Overnight Dexamethasone Suppression Test

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

Normally, when the Reference pituitary gland Opens New Window Reference Opens New Window make less Reference adrenocorticotropic hormone (ACTH) Opens New Window, the Reference adrenal glands Opens New Window Reference Opens New Window make less cortisol. 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. Sometimes 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.

An ACTH test is sometimes done at the same time as the cortisol test.

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