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 an illustration 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 decreasing 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.

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 regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The night before your blood is drawn (usually at 11:00 p.m.), you will swallow a pill containing 1 milligram (mg) of dexamethasone. The next morning (usually at 8:00 a.m.), a health professional will draw a sample of your blood. Take the pill with milk or an antacid to help prevent an upset stomach or heartburn.

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then put on a bandage.