Cushing's syndrome occurs when the body's tissues are exposed to high levels of cortisol for too long. Many people develop Cushing's syndrome because they take glucocorticoids-steroid hormones that are chemically similar to naturally produced cortisol-such as prednisone for asthma, rheumatoid arthritis (RA), lupus, and other inflammatory diseases. Glucocorticoids are also used to suppress the immune system after transplantation to keep the body from rejecting the new organ or tissue.
Other people develop Cushing's syndrome because their bodies produce too much cortisol. Normally, the production of cortisol follows a precise chain of events. First, the hypothalamus, a part of the brain about the size of a small sugar cube, sends corticotropin-releasing hormone (CRH) to the pituitary gland. CRH causes the pituitary to secrete adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands. When the adrenals, which are located just above the kidneys, receive ACTH, they respond by releasing cortisol into the bloodstream.
This answer is based on source information from National Institute of Diabetes and Digestive and Kidney Diseases.