Overnight dexamethasone suppression test: normal responses and the diagnosis of Cushing's syndrome.

Serum cortisol levels were measured the morning after the administration of 1 mg of dexamethasone. Only 5 of 190 subjects had serum cortisol levels greater than 2 micrograms/dL. Thus, the normal value after dexamethasone suppression is less than 2 micrograms/dL rather than less than 5 micrograms/dL as has generally been accepted. The distinction is important because some individuals with Cushing's syndrome partially suppress their cortisol levels to less than 5 but more than 2 micrograms/dL during the test procedure. Thus, the use of 5 micrograms/dL as the normal value may lead to an unnecessary delay in diagnosis.