metyrapone testmedical dictionary


<endocrinology> This test measures the ability of the pituitary gland to secrete ACTH in response to a low serum cortisol level. Under normal conditions a low serum cortisol level should stimulate the pituitary to produce ACTH, which stimulates the adrenal glands to produce more cortisol. This test is performed by injecting metyrapone into the bloodstream and then measuring cortisol levels over a period of time. Metyrapone is a substance which inhibits an enzyme (steroid 11-beta hydroxylase) involved in the synthesis of cortisol. This enzyme normally converts 11-deoxycortisol to cortisol. When the enzyme is inhibited, 11-deoxycortisol levels will increase in the blood and urine and cortisol levels will go down. This test is useful in differentiating the different forms of Cushing's syndrome: adrenal hyperplasia versus primary adrenal tumour. Those with an adrenal tumour (or carcinoma) generally have no response to metyrapone injection, since the dose is insufficient to inhibit cortisol synthesis.

(27 Sep 1997)

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