule in this conformation has no enzymatic activity. Less than 2% of plasma prorenin has an open conformation in which the pro segment no longer masks the active site. The open conformation is accessible to angiotensinogen and is enzymatically active (active prorenin) .
Two different methods are used to determine renin:
- Renin activity; the activity of renin to generate angiotensin I is determined. The renin activity assay is still considered the gold standard
- Renin mass; the concentration of renin (with or without including prorenin) in the plasma is determined.
Both methods provide different information. Whereas activity assays measure only active renin, immunoassays measure both active and inhibited renin (e.g. inhibited by renin inhibitors). Different conformations of renin and prorenin are measured using activity assays and immunoassays (
Plasma renin activity measurement
The assay comprises two steps. First, angiotensin I is generated by renin acting on endogenous angiotensinogen in plasma incubated at 37 °C. Inhibitors of angiotensinase and angiotensin converting enzyme (ACE) are added to the sample to prevent the degradation of angiotensin I and its conversion to angiotensin II. The concentration of angiotensin I is measured with a radioimmunoassay . Angiotensin I can also be quantified by liquid chromatography tandem mass spectrometry using online solid phase extraction (XLS-MS/MS) . Enzyme activity is expressed as ng (μg) angiotensin I generated per mL (liter) of plasma per hour. Expression as pmol or nmol per liter of plasma and hour is also common. Pre analytical factors influencing the result of the renin activity assay are shown in .
Plasma renin concentration measurement
- Renin and active prorenin (prorenin in open conformation). Total renin (renin and prorenin in the active form) is determined using sandwich immunoassays or competitive immunoassays.
- Renin without prorenin (prorenin in closed conformation). For the determination of renin it is essential to prevent inadvertent conversion of plasma prorenin from a closed to an open conformation during sample preparation and during the assay itself. This reportedly occurs to 5% if the immunoassay is performed at 22 °C for 24 h, but not at 37 °C for 6 h .
Calibration is performed to International Standard 68/356, in which 1 U is generally equivalent to 0.6 μg of active renin. The functional sensitivity of the assays is at the lower reference interval value and ranges between 2 and 4 mU/L, depending on the assay. The detection limit is 1 ng/L (1.7 mU/L) . Results are reported in mU/L, ng/L, or nmol/L.
EDTA blood: 5 mL
Centrifuge within 30 minutes, preferably within 10 minutes; freeze plasma, if assay is not performed immediately.
The physiologic activation of the renin-angiotensin-aldosterone system (RAS) is characterized by renin induced aldosterone increase and serves to maintain the intravascular volume and the blood pressure .
Low renin can be caused by physiological suppression of renin, in the context of intravascular volume expansion or a pathologic condition of aldosterone excess as described in primary aldosteronism.
Besides primary aldosteronism conditions that manifest with low renin and hypertension (low renin hypertension) can result from mineralocorticoid excess without aldosterone excess resulting from increased production of corticosteroids in the synthetic pathway of aldosterone from 11-deoxycorticosterone. Although these corticosteroids only have a mild mineralocorticoid effect, they have a clinical impact if produced in large amounts .
Method of determination
The detection of prorenin in the open conformation is a problem especially in the diagnosis of diseases in which low renin levels are expected, such as in primary aldosteronism.
Renin assays used for determining the ARR should be sufficiently sensitive to measure levels as low as 0.2–0.3 ng/mL/h; renin concentration assays should have a functional sensitivity of at least 2 mU/L .
In low renin states plasma renin activity may not ensure enough sensitivity in many patients. In a study a two step procedure is proposed: a brief 1.5 h enzymatic reaction time followed by a prolonged reaction (18 h) only if PRA is below 0.2 ng/mL/h.
The results obtained with assays from different diagnostics manufacturers are not comparable. Therefore, the conversion factors used to convert renin activity in ng/mL/h to renin concentration in ng/L or mU/L also differ. For example, in one commercial assay, a plasma renin activity level of 1 ng/mL/h converts to a direct renin concentration of 12 mU/L (7.6 ng/L) .
In patients with renin inhibitor therapy immunoassay measurement of renin can result in false results because binding of the renin inhibitor to the active site of prorenin molecules with an open conformation prevents refolding of the pro segment. Thereby the amount of prorenin recognized by the renin immunoassay is increased .
Samples may show a fast angiotensin I degradation activity during the incubation period for determination of plasma renin activity mediated by calcium. In a study most samples had normal calcium concentrations indicating that serum or heparinized but not EDTA plasma samples had been send to the laboratory .
