Aldosterone is the most important mineralocorticoid and it is produced by the zona glomerulosa of the adrenal cortex. Release of aldosterone is mainly controlled by the renin-angiotensin system (RAS) and by potassium levels in the blood.

Aldosterone levels may be useful in the differential diagnosis of primary and secondary Addison’s and to assess the degree of destruction of the mineralocorticoid areas of the adrenals. An ACTH stimulation test should be performed and cortisol and aldosterone measured in both samples.

Hyperaldosteronism may be primary or secondary. Primary hyperaldosteronism (Conn’s syndrome) is rare in the dog and cat and is usually caused by a small, solitary, aldosterone-producing adenoma of the adrenal cortex. Secondary hyperaldosteronism is more common and is caused by continued stimulation of the RAS.

Persistently low serum potassium levels <3.0 mmol/L (Hypokalaemia) may be due to hyperaldosteronism (aldosteronoma). Because exogenous glucocorticoids will have little or no effect on the RAS, aldosterone production is not affected by them. By measuring aldosterone response to ACTH instead of (or in addition to) cortisol we can confirm primary hypoadrenocorticism in dogs that have already had several or more days of glucocorticoid therapy. Measuring only cortisol in the same circumstance risks a false positive diagnosis of hypoadrenocorticism because of the negative feedback effects of exogenous steroid therapy (including topical skin, eye and ear preparations) on endogenous glucocorticoid production. Aldosterone levels can be measured in the same samples as cortisol and the measurement of aldosterone pre- and post- ACTH stimulation provides the most information on the mineralocorticoid producing abilities of the adrenals (see Hypoadrenocorticism: Diagnosis). The principle uses of aldosterone analysis therefore are:

  • Confirming primary hypoadrenocorticism in an animal which has already been receiving therapy prior to diagnosis
  • Differentiating primary from secondary hypoadrenocorticism
  • Investigating hypokalaemia (considering aldosteronoma)
  • Assessing renin-angiotensin dependency in cardiac patients prior to ACE inhibition
  • Assessing mineralocorticoid effect of anti-adrenal therapy (e.g. Lysodren, Vetoryl)

Example Aldosterone and Cortisol Interpretation:
Cortisol / Aldosterone Table

* In the case of trilostane (Vetoryl®) therapy, the ACTH response test results reflect the status of adrenal steroidogenesis only at the time of the test. Because of the temporary and reversible action of trilostane, the response may be greater later in the day, i.e., unlike in the case of mitotane, a flat aldosterone response does not necessarily mean that the trilostane patient is absolutely unable to make aldosterone.