31 Renin-angiotensin-aldosterone system (RAS)

Lothar Thomas

31.1 Hypertension

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 /1/. 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 /2/. 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 /3/. 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 /4/.

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 /5/. 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 /6/.

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 /7/.

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.

References

1. Williams B, Mania G, Spiering W, Agabiti Rosei E, Azizi M, Burnier M, et al. 2018 ESC/ESH guidelines on hypertension.Eur Heart J 2018; 39: 3021–3104.

2. Jordan J, Kurschat C, Reuter H. Arterial hypertension. Dtsch Arztebl Int 2018; 115: 557–68.

3. Lawes CM, Vander Hoorn S, Rodgers A. Global burden of blood pressure related disease 2001. Lancet 2008; 371: 1513–8.

4. Basile JN, Chrysant S. The importance of early antihypertensive efficacy: the role of angiotensin II receptor blocker therapy. J Human Hypertens 2006; 20: 169–75.

5. Hajjar J, Kotchen TA. Trends in the prevalence, awareness, treatment, and control of hypertension in the United States, 1988–2000. JAMA 2003; 290: 199–206.

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.

7. Baudrand R, Vaidya A. The low-renin hypertension phenotype: genetics and the role of the mineralocorticoid receptor. Int J Mol Sci 2018; 19: 546; doi: 10.3390/ijms19020546.

31.2 Renin-angiotensin-aldosterone system

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:

• Fig. 31.2-1 – Renin-angiotensin-aldosterone system
• Fig. 31.2-2 – The role of the renin-angiotensin-aldosterone system in maintaining blood pressure
• Section 8.1 – Water balance and fluid compartments.

31.2.1 Function of renin

Expression of the Renin gene leads to /1, 2/:

• 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.

Angiotensin II mediates the following effects /3/:

• 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).

31.2.2 Function of aldosterone

Aldosterone is produced in the zona glomerulosa of the adrenal cortex and regulates the body’s electrolyte and volume balance via its mineralocorticoid effect /4/. 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.

Refer to Fig. 31.3-2 – Synthesis of aldosterone.

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⁺ /5/.

Refer to Fig. 8.8-2 – Reabsorption of solutes in the collecting duct.

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.

31.2.3 Mineralocorticoid excess

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.

Refer to Tab. 31.2-1 – Disorders of the renin-angiotensin-aldosterone system.

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.

31.2.4 Mineralocorticoid deficiency

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 (P450_aldo, CYP11B2) deficiency. Refer to Fig. 31.3-2 – Synthesis of aldosterone, cortisol and andostendione).

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.

31.2.5 Testing for disorders of the RAS

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.

31.2.6 Biochemistry and pathophysiology of RAS

The RAS plays an important role in maintaining blood pressure and the balance of water, Na⁺ and K⁺ metabolism (Fig. 31.2-2 – The role of the RAS in maintaining blood pressure and Na⁺ balance in the case of decreased table salt intake). 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 /6/.

An overview of the genetic determinants of the RAS is shown in Tab. 31.2-2 – Genetic determinants of the RAS and diseases influenced by them.

References

1. Sparks MA, Crowley SD, Gurley SB, Mirotsou M, Coffman TM. Clasical renin-angiotensin system in kidney physiology. Compr Physiol 2014; 4: 1201–28.

2. Yang T, Xu C. Physiology and pathophysiology of the intrarenal renin-angiotensin system: an update. J Am Soc Nephrol 2017; 28: 1040–9.

3. Brown MJ. Renin: friend or foe? Heart 2007; 93: 1026–33.

4. Basile JN, Chrysant S. The importance of early antihypertensive efficacy: the role of angiotensin II receptor blocker therapy. J Human Hypertens 2006; 20: 169–75.

5. Fuller PJ. Adrenal diagnostics: an endocrinologist’s perspective focused on hyperaldosteronism. Clin Biochem Rev 3013; 34: 111–5.

6. Funder JW, Carey RM, Mantero F, Mura MH, 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.

31.3 Renin

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 /1/.

31.3.1 Indication

Patients with hypertension and suspicion of low-renin hypertension.

31.3.2 Method of determination

The analysis of plasma renin is challenging due to the low circulating concentration (10 × 10^–12 mol/L).

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) /2/.

