Adrenocorticotropic Hormone (ACTH): an essential pituitary hormone which causes the adrenal glands to release cortisol, the stress hormone. An ACTH-secreting pituitary adenoma is the cause of Cushing’s disease which is characterized by excess cortisol production. An ACTH deficiency may result in low cortisol levels and adrenal insufficiency which can be life threatening. Learn more.