Metyrapone inhibits 11-beta-hydroxylase, resulting in blockage of the conversion of 11-deoxycortisol (compound S) to cortisol. As cortisol serum levels fall, normal persons show a release of ACTH, which results in a further increase in serum 11-deoxycortisol levels. The test is used to differentiate ACTH-secreting pituitary tumor from adrenal tumors or from paraneoplastic syndromes due to ectopic ACTH secreting tumors. It also can be used to distinguish normal persons from patients with secondary adrenal insufficiency, but it will not differentiate primary from secondary adrenal insufficiency. A normal hypothalamic-pituitary axis is needed for a normal response.