The suppressibility of morning plasma 17-hydroxycorticosteroid (17-OHCS) levels by 1 mg dexamethasone given by mouth at 11 pm the preceding night was used as a screening test for Cushing's syndrome. Plasma 17-OHCS in 16 normal subjects, 20 with simple obesity and 10 with diseases other than Cushing's syndrome were suppressed to levels below 5μg/100 ml. In contrast, in 17 patients with Cushing's syndrome, the lowest observed value was 13μg/100 ml and no false-negative results were obtained. Two obese, hirsute women proved to be partial responders only, as their plasma levels were not suppressed below 10.5μg/100 ml. Mild abnormalities of adrenal cortical secretory activity may be present in such cases. Therefore, a single morning 17-OHCS value of less than 5μg/100 ml plasma should exclude Cushing's syndrome.
Pavlatos FC, Smilo RP, Forsham PH. A Rapid Screening Test for Cushing's Syndrome. JAMA. 1965;193(9):720–723. doi:10.1001/jama.1965.03090090026005