Cushing's syndrome is caused by contant, high levels of cortisol, a key glucocorticoid hormone. The growth of a tumor on one of the two adrenal glands causes about 15% of all cases of Cushing's syndrome. Adrenal tumors secrete cortisol and may be benign (noncancerous) or malignant (cancerous). Benign tumors that may cause Cushing's syndrome include adrenal adenomas or micronodular hyperplasia. Adrenal carcinomas are malignant tumors that may cause Cushing's syndrome. Other causes of Cushing's syndrome are secretion of ACTH by a pituitary tumor, by another organ (ectopic Cushing's syndrome), and regular use of corticosteroids such as prednisone, commonly used to treat chronic conditions such as rheumatoid arthritis and asthma. Although they may develop in anyone at any age, adrenal tumors are more common in adults -- 70% of all adrenal tumors occur in adults. They are found in women more often than men and are relatively rare. It occurs in 6 out of every 1,000,000 people.
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