Dexamethasone suppression test-long
The test is done as an inpatient procedure.
See Cortisol urine, Cortisol and ACTH.
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.
24 h urine collections for cortisol
Collection at 0900 hours of 5 mL blood in EDTA tube for cortisol and ACTH.
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Suppression is defined as a reduction of cortisol to < 50% of the basal value.
Definitive diagnosis of hypercorisolaemic syndromes in a patient with an abnormal (overnight) dexamethasone suppression test.
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.
Orth DN et al. In: Wilson JD and Foster DW eds. Williams Textbook of Endocrinology. 8th ed. WB Saunders, 1992.