Cushing's syndrome is the result of excess glucocorticoid hormones secreted over a long period of time. About 15% of Cushing's syndrome cases are due to ectopic Cushing's syndrome. Ectopic Cushing's syndrome is caused by tumors that secrete adrenocorticotropic hormone (ACTH). Other causes of Cushing's syndrome are oversecretion of ACTH by the pituitary gland, a tumor of the adrenal gland, or long-term administration of corticosteroid drugs commonly used to treat conditions such as rheumatoid arthritis and asthma. Small cell tumors of the lung, tumors of the thymus gland, islet cell tumors of the pancreas, and medullary carcinoma of the thyroid are tumors that can secrete ACTH, although this occurs rarely.
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