Metyrapone blocks 11β-hydroxylase activity in the adrenal cortex, this being the last step in Cortisol synthesis. The consequent decrease in Cortisol normally stimulates the feedback mechanism to the hypothalamus and anterior pituitary gland, resulting in increased production of ACTH. This, in turn, stimulates production of steroids up to 11-deoxycortisol (compound S), which may be measured directly in the serum or indirectly in the urine as 17-oxogenic steroids (17- OGS). In hypothalamic/pituitary disease there is failure of this response.