Exogenous vs. endogenous
Hormones that come from outside the body are called exogenous; hormones that come from within the body are called endogenous.The most common cause of Cushing's syndrome is ''exogenous'' administration of glucocorticoids prescribed by a health care practitioner to treat other diseases (called ''iatrogenic Cushing's syndrome''). This can be an effect of steroid treatment of a variety of disorders such as asthma and rheumatoid arthritis, or in immunosuppression after an organ transplant. Administration of synthetic ACTH is also possible, but ACTH is less often prescribed due to cost and lesser utility. Although rare, Cushing's syndrome can also be due to the use of medroxyprogesterone
''Endogenous'' Cushing's syndrome results from some derangement of the body's own system of secreting cortisol. Normally, ACTH is released from the pituitary gland when necessary to stimulate the release of cortisol from the adrenal glands.
* In ''pituitary Cushing's'', a benign pituitary adenoma secretes ACTH. This is also known as ''Cushing's disease'' and is responsible for 65% of endogenous Cushing's syndrome.
* In ''adrenal Cushing's'', excess cortisol is produced by adrenal gland tumors, hyperplastic adrenal glands, or adrenal glands with nodular adrenal hyperplasia.
* Finally, tumors outside the normal pituitary-adrenal system can produce ACTH that affects the adrenal glands. This final etiology is called ''ectopic'' or ''paraneoplastic Cushing's syndrome'' and is seen in diseases like small cell lung cancer.
Pseudo-cushing's syndrome
Elevated levels of total cortisol can also be due to estrogen found in oral contraceptive pills that contain a mixture of estrogen and progesterone. Estrogen can cause an increase of cortisol binding globulin and thereby cause the total cortisol level to be elevated. However, the total free cortisol, which is the active hormone in the body, as measured by a 24 hour urine collection for urinary free cortisol, is normal.Adapted from the Wikipedia article Cushing's syndrome, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki
