Apparently normal pituitary-adrenal suppressibility in Cushing's syndrome: dexamethasone metabolism and plasma levels.

In this study, we evaluated the responses of the pituitary-adrenal axis to the suppressive action of dexamethasone by relating plasma dexamethasone (Dex) and cortisol (F) levels in normal subjects and patients with Cushing's syndrome. Various doses of Dex were ingested at midnight as a single dose or at 6-hour intervals for 2 days beginning at 8 A.M. A nomogram was prepared by plotting the 8 A.M. plasma F concentration as a function of the plasma Dex value. Two patients with Cushing's syndrome had normal responses to Dex suppression as assessed by conventional tests. With the nomogram, we found that all patients with Cushing's syndrome, including these two, exhibited abnormally high plasma F values at some time during our evaluation. A decreased metabolic clearance rate of Dex appears to explain why one showed normal suppression. Another patient with bilateral adrenal hyperplasia showed sequentially over 13 months abnormal, normal, and abnormal suppression, which could not be attributed to an abnormality of Dex metabolism. We conclude that relating plasma Dex and F provides the investigator with a reliable index for evaluating resistance of the pituitary-adrenal axis to the suppressive effect of Dex.