Overnight Dexamethasone Suppression Test

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Test Overview

The overnight dexamethasone suppression test checks to see how taking a corticosteroid medicine (called dexamethasone) changes the levels of the hormone cortisol in the blood. This test checks for a condition in which large amounts of cortisol are produced by the adrenal glands (Cushing's syndrome).

Normally, when the pituitary glands make less adrenocorticotropic hormone (ACTH), the adrenal glands make less cortisol. (See a picture of the pituitary gland or the adrenal glands .) Dexamethasone, which is like cortisol, decreases the amount of ACTH released by the pituitary gland, which in turn decreases the amount of cortisol released by the adrenal glands.

After taking a dose of dexamethasone, cortisol levels often stay abnormally high in people who have Cushing's syndrome. Occasionally other conditions (such as major depression, alcoholism, stress, obesity, kidney failure, pregnancy, or uncontrolled diabetes) can keep cortisol levels from going down after taking a dose of dexamethasone.

The night before the blood test, you will take a pill containing dexamethasone. The next morning, the cortisol level in your blood will be measured. If your cortisol level remains high, Cushing's syndrome may be the cause.

Occasionally, an ACTH test may be done at the same time as the cortisol test.

Last updated:	June 26, 2008
Author:	Bets Davis, MFA
Reviewed By:	Caroline S. Rhoads, MD - Internal Medicine, David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
Editors:	Susan Van Houten, RN, BSN, MBA, Tracy Landauer