What Is the Dexamethasone Suppression Test?

What to expect when undergoing this test

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You may have been told that you need to have a dexamethasone suppression test if your doctor suspects you have a condition known as Cushing's syndrome. The test evaluates your body’s ability to respond normally to a type of hormone known as glucocorticoids. A dexamethasone suppression test can help your doctor diagnose Cushing's syndrome, or eliminate it as a possibility.

Purpose

To more deeply understand the purpose of the dexamethasone suppression test, you have to first learn about glucocorticoids and how they are regulated in your body. 

What Are Glucocorticoids?

Glucocorticoids are a group of compounds all with similar chemical structure and similar characteristics. They are members of a larger class of hormones called steroid hormones and members of a smaller steroid group called corticosteroids. Glucocorticoids bind to glucocorticoid receptors, which are found in many cells in the body. There, they have many different effects, including influencing how your body uses nutrients, modulating the immune system, and helping your body deal with stress.

Cortisol is a type of glucocorticoid naturally produced in your body by the adrenal gland (near the kidneys). Dexamethasone is another type of glucocorticoid that is very similar to cortisol in structure. However, it is made synthetically and not found naturally in the body. Various glucocorticoids are also sometimes given as medical treatments.

Pros and Cons of Glucocorticoids

How Are Glucocorticoids Regulated in the Body?

Your body works to maintain a balance of glucocorticoids in the body. That’s important because either too much or too little can cause problems.

The pituitary gland (found at the base of the brain) helps regulate how much cortisol is produced. It produces another hormone called ACTH. This hormone travels through the bloodstream and signals your adrenal gland to produce cortisol.

Various factors control how much ACTH the body produces. Normally, high amounts of cortisol will travel back in the bloodstream to the pituitary gland and inhibit release of ACTH. In other words, because cortisol is already high, the body downregulates secretion of ACTH. That helps prevent cortisol levels from getting too high in the body.

Normally, cortisol levels should also decrease if the body is exposed to another corticosteroid (like dexamethasone). The dexamethasone travels to the pituitary and causes the pituitary to downregulate secretion of ACTH. That leads to lower secretion of cortisol and lower levels of cortisol in blood tests. If that doesn’t happen, it’s a sign of some sort of problem in the body.

What Is Cushing's Syndrome? Is It Different From Cushing's Disease?

Cushing's syndrome describes a group of related medical conditions. Cushing's syndrome occurs when the body is exposed to abnormally high levels of glucocorticoid hormones over a prolonged period. That leads to the symptoms of Cushing's syndrome, which can include abdominal obesity, rounding of the face, elevated blood pressure, muscle weakness, increased blood glucose levels, and others.

The most common medical cause of Cushing's syndrome is a specific subtype: Cushing's disease. These sound very similar, but not everyone with “Cushing's syndrome” has “Cushing's disease.” Cushing's disease is caused specifically by a pituitary adenoma (a tumor that arises from the pituitary gland). This can cause the pituitary to release abnormally high amounts of ACTH and not respond to feedback in the right way. So, the adenoma might continue releasing ACTH even when the body is exposed to the glucocorticoid dexamethasone.

Cushing's syndrome also has other causes. For example, it can also occur when a person is given large doses of corticosteroids as a medical treatment. A variety of other problems can either cause the body to make too much ACTH (and therefore too much cortisol) or make too much cortisol even if ACTH is not high. Many of these types of Cushing's syndrome arise from specific kinds of cancer. For example, certain kinds of lung cancer or cancer of the adrenal glands can lead to Cushing's syndrome.

If you are diagnosed with Cushing's syndrome, your doctor may need to do other tests to find out the exact cause of your problem. This might be Cushing's disease (from a pituitary adenoma) or it might have another source. There are various types of treatment available, depending on the source of the disease.

An Overview of Cushing's Syndrome

Who Specifically Might Need the Test?

You might need a dexamethasone suppression test if you have some of the symptoms of Cushing's syndrome and your doctor wants to screen you for the disease. The test can be used to help diagnose the disease or help eliminate it as a possibility.

Many of the symptoms of Cushing's disease (like high blood pressure) are quite common in people who don’t have the disease. The dexamethasone suppression test can help pick out people who are having symptoms because of Cushing's syndrome. Your regular doctor or an endocrinologist might first suggest the test.

In some cases, the dexamethasone suppression test can be used if you already know you have Cushing's syndrome but your doctor is trying to figure out the exact source of your problem.

Are There Alternatives to the Test?

In some cases, yes. If your doctor is worried about the possibility of Cushing's syndrome, you might be able to have a test that assesses the cortisol in your saliva. Or you might be able to have a test in which you assess the amount of cortisol in your urine over a 24-hour period. You can talk to your doctor about these other possibilities.

Risks

The dexamethasone suppression test is generally a very safe test for most people. It can be given to pregnant women and people with many different health conditions. The low dose version of the test, in particular, is very unlikely to cause any problems. However, be sure to discuss all of your health conditions with your health provider before you have the test. 

