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 pictures of the pituitary gland and 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. 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.
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).
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 non-prescription and prescription medicines you take. You may be asked to stop taking some medicines (such as birth control pills, ASA, 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?).
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:
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having blood sample taken from a vein.
The overnight dexamethasone suppression test involves taking a dose of a corticosteroid medicine called dexamethasone to see how it affects the level of a hormone called cortisol in the blood. This test screens for Cushing's syndrome, a condition in which excess amounts of cortisol are being produced by the adrenal glands. Test results are usually available in 2 to 3 days.
High cortisol levels may be caused by:
Reasons you may not be able to have the test or why the results may not be helpful include:
Some people may quickly process (metabolize) the dose of dexamethasone. In these people, cortisol levels will not drop unless a higher dose of the medicine is given.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Primary Medical Reviewer||Anne C. Poinier, MD - Internal Medicine|
|Specialist Medical Reviewer||David C.W. Lau, MD, PhD, FRCPC - Endocrinology|
|Last Revised||August 17, 2010|
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