2. Campbell DJ, Nussberger J, Stowasser M, Danser AHJ, Morganti A, Frandsen E, et al. Activity assays and immunoassays for plasma renin and prorenin: information provided and precautions necessary for acute measurement. Clin Chem 2009; 867–77.
3. Carter S, Owen LJ, Kerstens MN, Dullaart RPF, Keevil BG. A liquid chromatography tandem mass spectrometry assay for plasma renin activity using online solid-phase extraction. Ann Clin Biochem 2012; 49: 570–9.
6. Funder JW, Carey RM, Mantero F, Murad HM, Reincke M, Shibata H, et al. The management of primary aldosteronism: case detection, diagnosis, and treatment: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2016, 101: 1889–1916.
9. Malik S, Wong ND, Franklin SS, Kamath TV, L’Italien GJ, Pio JR, et al. Impact on the metabolic syndrome on mortality from coronary heart disease, cardiovascular disease, and all causes in the United States adults. Circulation 2004; 110: 1245–50.
12. Eisenhofer G, Dekkers T, Peitzsch M, Dietz AS, Bidlingmaier M, Treitl M, et al. Mass spectrometry-based adrenal and peripheral venous steroid profiling for subtyping of primary aldosteronism. Clin Chen 2016; 62: 514–24
13. Fischer E, Reuschl S, Quinkler M, Rump LC, Hahner S, Bidlingmaier M, et al. Assay characteristics influence the aldosterone to renin ratio as a screening tool for primary aldosteronism: results of the German Conn’s Registry. Horm Metab Res 2013; 45: 526–31.
Mineralocorticoid excess syndrome results from excess adrenal production of aldosterone and is the most frequent cause of secondary arterial hypertension. The inappropriate increased production of aldosterone is associated with sodium retention, hypervolemia. Suppression of plasma renin, increased potassium excretion, which may lead to hypokalemia, and cardiovascular disease .
Mineralocorticoid deficiency is due to lack of aldosterone synthesis of the adrenal cortex or decreased action of aldosterone. The inappropriate decreased effect of aldosterone is associated with low aldosterone levels, sodium depletion, increase of plasma renin, decreased potassium excretion, which may lead to hyperkalemia , and hypovolemia.
Suspected excess of mineralocorticoids:
- Treatment-resistant hypertension
- Primary aldosteronism
Suspected mineralocorticoid deficiency (e.g., in hyperkalemia without renal failure).
HPLC tandem mass spectrometry
Radioimmunoassay and assays with non-radioactive tracers are used.
Measurement of urinary aldosterone
The major proportion of urinary aldosterone is tetrahydroaldosterone-3-glucuronide and aldosterone-18-glucuronide. In most cases, however, free aldosterone is measured, which only accounts for 0.2% of total aldosterone. To measure free aldosterone, the urine sample is subjected to acid hydrolysis, buffered, and then analyzed by gas chromatography mass spectrometry.
Serum or plasma (blood is collected in recumbent or seated position): 1 mL
24 h urine collection (neutral).
The RAS is often involved in the pathophysiology of hypertension, be it primary or secondary. The prevalence of primary aldosteronism increases with the severity of hypertension, from 2% in patients with low grade hypertension to 20% among resistant hypertensives.
Primary aldosteronism (PA) is a group of disorders in which aldosterone production is inappropriately high for Na+ status and non-suppressible by Na+ loading. PA is relatively autonomous for the most frequent regulators of the RAS. PA is the most frequent cause of secondary hypertension and is responsible for 5–15% of cases of increased blood pressure among hypertensive population. Laboratory findings are increased level of aldosterone, decreased plasma renin, and hypokalemia (9–37%) .
The two forms of PA are:
- Unilateral aldosterone secretion caused by excessive aldosterone producing adenoma (APA) and treated by adrenalectomy
- Bilateral adrenal hyperplasia (BAH) resulting from hyperplasia of the zona glomerulosa, also known as idiopathic hyperaldosteronism. BAH is treated with mineralocorticoid receptor antagonists.
Rare subtypes of PA are:
- Adrenocortical carcinoma
- Familial aldosteronism type II
- Ectopic aldosterone production
- Unilateral primary adrenal hyperplasia
- Glucocorticoid remediable aldosteronism.
