Dexamethasone Suppression Test

What is a dexamethasone suppression test (DST)?

A dexamethasone suppression test (DST) measures how your cortisol levels respond to a medication called dexamethasone. Healthcare providers mainly use a dexamethasone suppression test to diagnose Cushing syndrome.

Types of dexamethasone suppression test

There are two different types of tests: a low-dose and a high-dose. Healthcare providers can perform either type of test as an overnight test or as a three-day test. The low-dose overnight test is most common.

• Low-dose overnight. You’ll take a 1 milligram (mg) dexamethasone pill at 11 p.m., and have your blood drawn at 8 a.m. the next morning to check your cortisol levels.
• High-dose overnight. You’ll have your cortisol levels checked in the morning using a blood sample. At 11p.m. that evening, you’ll take 8 mg of dexamethasone by mouth. A provider will draw your blood at 8 a.m. the next morning to check your cortisol levels.
• Low-dose standard. You’ll collect your pee in special containers for three days. On the second day, you’ll take 0.5 mg of dexamethasone by mouth every six hours for 48 hours. On the third day, you’ll bring the containers to the lab for testing.
• High-dose standard. You’ll collect your pee in special containers for three days. On the second day, you’ll take 2 mg of dexamethasone by mouth every six hours for 48 hours. On the third day, you’ll bring the containers to the lab for testing.

How does a dexamethasone suppression test work?

Healthcare providers use the test to measure how well your body can suppress cortisol production. Cortisol is known as the stress hormone because your adrenal glands make it to respond to stress. But having chronically high cortisol levels can make you sick.

Dexamethasone is a human-made version of cortisol. If you take it, your adrenal glands should respond by making less cortisol. But, if you have Cushing syndrome, your body will not respond to the medication and your cortisol levels will stay high.

The test measures if your cortisol levels change after taking dexamethasone, and by how much. The test can also help determine if you have high cortisol due to excess adrenocorticotrophic hormone (ACTH) that your pituitary gland makes.

How do I prepare for the test?

Your healthcare provider may ask you to stop taking certain medications before the test because they can affect your results. These could include birth control pills, medications that contain corticosteroids or antiseizure medications. Your provider may also ask you to avoid eating and drinking before the test.

The best thing you can do to prepare is share your full health history with your healthcare provider, including any medications or supplements you’re taking.

What are the risks?

The test is safe, and there isn’t much risk. Since you only take dexamethasone for a short time, it’s not likely to cause side effects. The only other risks are excessive bleeding or bruising from blood draws. But these are rare.