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 concentration; 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 (Fig. 31.3-1 – Conformations of renin and prorenin).

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 /2/. Angiotensin I can also be quantified by liquid chromatography tandem mass spectrometry using online solid phase extraction (XLS-MS/MS) /3/. 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 Tab. 31.3-1 – Influence factors causing activation of prorenin and recommendations for specimen handling.

Plasma renin concentration measurement

Immunoassays are commercially available that measure /4/:

• 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 /2/.

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) /5/. Results are reported in mU/L, ng/L, or nmol/L.

31.3.3 Specimen

EDTA blood: 5 mL, refer to Section 31.3.6 – Comments and Problems.

31.3.4 Reference interval

Refer to Tab. 31.3-2 – Reference intervals for renin.

31.3.5 Clinical significance

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 /6/.

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. The determination of renin is important for determination of the renin/aldosteron ratio (ARR). The ARR is determined as screening for the detection of primary aldosteronism in patients with hypertension.

Refer to:

• Section 31.4.5.1 – Primary aldosteronism
• Tab. 31.4-6 – Aldosterone/renin ratio (ARR) cutoffs depending on renin assay based on renin activity or renin concentration.

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 /7/.

Refer to:

• Fig. 31.3-2 – Synthesis of aldosterone, cortisol and androstendione
• Tab. 31.3-3 – Low renin hypertension due to non aldosterone mineralocorticoids.

All drugs in use of hypertension have an influence on plasma renin and other components of the RAS /1/. Refer to Tab. 31.3-4 – Impact of different classes of antihypertensive agents on renin mass and plasma renin activity.

31.3.6 Comments and problems

Before measurement of renin pre analytical conditions are required to prevent unfolding and cleavage of the pro segment of prorenin (Tab. 31.3-1 – Measurement of renin by activity assay or immunoassay).

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 /6/.

Renin activity

In low renin states plasma renin activity may not ensure enough sensitivity in many patients. In a study /14/ 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.

Renin concentration

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) /13/.

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 /2/.

Method comparison

Different commercial assays for the determination of plasma renin activity in patient samples and in comparison to LC-MS/MS assays showed no harmonization. None of the samples measured by all assays showed overall coefficients of variation (CV) < 10% and 37% of samples showed overall CVs > 20%. The 95% confidence intervals (CIs) for slopes did not contain 1 for most assay pairs. Large relative biases (–85.1–104.2) were found, and 76% of samples had unacceptable biases /16/.

Standardization

WHO standard renin 68/356

Stability

Stability of plasma renin activity and renin concentration in whole blood was investigated /15/. After sample processing, plasma can be kept at room temperature (15–25 °C) for up to 24 h for determination of renin concentration.

For determination of renin activity separated plasma should be analysed or frozen as soon as possible. Plasma renin activity increases when plasma is stored at 2–8 °C. Studies showed that prorenin is reversibly activated at low temperature or by acidic conditions allowing proteolytic conversion of prorenin to renin.

After blood collection EDTA-samples can be collected in primary care and left unspun, provided they reach the laboratory for processing within 24 h.

References

1. Brown MJ. Renin: friend or foe? Heart 2007; 93: 1026–33.

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.

4. Deinum J, Derkx FHM, Schalekamp MADH. Improved immunoradiometric assay for plasma renin. Clin Chem 1999; 45: 847–54.

5. Ferrari P, Shaw SG, Nicod J, Saner E, Nussberger J. Active renin versus plasma renin activity to define aldosterone-to-renin ratio for primary aldosteronism J Hypertens 2004; 22: 377–81.

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.

7. Baudrand R, Vaidya A. The low-renin hypertension phenotype: genetics and the role of the mineralocorticoid receptor. Int J Mol Sci 2018; 19: 546; doi: 10.3390/ijms19020546.

8. Hajjar J, Kotchen TA. Trends in the prevalence, awareness, treatment, and control of hypertension in the United States, 1988–2000. JAMA 2003; 290: 199–206.

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.

10. Corry DB Tuck ML. Secondary aldosteronism. Endocrinol Metab Clin North Am 1995; 24: 511–27.

11. Gupta V. Mineralocorticoid hypertension. Indian J Endocrinol 2011; 15 (Suppl 4): S298–S312.

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.

