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.

What do the results of a dexamethasone suppression test mean?

It may take several days for your healthcare provider to get the results of your test. They use the results to determine if you have Cushing syndrome and what could be causing it.

Normal results

Your cortisol levels should decrease after taking dexamethasone. The following results are considered normal results:

• For a low-dose overnight DST, a fasting cortisol level of less than 1.8 micrograms per deciliter (mcg/dL) at 8 a.m. is considered normal.
• For a high-dose overnight DST, a greater than 50% decrease in cortisol level after taking dexamethasone is considered normal.
• For a low-dose standard DST, a urinary free cortisol level of less than 10 mcg/day on day 3 is considered normal.
• For a high-dose standard DST, a greater than 90% decrease in urinary free cortisol level on day 3 is considered normal.

Normal result ranges can vary between labs. Be sure to talk to your healthcare provider about what the results mean for you.

Abnormal results

• A cortisol level greater than 1.8 mcg/dL may indicate hypercortisolism.
• A cortisol level greater than 5 mcg/dL may indicate significant hypercortisolism.

You should always have a conversation with your healthcare provider about your specific results and what they mean for you.

When should I call my healthcare provider?

Contact your provider if you have any questions about the test or the next steps. Depending on the results, your provider may order additional tests.