Dexamethasone tests

Dexamethasone is a potent synthetic glucocroticoid which in a normal person produces feedback suppression of CRH and ACTH and consequently, serum cortisol.

Low dose suppression tests are used in screening and as a confirmatory test of Cushing's syndrome:

• oral dexamethasone 0.5 mg qds for 48 hr - start 0900
• measure serum cortisol at 0900 hrs, basally and at 48 hr
• serum cortisol level should fall to less than 50 nmol per litre by radioimmunoassay in normal patients. Non-suppression suggests Cushings syndrome

High dose tests help to differentiate the cause:

• oral dexamethasone 2.0 mg qds for 48 hr - start 0900
• measure serum cortisol at 0900 hrs, basally and at 48 hr
• fall in cortisol to less than 50% of the basal value at 48 hrs suggests pituitary Cushing's disease. Ectopic ACTH secretion and adrenal tumours are not suppressed

Some patients with psychiatric disorders such as depression, mania and chronic schizophrenia fail to show cortisol suppression following dexamethasone administration.