What Is an Overnight Dexamethasone Suppression Test?

Some illnesses have clear signs that tell you what they are, like the bull’s-eye rash of Lyme disease or the high-pitched whoop of whooping cough. But others, such as Cushing syndrome, make you work to figure it out.

When you have Cushing syndrome, you have high levels of cortisol. That’s a hormone made by your adrenal glands. It affects everything from blood pressure to memory.

The challenge with Cushing syndrome is that the signs and symptoms can look a lot like many other diseases. So doctors typically do more than one test to make sure you have it.

One of those tests has a mouthful of a name -- it’s an “overnight dexamethasone suppression test.” It can help you learn whether you have Cushing syndrome and what’s causing it if you do.

What’s Cushing Syndrome?

You get this syndrome when you have high levels of cortisol for a long time. It can cause weight gain, thining skin, easy bruising, and other issues. Longer term, it can lead to conditions such as:

When your body makes too much cortisol, it’s often because of a tumor. In some cases, these tumors are cancerous, but usually they’re not.

A tumor on your pituitary gland is the most common one that causes Cushing syndrome. It’s not cancer, but it causes the pituitary to make too much adrenocorticotropic hormone (ACTH), which controls how much cortisol your body makes. More ACTH means more cortisol.

Adrenal tumors are the second most common cause of Cushing syndrome. The may or may not be cancerous.

You can also get tumors on other organs where the tumor itself makes ACTH. This is called an ectopic ACTH-secreting hormone. Again, it leads to too much cortisol.

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How Does the Test Work?

Dexamethasone is a manmade version of cortisol. After you take a dose of it, your body should make less cortisol. That’s the idea behind the test -- take some dexamethasone and see whether your cortisol level drops. Usually, the test is done overnight, but it can also be done over 2 days.

There are two doses you can take for the test: low dose and high dose. The low-dose test helps you find out if you have Cushing syndrome or not. You typically get 1 mg of dexamethasone.

You’d get the high-dose test once you know you have Cushing syndrome. It’s done to find out whether it’s caused by a tumor on your pituitary gland. The high dose is typically 8 mg.

Why Would I Need It?

You’d get this test if you show signs and symptoms of Cushing syndrome, like bruising easily, weight gain around your belly, and a very round face. Your doctor might also order this test if you have problems that aren't typical for your age but could mean you have Cushing syndrome: For example, if you’re young but have weak bones (osteoporosis), high blood pressure, or diabetes.

This test isn’t ideal for everyone. For example, pregnancy and seizure medicines can both affect your results, so you’d likely get one of the other tests for Cushing syndrome.

How Do I Get Ready for It?

Your doctor may tell you to avoid taking any food or drink 10 to 12 hours before the morning blood test, but that’s about it.

Make sure to tell your doctor about any medicines and supplements you’re taking -- some medications, such as birth control pills and seizure medicine, can affect your results.

What Happens During the Test?

Between 11 p.m. and midnight the night before, you’ll take the dexamethasone. That’s all you need to do until the next morning. Between 8 a.m. and 9 a.m., you’ll get a blood draw to measure your cortisol level.

A blood draw takes just a few minutes. Your doctor will:

  • Clean the skin where the needle goes in
  • Wrap a rubber strap around your upper arm -- this creates pressure to make your veins swell with blood
  • Put a thin needle into a vein, usually on the inside of your arm at your elbow or in the back of your hand
  • Draw the blood
  • Remove the rubber strap and put a bandage on your arm or hand

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What Does the Result Mean?

Results can vary with different labs, so be sure to talk to your doctor to learn exactly what your numbers mean and what your next steps are.

For a low-dose test, a healthy cortisol level is below 1.8 mcg/dL (micrograms per deciliter) or 50 nmol/L (nanomoles per liter). These two numbers say the same thing, they just have different units -- just like 12 inches is the same as 1 foot. If your result is higher than that, there’s a good chance you have Cushing syndrome. If it’s lower, something else may be triggering high cortisol levels.

For a high-dose test, your doctor looks for a 50% drop in cortisol. If your level drops, it’s likely that a pituitary tumor is causing Cushing syndrome. If your level doesn’t drop, you may have a tumor somewhere else.

Will I Need Any Other Tests?

You may get other tests to check for Cushing syndrome and what’s causing it, such as:

  • Corticotrophin releasing hormone (CRH) stimulation test to learn more about the possible causes of Cushing syndrome
  • Imaging, such as CT or MRI, to look for tumors
  • Late-night saliva cortisol or 24-hour urine free cortisol tests to check cortisol level
  • 24-hour urinary cortisol excretion test
WebMD Medical Reference Reviewed by Sabrina Felson, MD on February 23, 2019

Sources

SOURCES:

CDC: “Lyme Disease,” “Pertussis (Whooping Cough).”

Hormone Health Network: “Diagnosis of Cushing Syndrome,” “What Does Cortisol Do?”

Lab Test Online: “Cushing Syndrome.”

UpToDate: “Dexamethasone Suppression Tests.”

Mayo Clinic: “Cushing Syndrome.”

Dartmouth-Hitchcock: “Dexamethasone Suppression Test.”

KidsHealth: “Blood Test: Partial Thromboplastin Time (PTT).”

University of Rochester Medical Center: “Activated Partial Thromboplastin Clotting Time.”

American Board of Internal Medicine: "ABIM Laboratory Test Reference Ranges ̶ August 2018" (pdf).

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