It is important to diagnose PA, since aldosterone producing adenoma induced hypertension is treated surgically while bilateral hyperplasia induced hypertension can only be treated with mineralocorticoid receptor antagonists. Patients with PA have a higher incidence of cardiovascular events than those with essential hypertension. The Endocrine Society Clinical Practice Guideline of the USA recommends for the diagnosis of PA three diagnostic steps :
- Case detection
- Case confirmation
- Subtype classification.
Case detection test: Aldosterone/renin ratio (ARR)
Drugs that may lead to false-positive or false-negative ARR results should be discontinued.
The ARR test is most sensitive when samples are collected in the morning after patients have been out of bed for at least 2 hours, usually after they have been seated for 5–15 minutes . A detailed approach is shown in Ref. .
The ARR is the best screening test for suspected PA-induced hypertension and its differentiation from essential hypertension. The ARR can be used under random conditions because it is less affected by diurnal variations, posture and gender. Prior to blood collection the K+ level should be normal. The diagnostic sensitivity of the ARR for PHA is 64–100%, with a specificity of 87–100% . Influencing factors include medication, hormonal contraceptives, antidepressants, hypokalemia, and increased table salt intake. The cutoff values are depending on the renin assay used and are shown in .
Case confirmation of primary aldosteronism
Patients with a positive ARR should undergo one or more confirmatory tests to definitively confirm or exclude the diagnosis of PA. However in the setting of spontaneous hypokalemia, plasma renin below detection levels, plus aldosterone > 20 ng/dL (550 pmol/L) the Endocrine Society suggests that there may be no need for further confirmation. There is no gold standard confirmatory test.
Subtype classification of primary aldosteronism (PA)
For treatment strategy subtype classification of PA is needed independent of etiology of PA. Lateralization of the source of the excessive aldosterone production is critical to guide the management of PA. The two main forms of PA the unilateral aldosterone producing adenoma and the bilateral adrenal hyperplasia are differentiated by imaging. However imaging cannot reliably visualize micro adenoma or distinguish non-functioning incidentaloma from aldosterone producing adenoma. Adrenal vein sampling is the most accurate means of differentiating unilateral from bilateral forms of PA. Distinguishing between unilateral and bilateral abnormalities is critical in deciding whether surgical intervention is indicated, because unilateral adrenalectomy in patients with adenoma, unilateral hyperplasia, adrenal carcinoma, ectopic ACTH production and renin- and 11-deoxycorticosterone producing tumors results in normalization of K+ levels in 30–60% of the hypertensive patients. In bilateral idiopathic aldosteronism and glucocorticoid remediable hyperaldosteronism, unilateral and bilateral adrenalectomy rarely corrects the hypertension, and a conservative approach is necessary.
Adrenal venous sampling (AVS) is the most accurate means of differentiating unilateral from bilateral forms of PA. The diagnostic sensitivity and specificity of AVS for detecting unilateral aldosterone excess are 95% and 100%, respectively. However, in patients younger than 35 years with marked PA (e.g., spontaneous hyperkalemia, aldosterone > 30 ng/dL, 831 pmol/L) and solitary unilateral apparent adenoma on CT scan, a case can be made to proceed directly to unilateral adrenalectomy without prior AVS .
Secondary aldosteronism, occurs in states of low effective blood volume, which activates the RAS. The resultant increase in renin and aldosterone stimulates the distal reabsorption of Na+ by the kidney to restore blood volume . This Na+ retaining effect occurs in association with an increase in K+ excretion. Because hyponatremia and hypovolemia increase renin release the concentrations of aldosterone and renin are increased in secondary aldosteronism. The aldosteronism results in hyperkaluresis, which also is present when there is hypokalemia.
Secondary aldosteronism is a common finding in both normotensive and hypertensive patients. The primary stimulus is caused by hypovolemia and hyponatremia that stimulate renin release, which acts as proteolytic enzyme on angiotensinogen to produce angiotensin I . In edema, for example, water and Na+ are conserved at the expense of K+. This is also the case in pregnancy, although there is no loss of K+, since the effect of aldosterone is overridden by pregnancy hormones. In chronic kidney disease, the secondary aldosteronism counteracts hyperkalemia. Secondary aldosteronism is most common in renal artery stenosis, diuretic abuse, or renin secreting tumors .
Pseudo hyperaldosteronism is a condition of mineralocorticoid excess and normal aldosterone characterized by hypertension, hypokalemic alkalosis, and suppressed renin . Some of the cases include Liddle’s syndrome, Cushing’s syndrome, congenital adrenal hyperplasia, and 11β-hydroxysteroid dehydrogenase deficiency.
