Hormone, aldosterone: A hormone made by the outer portion (cortex) of the adrenal gland that regulates the balance of salt and water in the body. Pronounced AL·do·ste·rone.
Aldosterone is secreted in response to low salt levels. Aldosterone then activates the MR (for mineralocorticoid receptor) gene. (Aldosterone is classified as a mineralocorticoid hormone.) The product of the MR gene, in turn, stimulates the kidney to reabsorb and retain salt, thereby retaining water.