Dexamethasone suppression test (long)

Last updated: Thursday, 15, April, 2004

MBS Online Search Page: Click here for details

Item Process
Specimen

The test is done as an inpatient procedure.

Method

See Cortisol - urine, Cortisol - plasma or serum and Adrenocorticotrophic hormone.

Protocol

Two days basal (no dexamethasone); then dexamethasone at low dose (0.5 mg qid) for two days; and high dose (2 mg qid) for a further two days.
Specimens are collected daily for 7 days.

  1. 24 hour urine collections for cortisol
  2. Collection at 0900 hours of 5 mL blood in EDTA tube for cortisol and ACTH.
Reference Interval

Suppression is defined as a reduction of cortisol to <50% of the basal value.

Application

Definitive diagnosis in a patient with an abnormal (overnight) dexamethasone suppression test.

Interpretation

Suppression of cortisol levels on dexamethasone 2 mg/day excludes Cushing’s syndrome.

Failure to suppress cortisol after 2 mg/day, with suppression on 8 mg/day, indicates pituitary Cushing’s syndrome (Cushing’s disease).

Failure to suppress cortisol on 8 mg/day indicates adrenal neoplasm or ectopic ACTH syndrome.

In Cushing’s disease, ACTH is suppressed by high dose dexamethasone; in the ectopic ACTH syndrome, ACTH is not suppressed; in adrenal neoplasia, ACTH levels are low in the baseline specimen.

Reference

Orth DN et al. In: Wilson JD and Foster DW eds. Williams Textbook of Endocrinology. 8th ed. WB Saunders 1992.