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Test Overview
The overnight dexamethasone suppression test involves taking a dose of a corticosteroid medication called dexamethasone to see how it affects the level of a hormone called cortisol.
Normally, the pituitary gland's production of adrenocorticotropic hormone (ACTH) decreases after taking a dose of dexamethasone. A decrease in ACTH levels triggers the adrenal glands to decrease production of cortisol. (See an illustration of the pituitary gland or the adrenal glands.)
After taking a dose of dexamethasone, cortisol levels often remain 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 decreasing after taking a dose of dexamethasone.
This test is done on a sample of blood from a vein. 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.