Laboratory findings are hyponatremia, hyperkalemia, hypermagnesemia, metabolic acidosis, significantly reduced aldosterone, significantly elevated renin.
Hypoaldosteronism in primary adrenal insufficiency
Laboratory findings are reduced cortisol and aldosterone, and elevated renin. Normalization of renin can be a sensitive indicator for monitoring the treatment of adrenal insufficiency.
Aldosterone synthase type I deficiency with 18-hydroxylase deficiency
Laboratory findings are reduced aldosterone, elevated renin, reduced levels of the mineralocorticoids corticosterone, 11-deoxycorticosterone, and 18-hydroxycorticosterone. As a further step, the ACTH test should be performed to check that cortisol synthesis is intact.
Aldosterone synthase type II deficiency with 18-oxidase deficiency
Findings are elevated 18-hydroxycorticosterone levels in relation to aldosterone and/or an increased ratio of 18-OH-tetrahydroaldosterone to tetrahydroaldosterone excretion in urine . The diagnosis can be confirmed by molecular genetic investigations. Patients have low aldosterone, hyperkalemia, renal salt loss, and metabolic acidosis.
Besides the different 18-hydroxycorticosterone concentrations, the ratio of 18-hydroxycorticosterone to aldosterone is also important in the differential diagnosis of aldosterone synthase deficiency. Ratios less than 10 are found in type I, ratios greater than 100 in type II.
Secondary mineralocorticoid deficiency is associated with low renin and low aldosterone levels and occurs in Liddle’s syndrome, congenital adrenal hyperplasia, apparent mineralocorticoid excess syndrome, and licorice abuse.
Pseudo hypoaldosteronism represents a disorder associated with functional secretion of aldosterone however, clinical symptoms are comparable to hypoaldosteronism. Pseudo hypoaldosteronism is a group of hereditary or acquired disorders due to a defect in the loop of Henle or the distal tubule. Secondary forms of pseudo hypoaldosteronism are seen in kidney diseases, such as diabetic nephropathy and analgesic nephropathy.
- The congenital forms are characterized by hypokaliuric hyperkalemia
- Pseudo hypoaldosteronism type I is characterized by abnormal function of the amiloride sensitive Na+- K+ channel ENaC ().
- In pseudo hypoaldosteronism type II there is thought to be increased chloride reabsorption instead of K+ efflux . For further information refer to .
Manifestation in childhood, failure to thrive, and hypotension.
High renal sodium chloride excretion, hyperkalemia, dramatically increased aldosterone and renin.
Because ARR is mathematically highly dependent on renin, assays for the determination of renin should be sufficiently sensitive to measure levels as low as 0.2–0.3 ng/mL/h or 2 mU/l using immunoassays .
Aldosterone is the most important naturally occurring mineralocorticoid. Aldosterone is produced exclusively in the zona glomerulosa of the adrenal cortex from cholesterol via a series of hydroxylations and oxidations ().
The final steps of this pathway, the conversion of 11-deoxycorticosterone (DOC) to aldosterone, require conversion via the intermediates to 18-hydroxy-DOC or corticosterone and 18-hydroxycorticosterone . Aldosterone is present in serum in the isomeric forms through formation of a cyclic hemiacetal bond between the positions C11 and C18.
The enzyme P450aldo (aldosterone synthase, CYP11B2) catalyzes the last three steps (11β-hydroxylation, 18-hydroxylation and 18-oxidation) of the aldosterone biosynthesis pathway. The enzyme is encoded by the gene CYP11B2.
Aldosterone is excreted into the urine as aldosterone-18-glucuronide, tetrahydroaldosterone, and free aldosterone. Aldosterone is glucuronidated in the liver and kidneys at position C18. Only about 0.2% of secreted aldosterone is excreted as free aldosterone. The remainder is converted in the liver to tetrahydroaldosterone by reduction of the ring A, and glucuronidated at position C3 ().
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18-hydroxycortisol (18-OHF) is known as hybrid steroid, because it has structural characteristics of both cortisol and aldosterone (). 18-OHF is synthesized by aldosterone synthase using 11-deoxycortisol as substrate. Small amounts are also synthesized by 11β-hydroxylase. Because aldosterone synthase expression is normally limited to the zona glomerulosa of the adrenal cortex and 17α-hydroxylase and 11β-hydroxylase necessary for cortisol synthesis occur in the zona fasciculate, production of 18-OHF is normally very low .
Diagnosis of primary aldosteronism.
Collect urine over 24 hours, keep sample frozen until it is analyzed.
