cortisol (kôrˈtĭsôlˌ) [key] or hydrocortisone, steroid hormone that in humans is the major circulating hormone of the cortex, or outer layer, of the adrenal gland. Like cortisone, cortisol is classed as a glucocorticoid; it stimulates liver glycogen formation while it decreases the rate of glucose utilization in body cells. A main effect of cortisol is to reduce the reserves of protein in all body cells except cells of the liver and gastrointestinal tract. It also makes fatty acids available for metabolic use. Cortisol is synthesized and secreted by the adrenal cortex in response to the stimulating substance adrenocorticotropic hormone (ACTH). In turn, cortisol is the major regulator of ACTH production in the pituitary gland; it acts by negative feedback inhibition, i.e., a rise in the level of cortisol in the blood inhibits ACTH secretion by the pituitary. Cortisol, usually referred to as hydrocortisone when used medicinally, is more potent than cortisone with respect to metabolic and anti-inflammatory effects. See also corticosteroid drug; steroids.