Serum cortisol levels in Cushing's syndrome after low- and high-dose dexamethasone suppression.

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 hours and 1600 hours) and urinary 17-hydroxycorticosteroids were compared (Study A). Forty-seven subjects were studied (11 normal subjects, 15 patients with Cushing's disease, five patients with adrenal adenoma, and 16 subjects with suspected Cushing's syndrome). A serum cortisol measurement at 1600 hours of more than 5 micrograms/dL on low-dose dexamethasone suppression and more than 10 micrograms/dL on high-dose dexamethasone were ascertained to be nonsuppressed values. A baseline dehydroepiandrosterone-sulfate value less than 0.4 microgram/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 in 1600 hours on the second 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.