In patients with elevated aldosterone/renin ratio (ARR) the determination of 18-OHF urinary excretion is recommended. Levels below 130 ug/24 h exclude primary aldosteronism and are secreted from patients with non secreting cortical adrenal tumors e.g., in patients with essential hypertension. In the range of 130–510 ug/24 h no assessment is possible and confirmatory tests are needed. Levels higher than 510 ug/24 h indicate primary aldosteronism, however differentiation between adenoma and glucocorticoid remediable aldosteronism is not possible .
1. Mulatero P, Morra di Cella S, Monticone S, Schiavone D, Manzo M, Mengozzi G, et al. 18-hydroxycorticosterone, 18-hydroxycortisol, and 18-oxocortisol in the diagnosis of primary aldosteronism and its subtypes. J Clin Endocrinol Metab 2012; 97: 881–9.
18-Hydroxycorticosterone (18-OHB) is formed by 18-hydroxylation of corticosterone. 18-OHB is an intermediate precursor in aldosterone biosynthesis that originates from the conversion of corticosterone by the aldosterone synthase (). Although small amounts may be produced by the 11β-hydroxylase, 18-OHB has only low affinity for the mineralocorticoid receptor.
Differentiation of aldosterone producing adenoma and bilateral adrenal hyperplasia.
Heparinized and EDTA plasma: 1 mL
Urine: collect urine (neutral) over 24 hours, take sample to laboratory or measure volume and ship in 10 mL container.
Patients with aldosterone producing adenoma generally have recumbent 18-OHB levels higher than 1,000 ng/L at 8.00 a.m., whereas patients with idiopathic hyperaldosteronism have 18-OHB levels that are usually below 1,000 ng/L /, /.
1. Abdelhamid S, Vecsei P, Haak D, Gless KH, Walb D, Fiegel P, Lichtwald K. Elevated free 18-OH-corticosterone excretion as a possible indicator for early diagnosis of primary aldosteronism. J Steroid Biochem 1981; 14: 913.
2. Hornung J, Gless KH, Abdelhamid S, Vielhauer W, Vecsei P. Radioimmunoassay of free urinary 18-hydroxy-deoxycorticosterone (18-OH-DOC) in patients with essential hypertension. Clin Chim Acta 1978; 87: 181.
Plasma renin activity (PRA)
Values expressed are median and 2.5th and 97.5th percentiles
Values expressed are 2.5th and 97.5th percentiles
Renin concentration (immunoassay)
Renin plus prorenin concentration (immunoassay)
Values expressed are median and 2.5th and 97.5th percentiles
Conversion renin concentration: 1 pmol/L = 1.296 ng/L; 1 ng/L = 1.7 mU/L
Clinical and laboratory findings
Legend: I, increase; D, decline; 1= low; 2 = moderate; 3 = significant
Aldosterone in plasma and serum
Aldosterone in urine
Conversion of aldosterone: ng/L × 2.77 = pmol/L; μg/L × 2.77 = nmol/L
Clinical and laboratory findings
Values shown are on the basis of a conversion factor of 8.2 for renin activity assay to renin immunoassay. The most common cutoffs are 30 for conventional units and 750 for SI units.
Figure 31.2-2 The role of the RAS in maintaining blood pressure and Na+ balance in the case of decreased sodium chloride (table salt) intake. The kidney synthesizes renin; angiotensin II, which is produced in subsequent reaction steps, causes elevated blood pressure by arteriolar vasoconstriction and the aldosterone stimulated retention of Na+.
Figure 31.3-1 Different conformations of renin and prorenin, and specificities of the different activity assays and assays measuring the concentration (e.g., immunoassays). Assays 1 and 2 are commonly used in routine diagnostics. Modified from Ref. .
Figure 31.3-2 Aldosterone is synthesized from cholesterol, as are the other adrenal steroids. The enzymes catalyzing each of the synthetic reactions are printed in italics. The dotted arrows indicate those biochemical transformations that are catalyzed by each of the corresponding enzymes. AS, aldosterone synthase (P450aldo, CYP11B2). The steroid molecule depicted in the right lower portion shows the biologically important positions on the steroid molecule.
Figure 31.4-1 Diagnostic approach for suspected primary aldosteronism (PA) and possible diagnoses. Modified from Ref. . A positive aldosterone/renin ratio (ARR) must be confirmed by functional tests. Adrenal venous sampling (AVS) is used to identify the subtype of PA. PAC, concentration of aldosterone in plasma; CT, computer tomography; MR, mineralocorticoid receptor.