Low-dose dexamethasone suppression of urinary free cortisol in the differential diagnosis between Cushing's syndrome and obesity.

Urinary free cortisol (UFC) was measured in the basal state and on 2nd day of the low-dose dexamethasone (2 mg daily for 2 days) suppression test (LDDT) in 182 subjects. At first examination the results were in good agreement with the definite diagnosis in 21 of 27 patients with Cushing's syndrome (CS) and in 145 out of 155 obese subjects. In two obese subjects the basal UFC was increased and the response to the test insufficient. At re-examination their laboratory findings became normal. In 14 subjects the basal UFC was increased but the response to the test was normal. With nine of them, repeated examinations were performed over a period of 1-7 years. Six subjects progressed into obvious CS, while in two hypercortisolism spontaneously regressed and in one it remained unchanged. The combination of elevated basal UFC with a normal suppressibility during the LDDT is considered a borderline adrenocortical impairment.