Aldosterone is a steroid hormone produced by the outer layer (zona glomerulosa) of the adrenal cortex. It is the primary mineralocorticoid in humans and plays a central role in regulating sodium and potassium balance, and consequently blood pressure and fluid volume. Aldosterone acts on the kidneys to increase sodium reabsorption and potassium excretion — retaining water and raising blood pressure.

Aldosterone is tested most often alongside renin (the aldosterone-to-renin ratio) to investigate primary hyperaldosteronism (Conn's syndrome) — a common but underdiagnosed cause of secondary hypertension. It is also tested in people with unexplained low potassium, abnormal blood pressure, or symptoms of adrenal disorders. Low aldosterone is associated with adrenal insufficiency (Addison's disease).