Aid in diagnosing disorders of the hypothalamic-pituitary system including ACTH-secreting tumors. In adrenal insufficiency, a concentration of ACTH greater than 100 pg/mL is diagnostic of primary adrenal insufficiency; whereas, patients with pituitary causes of adrenal insufficiency have ACTH concentrations in the normal range. In hypercortisolism (plasma cortisol greater than 15 ug/dL), an ACTH concentration of less than 5 pg/mL indicates that the cortisol secretion is ACTH-independent, usually an adrenal tumor or hyperplasia. An ACTH concentration of greater than 15 pg/mL indicates ACTH-dependent hypercortisolism, usually of pituitary origin, or occasionally ectopic ACTH secretion.