Dexamethasone suppression test-long

Specimen:

The test is done as an inpatient procedure.

Method:

See Cortisol urine, Cortisol and ACTH.

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 h 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 of hypercorisolaemic syndromes 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.