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. This is more cumbersome than the single dose metyrapone test, but results in more complete enzyme inhibition.

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