The corticotropin-releasing hormone (CRH) stimulation test helps determine whether a pituitary tumor may be causing Cushing's syndrome. It is usually done with inferior petrosal sinus sampling (IPSS). In this test, a small tube (catheter) is used to collect samples from blood vessels near the brain. If these blood samples show high levels of adrenocorticotropic hormone (ACTH), it usually indicates that the pituitary gland is the source of excess ACTH.

Alternatively, you may be given an intravenous (IV) injection of corticotropin-releasing hormone. If blood levels of ACTH and cortisol rise excessively, a pituitary tumor is probably causing Cushing's syndrome. If blood levels of ACTH and cortisol do not rise, your doctor may then look for an adrenal tumor or a cancerous tumor elsewhere in your body.