Dexamethasone suppression test (overnight)

Last updated: Monday, 06, August, 2007

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Item Process
Specimen

The test is done as an outpatient screen for Cushing’s syndrome.

Method

See Cortisol - plasma or serum.

Protocol

Fasting not required.

Day 1
0900 hours: 5 mL blood for cortisol (optional baseline)
2300 hours: give 1 mg dexamethasone orally.

Day 2
0900 hours: 5 mL blood for cortisol.

Reference Interval
Day 1
0900 hours cortisol: 200-650 nmol/L

Day 2
0900 hours cortisol: <50 nmol/L.

Application

Initial investigation when Cushing’s syndrome is suspected.

Interpretation

Day 2 cortisol should suppress to less than 50 nmol/L and should be less than 50% of the baseline level.

A normal result excludes Cushing’s syndrome.

Failure to suppress indicates that cortisol production is not responsive to normal feedback inhibitory mechanisms.

In addition to Cushing’s syndrome, failure to suppress may be seen in endogenous depression, stress, obesity, chronic alcoholism and with certain drugs eg, phenytoin.

Failure to take the dexamethasone and abnormalities of dexamethasone absorption or metabolism should also be considered if there is failure to suppress. These can be detected by an assay of dexamethasone.

A modified test has been used in the diagnosis of endogenous depression, but is not recommended.

Reference

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