Dexamethasone Suppression Test: Why It Is Done, How It Works, and What to Expect Before and After

What is this test? The dexamethasone suppression test is a blood test that measures cortisol, a hormone made by your adrenal glands, after you take a dose of dexamethasone. Doctors use it to find out why your body may be producing too much cortisol and to help diagnose conditions such as Cushing's syndrome.

The 17-Ketogenic Steroids test measures the amount of certain steroid hormones (called 17-ketogenic steroids) in your urine over a 24-hour period. These hormones come from your adrenal glands, which sit on top of your kidneys. Before collecting the urine, you will take dexamethasone medication—a synthetic steroid that normally suppresses natural hormone production in healthy individuals.

Why do I need to take dexamethasone first? Dexamethasone is a man-made corticosteroid that resembles cortisol. In a healthy body, taking it signals your pituitary gland to cut back on the hormones that tell your adrenal glands to make cortisol. If your cortisol level stays high after taking dexamethasone, it suggests your body is not suppressing cortisol normally, which can point to Cushing's syndrome.

Dexamethasone is a medication that normally tells your adrenal glands to stop producing cortisol (the stress hormone). By taking dexamethasone before the test, doctors can see how your body responds. In a healthy person, the hormone levels will drop. If the levels remain high, it may indicate a problem like Cushing's syndrome. This is called a dexamethasone suppression test.

How is the test performed? In the most common version, you take a small tablet of dexamethasone by mouth late in the evening, often around 11 p.m., and a blood sample is drawn the next morning between 8 a.m. and 9 a.m. to measure your cortisol level. The blood draw is quick: a clinician cleans your skin, inserts a small needle into a vein in your arm, collects the sample, and then applies pressure and a bandage.

First, you will take dexamethasone pills (500 micrograms) every 6 hours for 2.5 days as prescribed by your doctor. On the third day, you will begin collecting all your urine in a special container provided by the lab. You must collect every drop for exactly 24 hours. Keep the container refrigerated or on ice during collection. After 24 hours, return the container to the lab as instructed.

What should I avoid before the test? Tell your doctor about all medicines, vitamins, and supplements you take, because some drugs, such as birth control pills, phenytoin, carbamazepine, and certain steroids, can affect the results. Avoid alcohol, caffeinated drinks, and strenuous exercise the day before and the day of the test, and follow any fasting instructions your doctor gives you before the morning blood draw.

Tell your doctor about all medications you are taking, as some drugs can affect the results. You may need to stop certain medicines, especially steroids, hormones, or diuretics. Avoid strenuous exercise during the collection period, as it can affect hormone levels. Follow your doctor's specific instructions about diet and medications.

Are there any risks or side effects? The blood draw is very safe, but you may have brief pain, minor bruising, or slight bleeding where the needle was inserted, and rarely dizziness or infection can occur. Dexamethasone can cause temporary effects in some people, such as trouble sleeping, feeling jittery, an upset stomach, or mood changes. Serious problems are uncommon, but contact your doctor if you notice unusual symptoms.

The dexamethasone used in this test is a low dose and is generally safe. Some people may experience mild side effects like nausea, trouble sleeping, or slight mood changes. The urine collection itself has no risks. If you have diabetes, monitor your blood sugar closely as dexamethasone can affect glucose levels.