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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 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 gland camera.gif make less adrenocorticotropic hormone (ACTH), the adrenal glands camera.gif 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.

Why It Is Done

An overnight dexamethasone suppression test is done to check for a condition in which large amounts of cortisol are produced by the adrenal glands (Cushing's syndrome).

How To Prepare

You will not be able to eat or drink anything for 10 to 12 hours before the morning blood test.

Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. You may be asked to stop taking some medicines (such as birth control pills, aspirin, morphine, methadone, lithium, monoamine oxidase inhibitors [MAOIs], and diuretics) for 24 to 48 hours before your blood is drawn.

Talk to your doctor about any concerns you have about the need for the test, its risks, how it will be done, or what the results will mean. To help you learn about this test and how important it is, fill out the medical test information form(What is a PDF document?).

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WebMD Medical Reference from Healthwise

Last Updated: May 17, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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