In normal individuals, serum cortisol levels should be less than 100nmol/l at midnight if the patient was asleep just before the blood was taken; after forty-eight hours of dexamethasone 2mg/d, 0900 serum cortisol levels should be less than 50nmol/l.
Single-spot measurement of serum cortisol levels is of little diagnostic value. Twenty-four hour urinary-free cortisol excretion is a useful screening investigation for Cushing's syndrome.
