Serum Cortisol Levels in Cushing's Syndrome After Low- and High-Dose Dexamethasone Suppression

Abstract

To standardize the cutoff points of serum cortisol values in the evaluation of Cushing''s syndrome during a low- and high-dose dexamethasone suppression test, daily serum cortisol measurements (0800 h and 1600 h) and urinary 17-hydroxycorticosteroids were compared (Study A). Forty-seven subjects were studied (11 normal subjects, 15 patients with Cushing''s disease, 5 patients with adrenal adenoma, and 16 subjects with suspected Cushing''s syndrome). A serum cortisol measurement at 1600 h of more than 5 .mu.g/dl on low-dose dexamethasone suppression and more than 10 .mu.g/dl on high dose dexamethasone were ascertained to be nonsuppressed values. A baseline dehydroepiandrosterone-sulfate value less than 0.4 .mu.g/ml indicated patients with an adrenocorticol adenoma. Study B was a prospective study of 17 patients in which no urine samples were collected. Serum cortisol levels, obtained at 1600 h on the 2nd day of low- and high-dose dexamethasone, accurately allowed a differential diagnosis of suspected Cushing''s syndrome. Serum cortisol measurements can replace the urinary 17-hydroxycorticoid measurements in a cost-effective manner without a decrease in the degree of accuracy.