14. Brossaud J, Corcuff JB. Pre-analytical and analytical considerations for the determination of plasma renin activity. Clin Chim Acta 2009; 410: 90–2.

15. Hepburn S, Munday C, Taylor K, Halsall DJ. Stability of direct renin concentration and plasma renin activity in EDTA whole blood and plasma at ambient and refrigerated temperatures from 0 to 72 hours. Clin Chem Lab Med 2022; 60 (9): 1348–92.

16. Liu Z, Jin L, Zeng J, Zhang T, Zhang J, Zhou W, et al. Poor comparability of plasma renin activity measurement in determining patient samples: the status quo and recommendations for harmonization. Clin Chem Lab Med 2023; 61 (10): 1770–9.

31.4 Aldosterone

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 /1/.

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.

31.4.1 Indication

Suspected excess of mineralocorticoids:

• Hypertension > 150/100 mm Hg
• Treatment-resistant hypertension
• Primary aldosteronism

Suspected mineralocorticoid deficiency (e.g., in hyperkalemia without renal failure).

31.4.2 Method of determination

HPLC tandem mass spectrometry

This is the reference method. Principle and detailed procedure see Ref. /2/.

Immunoassay

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.

31.4.3 Specimen

Serum or plasma (blood is collected in recumbent or seated position): 1 mL

24 h urine collection (neutral).

31.4.4 Reference interval

The reference interval for aldosterone in plasma and the different forms of aldosterone in urine are shown in Tab. 31.4 -1– Reference intervals for aldosterone.

31.4.5 Clinical assessment

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.

Mineralocorticoid hypertension comprises the etiological spectrum listed in Tab. 31.4-2 – Mineralocorticoid hypertension disorders.

Indications for testing for mineralocorticoid hypertension are listed in Tab. 31.4-3 – Case detection of primary aldosteronism.

31.4.5.1 Primary aldosteronism

Primary aldosteronism (PA) is a group of disorders in which aldosterone production is inappropriately high for Na+ status, relatively autonomous of the major regulators of secretion (angiotensin I, plasma K⁺ and non-suppressible by Na⁺ loading). PA is the most frequent cause of secondary hypertension and is responsible for 5–15% of hypertensive patients. Laboratory findings are increased level of aldosterone, decreased plasma renin, and hypokalemia (9–37%)  /3/.

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.

Refer to Tab. 31.4-4 – Primary aldosteronism, mineralocorticoid excess and hypertension.

31.4.5.1.1 Diagnosis of primary 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 /3/:

• Case detection
• Case confirmation
• Subtype classification.

The plasma aldosterone/renin ratio (ARR) is recommended to detect possible cases of PA /3/.

Case detection investigations

• Blood pressure (BP) measurement: sustained BP above 150/100 mm Hg or BP > 140/90 mm Hg resistant to three conventional antihypertensive drugs (including a diuretic) or controlled BP > 140/90 mm Hg on four or more antihypertensive drugs
• Plasma aldosterone/renin ratio (ARR): drugs that may lead to false-positive or false-negative ARR results should be discontinued. Refer to Tab. 31.4-5 – Medications to be discontinued prior to testing for mineralocorticoid induced hypertension.

Determination of ARR

The ARR is recommended to detect possible cases of PA. Blood 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 min. /3/. A detailed approach is shown in Ref. /3/.

A positive ARR (> 20) is the best screening test to detect 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 PA is 64–100% at specificity of 87–100% /4/. Influencing factors include medication, hormonal contraceptives, antidepressants, hypokalemia, and increased table salt intake. The cutoff values are depending on the renin assay used /3/ and are shown in Tab. 31.4-6 – Aldosterone/renin-ratio (ARR) cutoff values depending on the renin assay.

Limitations of the ARR is that in the presence of very low renin levels e.g., below the activity of 0.1 ng/mL/h or a direct value (concentration) below 1 ng/L. The ARR may be elevated even when aldosterone is also low e.g., 40 ng/L or 110 pmol/L /3/.

A confirmatory test should confirm or exclude the result of the ARR.

Confirmatory tests 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 limit, plus aldosterone > 200 ng/L (550 pmol/L) the Endocrine Society /3/ suggests that there may be no need for further confirmation.

