Chemistry
Adrenal related
Aldosterone

Author: Renu Virk, M.D. (see Authors page)

Revised: 2 February 2016, last major update January 2010

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Aldosterone [title] adrenal
Definition / General
  • 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
Pathophysiology
  • 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)
Clinical Features
Function:
  • 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)
Laboratory
Methodology to measure levels:
  • 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:
  • 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:
  • To diagnose hyperaldosteronism
  • To differentiate aldosteronoma from idiopathic hyperaldosteronism

Limitations:
  • 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:
  • 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:
  • High values: primary hyperaldosteronism, aldosterone secreting adrenal tumor, idiopathic adrenal hyperaldosteronism
  • Low values: adrenal insufficiency, congenital adrenal hyperplasia due to 11-beta-hydroxylase deficiency