A small tumor in the pituitary gland (the master gland that lies at the base of the brain), can secrete too much ACTH (adrenocorticotropic hormone), a hormone that controls the activity of the adrenal glands. The ACTH signal travels through the bloodstream and causes the normal adrenal glands to make too much cortisol. This can lead to hyperactivity of both adrenal glands called bilateral adrenal hyperplasia. Pituitary tumors are the most common cause of endogenous Cushing's syndrome, making up about 70% of cases. When the pituitary is the source of ACTH, it is actually called Cushing's disease rather than Cushing's syndrome.