Clinical Chemistry - Adrenal Tests

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Clinical Chemistry

Adrenal Tests

Aldosterone

Author: Renu Virk, M.D., University of Massachusetts Memorial Hospital (see Authors page)

Revised: 21 September 2012, last major update January 2010

Definition

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● Hormone that increases the reabsorption of sodium and water and the release (secretion) of potassium in the kidneys; this increases blood volume and, therefore, increases blood pressure

Physiology

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● Synthesized and released into bloodstream by zona glomerulosa of the adrenal gland

● Average daily production is 0.1 to 0.7 μml (50 to 250 μg) with normal salt intake.

● Inactivated in liver by conjugation with glucuronic acid or sulfuric acid

● Secretion is regulated by renin-angiotensin system (main regulator), serum potassium concentration and ACTH levels

●.Decrease in effective circulating blood volume increases aldosterone via renin-angiotensin axis

● Increase in serum potassium increases aldosterone levels

● ACTH plays a minor role in regulation of aldosterone synthesis

● Aldosterone secretion in healthy subjects is increased by low-salt diet (< 2 gm/day), stress, upright posture and diuretic therapy, and is decreased by high salt diet or lying supine (lying down with face up)

Diagrams of feedback mechanisms

Function

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● Major regulator of effective circulating blood volume

● Increases renal reabsorption of sodium and water to maintain blood pressure and tonicity

● Acts through mineralocorticoid receptors in renal cortical collecting ducts

● Also increases urinary excretion of potassium and hydrogen ions

● Recently discovered to have an effect on endothelium and coronary relaxation (Pharmacol Rep 2009;61:58)

Methodology to measure levels

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● Radioimmunoassay (RIA) is most common method

● Extraction of aldosterone from plasma or urine followed by chromatography is done if levels of deoxycorticosterone or cortisterone are high, as antialdosterone antibodies may cross-react with them

● Note: It is important to control the time of sampling, posture and dietary sodium while collecting the sample; ideally sample should be obtained at 8 am after 4 hours of lying down (recumbency) and on unrestricted salt diet

Specimen collection

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● Urine or serum

● Same laboratory methods can be used for both specimens, except that urine is acid hydrolyzed before assay

● 24 hr urine sample is ideal specimen - urine is collected with boric acid as preservative; urine is acidified by adding acetic acid; urine aldosterone is measured in the form of 18-glucuronide by radioimmunoassay

Indications for serum or urine aldosterone testing

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● To diagnose hyperaldosteronism

● To differentiate aldosteronoma from idiopathic hyperaldosteronism

Limitations

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● Isolated serum aldosterone measurement has little clinical value; serum aldosterone levels should be interpreted in conjunction with plasma renin activity and serum potassium concentration

● Note: Urine aldosterone measurement is more useful clinically than random plasma aldosterone levels

Reference ranges

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● Plasma aldosterone - supine position and normal sodium diet: 209 ng/dl (55-250 pmol/L)

● Plasma aldosterone - upright position (standing/seated for at least 2 hr) and normal sodium diet: 2-5x supine value

● Urine aldosterone: 5-20 μmg/24 hr (14-56 nmol/24 hr)

● Conversion factor: multiply by 0.0277 to convert from ng/dl to nmol/L

Interpretation

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● High values: primary hyperaldosteronism, aldosterone secreting adrenal tumor, idiopathic adrenal hyperaldosteronism

● Low values: adrenal insufficiency, congenital adrenal hyperplasia due to 11-beta-hydroxylase deficiency

Additional references

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● eMedicine, Wikipedia, Endotext

End of Clinical Chemistry > Adrenal Tests > Aldosterone

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