The overnight dexamethasone suppression test checks to see how taking a steroid 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 makes less adrenocorticotropic hormone (ACTH), the adrenal glands make less cortisol. Dexamethasone, which is like cortisol, lowers the amount of ACTH released by the pituitary gland. This in turn lowers the amount of cortisol released by the adrenal glands.
After a dose of dexamethasone, cortisol levels often stay very high in people who have Cushing's syndrome. Sometimes other conditions can keep cortisol levels high during this test. Examples include major depression, alcoholism, stress, obesity, kidney failure, pregnancy, and uncontrolled diabetes.
The night before the blood test, you will take a dexamethasone pill. The next morning, the cortisol level in your blood will be measured. If your cortisol level stays 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
This test is done to check for Cushing's syndrome. It is a condition in which large amounts of cortisol are produced by the adrenal glands.
How To Prepare
You will not be able to eat or drink anything for 10 to 12 hours before the morning blood test.
Be sure to tell your doctor about all the medicines you take, even over-the-counter ones. Many medicines can change the results of this test. You may be asked to stop taking some medicines for 24 to 48 hours before your blood is drawn. These medicines include birth control pills, aspirin, morphine, methadone, lithium, monoamine oxidase inhibitors (MAOIs), and diuretics.
Talk to your doctor if you have any concerns 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?).
How It Is Done
The night before the test (usually at 11:00 p.m.), you will swallow a pill that contains 1 milligram (mg) of dexamethasone. The next morning (usually at 8:00 a.m.), you will have a sample of your blood drawn. Take the pill with milk or an antacid. This can help prevent an upset stomach or heartburn.
The health professional taking a sample of your 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 on the site and then put on a bandage.
Sometimes a more complete dexamethasone suppression test may be done. For this test, you will take up to 8 pills over 2 days. Then the cortisol levels in your blood and urine will be measured.
How It Feels
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.
Risks of a blood test
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Bruising may be more likely in people with high ACTH and cortisol levels.
Bleeding can be a problem for people who have bleeding disorders or take blood-thinning medicines such as aspirin or warfarin (Coumadin). If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
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 checks for Cushing's syndrome. In this condition, large amounts of cortisol are produced by the adrenal glands. Test results are usually ready in a few days.
An abnormal test result may mean that more tests are needed. A normal test result means that you do not have Cushing's syndrome. This syndrome can be hard to diagnose. So if the test results aren't clear or if they don't help explain your symptoms, you may be referred to see an endocrinologist.
These numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you.
High cortisol levels may be caused by:
- Cushing's syndrome.
- Other health problems. Examples include heart attack or heart failure, fever, poor diet, an overactive thyroid gland (hyperthyroidism), depression, anorexia nervosa, uncontrolled diabetes, and alcoholism.
- Cancers that make ACTH, such as lung cancer.
What Affects the Test
You may not be able to have the test, or the results may not be helpful, if:
- You are pregnant or extremely overweight.
- You have had severe weight loss, dehydration, or acute alcohol withdrawal.
- You have had a severe injury.
- You have diabetes.
- You take certain medicines. These medicines include barbiturates, phenytoin (Dilantin), birth control pills, aspirin, morphine, methadone, lithium, monoamine oxidase inhibitors (MAOIs), spironolactone (Aldactone), or diuretics.
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.
What To Think About
- Some doctors think that a 24-hour urine free cortisol test is more accurate than an overnight dexamethasone suppression test. A 24-hour urine free cortisol test is also used to look for Cushing's syndrome. To learn more, see the topic Cortisol in Urine.
- An adrenocorticotropic hormone (ACTH) test may be done at the same time as the cortisol test. To learn more, see the topic Adrenocorticotropic Hormone.
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.
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerAlan C. Dalkin, MD - Endocrinology
Current as ofNovember 20, 2015