For confirmatory laboratory test refer to Tab. 31.4-7 – Primary aldosteronism: confirmatory tests.

Subtype tests for differentiation of primary aldosteronism

• Adrenal computed tomography
• Establishment or exclusion of unilateral PA
• Bilateral adrenal venous sampling.

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 > 300 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 /3/.

The protocol for adrenal venous sampling (AVS) test is shown in Tab. 31.4-8 – Adrenal venous sampling test.

An algorithm for the workup of hypertension in the case of a suspected mineralocorticoid related etiology is shown in Fig. 31.4-1 – Diagnostic approach for suspected primary aldosteronism and possible diagnoses.

31.4.5.2 Secondary aldosteronism

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 /5/. 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 /5/. 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 /5/.

Findings in different forms of secondary aldosteronism are shown in Tab. 31.4-9 – Secondary hyperaldosteronism, mineralocorticoid excess and hypertension.

31.4.5.3 Pseudo hyperaldosteronism

Pseudo hyperaldosteronism is a condition of mineralocorticoid excess and normal aldosterone characterized by hypertension, hypokalemic alkalosis, and suppressed renin /6/. Some of the cases include Liddle’s syndrome, Cushing’s syndrome, congenital adrenal hyperplasia, and 11β-hydroxysteroid dehydrogenase deficiency.

Refer to Tab. 31.3-3 – Disorders associated with low renin hypertension.

31.4.5.4 Primary hypoaldosteronism

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 /6/. 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.

31.4.5.5 Secondary hypoaldosteronism

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.

Refer to Tab. 31.3-3 – Disorders associated with low renin hypertension.

31.4.5.6 Pseudo hypoaldosteronism

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.

Disorders

• 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 (Fig. 8.8-2 – Reabsorption of solutes in the collecting duct).
• In pseudo hypoaldosteronism type II there is thought to be increased chloride reabsorption instead of K⁺ efflux /7/. For further information refer to Section 8.8 – Renal electrolyte excretion.

Clinical findings

Manifestation in childhood, failure to thrive, and hypotension.

Laboratory findings

High renal sodium chloride excretion, hyperkalemia, dramatically increased aldosterone and renin.

31.4.6 Comments and problems

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 /3/.

The available radioimmunoassays for aldosterone overestimate levels by 50–100% in the range below 200 pmol/l /3/.

Standardization

There is no international standard protocol.

31.4.7 Biochemistry

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 (Fig. 31.3-2 – Synthesis of aldosterone).

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 /5/. Aldosterone is present in serum in the isomeric forms through formation of a cyclic hemiacetal bond between the positions C11 and C18.

Refer to Fig. 31.4-2 – Isomeric forms of aldosterone in plasma.

The enzyme P450_aldo (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 (Fig. 31.4-3 – Conjugation of aldosterone to aldosterone 18-glucuronide and tetrahydroaldosterone 3-glucuronide).

References

1. Dick SM, Queiroz M, Bernardi BL, Dall´Agnol A, Brondani LA Silveiro SP. Update in diagnosis and management of primary aldosteronism. Clin Chem Lab Med 2018; 56: 360–72.

2. Taylor PJ, Cooper DP, Gordon RD, Stowasser M. Measurement of aldosterone in human plasma by semiautomated HPLC-tandem mass spectrometry. Clin Chem 2009; 55: 1155–62.

3. 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.

4. Gupta V. Mineralocorticoid hypertension. Indian J Endocrinol 2011; 15 (Suppl 4): S298–S312.

5. Corry DB Tuck ML. Secondary aldosteronism. Endocrinol Metab Clin North Am 1995; 24: 511–27.

6. Hauffa BP, Solyom J, Glaz E, Shackleton CHL, Wambach G, Vecsei P, Stolecke H, Homoki J. Severe hypoaldosteronism due to corticosterone methyl oxidase type II deficiency in two boys: metabolic and gas chromatography-mass spectrometry studies. Eur J Pediatr 1991; 150: 149–53.

7. Sayer JA, Pearce SAS. Diagnosis and clinical biochemistry of inherited tubulopathies. Ann Clin Biochem 2001; 38: 459–70.

