Dexamethasone Suppression Test

The dexamethasone suppression test (DST) is what your doctor orders to check how much cortisol your body is producing. The test helps determine if you're suffering from a condition called Cushing syndrome. There are two different types of DST tests being used by healthcare providers. You may have to go for the low-dose test first that helps confirm if your body is producing too much cortisol. Your doctor will also order the high-dose test that helps specify if the increase in cortisol is due to malfunctioning pituitary gland (Cushing's disease). Keep reading to learn more details about dexamethasone suppression test.

What Is Dexamethasone Suppression Test?

It is basically a blood test that healthcare providers order to diagnose Cushing syndrome. You will develop Cushing syndrome when your pituitary gland doesn't function properly and produces too much cortisol. Your pituitary gland releases a hormone called adrenocorticotropic hormone or ACTH. There is a negative relationship between ACTH and cortisol. It means that if your pituitary gland is producing enough ACTH, your cortisol levels will be on the lower side. The dexamethasone suppression test takes advantage of this relationship to help determine the levels of cortisol in your bloodstream.

Dexamethasone is basically a synthetic steroidal hormone and it functions quite like cortisol. Your healthcare provider will give you dexamethasone that will reduce ACTH in your body. They will then measure the ability of this synthetic steroidal hormone to suppress cortisol production. There will an abnormal response to dexamethasone if your pituitary gland isn't functioning properly.

How to Prepare Dexamethasone Suppression Test

Before the test, you may have to stop taking certain medications such as barbiturates, oral contraceptives, corticosteroids, estrogens, tetracyclines, phenytoin and spironolactone, which are used to treat patients with cirrhosis, congestive heart failure or kidney disease.

How Is Dexamethasone Suppression Test Performed?

The test involves giving you a dose of Dexamethasone, a strong synthetic glucocorticoid medication. After some time, your healthcare provider will draw some blood and use this blood sample to measure the cortisol levels in your blood. As mentioned, there are two types of dexamethasone suppression tests. Both low and high dose tests can be done in an overnight or 3-day. Different laboratories will use different methods to perform these tests.

  • Low-dose overnight: The test involves giving you 1 mg of dexamethasone at 11 pm and drawing out a blood sample the next morning at 8 am.
  • Standard low-dose: The test involves collecting a urine sample over 3 days to measure the levels of cortisol in your body. You will get 0.5 mg of dexamethasone on day 2 every 6 hours for next 48 hours.
  • High dose overnight: You will receive a high dose (8 mg) of dexamethasone at 11 pm, whereas the blood sample will be taken at 8 am next morning.
  • Standard high-dose: Just like standard low-dose test, your urine will be collected over 3 days, with 2 mg of dexamethasone given after every 6 hours on day 2 that will continue for 48 hours.

Understanding the Results

Although there are some variations in results obtained through different laboratories, you can still use the following for reference.

1.   Normal Results

During a dexamethasone suppression test, you will receive dexamethasone that will lower your cortisol levels. Here are the values that are considered normal by healthcare providers.

Low dose

  • Overnight: 8 a.m. plasma cortisol < 1.8 mcg/dl
  • Standard: Urinary free cortisol on day 3 < 10 mcg/day

High dose

  • Overnight: > 50% reduction in plasma cortisol
  • Standard: > 90% reduction in urinary free cortisol

2.   Abnormal Results

If your test shows abnormal results, this may indicate several issues, including Cushing syndrome. You can get abnormal results mainly due to a tumor in your pituitary gland that produces ACTH, adrenal tumor that releases cortisol, or tumor in the body that leads to the excessive production of ACTH.

In this situation, you will have to go for the high-dose test if you haven't already done it. This will help confirm if your pituitary gland is malfunctioning or there are other causes of abnormal results. An ACTH blood test may tell more about certain causes of high cortisol levels.

Even if you have Cushing syndrome, you may get different results depending on the condition that's causing the problem.

  • ŸYou may notice no change in low-dose test with low ACTH level if your Cushing syndrome is the outcome of an adrenal tumor. You don't need the high-dose test in this case.
  • ŸYour low-dose test will show no change but your ACTH level will be on the higher side if your Cushing syndrome is the outcome of an ectopic ACTH-producing tumor, and the high-dose test will show no change in this case.
  • ŸIf your Cushing syndrome is due to a pituitary tumor, your low-dose test won't show any change and your high-dose test will have normal suppression. This usually indicates Cushing's disease.

After the Dexamethasone Suppression Test

Even when your test gives abnormalresult, your doctor may still order further tests to confirm you're a victim of Cushing's disease. Once they have confirmed the disorder, they will prescribe certain medications to help control your high cortisol levels.

In some cases when your high cortisol levels are due to a tumor, your doctor will ask for further tests to determine the type of tumor. Once he knows its type and severity of the issue, then he will be able to select a treatment option. However, you will require a different course of treatment if your cortisol levels are on the higher side due to other disorders.

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