During the Test

The dexamethasone suppression test is relatively simple to perform. It does not require medical imaging.

In the test, a person takes a dose of dexamethasone orally at a specific time. Often, this can be done as an outpatient. Usually, there are no special preparations to make ahead of time. You can eat and drink normally unless your doctor tells you otherwise. Many people don't notice anything at all, or you might experience a mild response to the dexamethasone, like a headache.

The dexamethasone acts in the body very similarly to cortisol. After a period of time (often overnight), the amount of cortisol in your body is assessed. This could be through blood or urine samples taken once or a few times, so you might come into an outpatient clinic to have your blood drawn. The sample is then analyzed for the amount of cortisol.

In a normal person, the pituitary gland responds to the extra dexamethasone by decreasing secretion of ACTH. This decreases the amount of cortisol found in the blood or urine. If cortisol doesn’t decrease as much as expected, then the patient may have a form of Cushing's syndrome.

In some versions of the test, you might need to take multiple doses of dexamethasone. Your doctor can let you know what to expect in your particular situation.

What Are the Types of the Test?

In the high-dose dexamethasone suppression test, you receive a higher amount of dexamethasone. In the low-dose version of the test, you would receive a somewhat lower dose.

Typically, the low-dose test is the version given if Cushing's syndrome is suspected. For example, with a low-dose test, people with all different types of Cushing's syndrome will show no suppression of cortisol (e.g., cortisol will be higher than it is in normal people). That’s why it is often used as an initial screening test.

The high-dose test is more likely to be given if Cushing's syndrome has already been diagnosed, to help identify the specific cause of the problem. With a high-dose test, people with different types of Cushing's syndrome can have different responses. This can help doctors tease apart the underlying cause of your Cushing's syndrome.

There are several existing protocols for the dexamethasone suppression test. Both the low dose and the high dose version can be given either overnight (more commonly) or as part of a two-day test.

After the Test

There generally aren't any special precautions to follow after having your dexamethasone suppression test.

Potential Side Effects

There can be side effects from receiving high doses of corticosteroids over the long-term. However, in the most common version of the test, you receive only a single low dose of dexamethasone. In this case, you are unlikely to experience side effects. If present, they are likely to be mild and temporary (e.g., poor sleep the night of test, headache), and they are likely to go away without intervention. If you have severe symptoms such as chest pain, contact a healthcare professional right away. 

Interpreting the Results

Because the dexamethasone suppression test most often relies on a simple blood test of cortisol levels, results are usually available relatively quickly. 

Healthy people show suppression of cortisol release as part of the dexamethasone suppression test. That is, in response to the dexamethasone which they receive, their body downregulates the amount of cortisol it releases. In people with Cushing's syndrome, there is no suppression of cortisol or only limited suppression. Cortisol levels remain high, even though the body received a dose of the dexamethasone. 

Interpreting the results of the dexamethasone suppression test can be complicated. Exact levels of cortisol will vary based on the version of the test you have. However, for the low-dose overnight version of the test, a serum cortisol of fewer than 1.8 micrograms per decilitre is considered normal.

Can the Test Have Incorrect Results?

Yes. A false positive might happen when a person’s cortisol does not appear to go down normally, but they don’t really have Cushing's syndrome. This might happen, for example, with:

  • Women taking contraceptive medications
  • People taking certain other medications (like phenytoin)

Other times, the test might have a false negative. In other words, the person might really have Cushing's syndrome, but the test doesn’t pick it up. For example, this might happen, for example, with:

  • People with kidney problems
  • People with liver problems
  • People taking other medications (like fluoxetine and certain HIV medications)

You also might have incorrect results if you don’t follow the instructions correctly. For example, it is important to take the dexamethasone at exactly the time that your doctor instructs, or you can have incorrect results. Be sure to talk to your doctor about all your medications before you have your test. That will help make sure that your test can be interpreted correctly. 

Will I Need Additional Tests?

You might need other tests after having a dexamethasone suppression test, especially if your test result is abnormal. Cushing's syndrome can be challenging to diagnose. Even once you are diagnosed with Cushing's syndrome, your doctor might request follow-up tests to diagnose the specific cause. Some additional tests you might need include:

  • CT scan of the abdomen
  • Salivary tests for cortisol
  • Blood tests (like for levels of ACTH)
  • MRI of the brain
  • Repeat low-dose dexamethasone test
  • High-dose dexamethasone test (after an initial low-dose test)

A Word From Verywell

The dexamethasone suppression test is generally a very safe test that requires little preparation on your part. You should be able to discuss the results of your test relatively soon. If you have Cushing's syndrome, your doctor may need to do follow up tests to evaluate the cause. Though it can be anxiety-provoking to wait for your results, know that there are many people available to help you through your potential diagnosis and beyond.

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