8. Hubl W. Aldosterone. In: Bergmeyer HU. Methods of enzymatic analysis, Vol VIII. Weinheim: VCH, 1985: 256–66.

9. Taylor PJ, Cooper DP, Gordon RD, Stowasser M. Measurement of aldosterone in human plasma by semiautomated HPLC-tandem mass spectrometry. Clin Chem 2009; 55: 1155–62.

10. Demers LM, Whitley RJ. Function of the adrenal cortex. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical chemistry. Philadelphia; Saunders 1998: 1561.

11. 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.

12. Abdelhamid S, Thomas L, Neusel U, Lorenz H, Rückel A, Bönhof JA, Müller-Lobeck H. Studies on the diagnostic value of aldosterone precursor 18-hydroxycorticosterone in the diagnosis and differential diagnosis of primary hyperaldosteronism. Lab Med 1994; 18: 275–84.

13. Kohl KH, Vecsei P, Abdelhamid S. Radioimmunoassay of tetrahydroaldosterone (TH-aldo) in human urine. Acta Endocrinol 1978; 87: 596–608.

14. Miller WL, Auchus RJ. The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr Rev 2011; 32: 81–151.

15. Fuller PJ. Adrenal diagnostics: an endocrinologist’s perspective focused on hyperaldosteronism. Clin Biochem Rev 2013; 34: 111–5.

16. 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.

17. 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. Jin S, Wada N, Takahashi Y, Hui SP, Sakurai T, Fuda H, et al. Quantification of urinary 18-hydroxycortisol using LC-MS/MS. Ann Clin Biochem 2013; 50: 450–6.

19. 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.

20. Wilson M, Morganti AA, Zervoudakis I, et al. Blood pressure, the renin-aldosterone system and sex steroids throughout normal pregnancy. Am J Med 1980; 68: 97.

21. Nicod J, Bruhin D, Auer L, et al. A biallelic gene polymorphism of CYP11B2 predicts increased aldosterone to renin ratio in selected hypertensive patients. J Clin Endocrinol Metab 2003; 88: 2495–500.

22. Dworkin LD, Cooper CJ. Renal-artery stenosis. N Engl J Med 2009; 361: 1972–8.

23. Amirlak I, Dawson KP. Bartter syndrome: an overview. Q J Med 2000; 93: 207–15.

24. Kebebew E. Adrenal incidentaloma. N Engl J Med 2021; 384 (16): 1542–51.

25. Kebebew E. Adrenal incidentaloma. N Engl J Med 2021; 385 (8): 768.

31.5 18-Hydroxycortisol

18-hydroxycortisol (18-OHF) is known as hybrid steroid, because it has structural characteristics of both cortisol and aldosterone (Fig. 31.3-2 – Synthesis of 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 /1/.

31.5.1 Indication

Diagnosis of primary aldosteronism.

31.5.2 Method of determination

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) /2/.

31.5.3 Specimen

Collect urine over 24 hours, keep sample frozen until it is analyzed.

31.5.4 Reference interval

Urine: 28–485 nmol/L /2/.

31.5.5 Clinical significance

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/.

References

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.

2. Jin S, Wada N, Takahashi Y, Hui SP, Sakurai T, Fuda H, et al. Quantification of urinary 18-hydroxycortisol using LC-MS/MS. Ann Clin Biochem 2013; 50: 450–6.

31.6 18-Hydroxycorticosterone

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 (Fig. 31.3-2 – Synthesis of aldosterone). Although small amounts may be produced by the 11β-hydroxylase, 18-OHB has only low affinity for the mineralocorticoid receptor.

31.6.1 Indication

Differentiation of aldosterone producing adenoma and bilateral adrenal hyperplasia.

31.6.2 Method of determination

Radioimmunoassay /1/, HPLC or gas chromatography-tandem mass spectrometry.

31.6.3 Specimen

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.

31.6.4 Reference interval

• Plasma: 115–550 ng/L (317–1.418 pmol/L)
• Urine: 1.5–6.5 μg/24 h /2/.

31.6.5 Clinical significance

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 /3, 4/.

References

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.

3. Biglieri EG, Schambelan M. The significance of elevated levels of plasma 18-hydroxycorticosterone in patients with primary aldosteronism. J Clin Endocrinol Metab 1979; 49: 87–91.

4. Miller WL, Auchus RJ. The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr Rev 2011; 32: 81–151.