Cushing's Syndrome – Serum Cortisol Test	

The single most reliable test for pituitary-dependent disease is the high-dose dexamethasone suppression test (2mg given exactly every  six hours for two days), during which time patients with Cushing's disease normally show at least fifty per cent suppression of baseline serum or urinary cortisol levels. Cortisol levels in patients with ectopic sources or adrenal tumours are rarely suppressed by dexamethasone, although exceptions do occur (in approximately ten per cent of cases).