a polypeptide hormone formed in the pituitary gland that regulates the activity of the outer region (cortex) of the adrenal glands. In mammals the action of ACTH is limited to those areas of the adrenal cortex in which the glucocorticoid hormones—cortisol and corticosterone (see corticoid)—are formed. The secretion of ACTH by the pituitary is itself regulated by another polypeptide, a so-called corticotropin-releasing hormone (CRH), that is discharged from the hypothalamus in response to impulses transmitted by the nervous system.