A dexamethasone suppression test is primarily used to help diagnose Cushing syndrome. Cushing syndrome indicates that you have an abnormally high level of cortisol. Cortisol is a steroid hormone produced by the body during high levels of stress. (Abnormally low cortisol levels can be a sign of Addison’s disease, which is not diagnosed by this test.)
A dexamethasone suppression test measures how your cortisol levels are affected by taking dexamethasone. Dexamethasone is a manmade corticosteroid similar to one produced naturally by your adrenal glands. It’s prescribed to replace the natural chemical if your body isn’t producing enough of it. It may also be prescribed as an anti-inflammatory agent that’s used to treat arthritis and various blood, kidney, and eye disorders.
Your adrenal glands are located on top of your kidneys. In addition to producing cortisol, they produce steroid hormones such as:
- androgens, which are male sex hormones
The test is also used to determine how well the adrenal glands respond to adrenocorticotropic hormone (ACTH). ACTH is a hormone produced by the brain’s pituitary gland. It has a number of functions, including the production of corticosteroids. Too much ACTH can cause Cushing syndrome. In a healthy person, as the pituitary glands make less ACTH, the adrenal glands make less cortisol. Dexamethasone should decrease the amount of ACTH, which should then cause the amount of cortisol to decrease.
If you’re currently taking the corticosteroid medicine dexamethasone, your doctor may recommend a dexamethasone suppression test to determine how it’s affecting cortisol levels in your blood.
Dexamethasone relieves inflammation related to arthritis and severe allergies, among other conditions. When you take dexamethasone, which is very similar to cortisol, it should decrease the amount of ACTH released into your blood. If your cortisol level is high after taking a dose of dexamethasone, this is a sign of an abnormal condition.
Before the test, your doctor will tell you to stop taking certain prescription medications that may affect the results. These include:
- birth control pills
- phenytoin, which is used to treat seizures
- spironolactone, which is used to treat congestive cirrhosis, ascites, or kidney problems
- tetracycline, which is an antibiotic
Two variations of the dexamethasone suppression test are the low-dose test and the high-dose test. Both forms of the test can be done overnight or over the course of a three-day period. The standard test for both is the test that spans three days. During both forms of the test, your doctor will give you a certain amount of dexamethasone and will later measure your levels of cortisol. A blood sample is also needed.
Blood will be drawn from a vein in the inside of your lower arm or the back of your hand. First, your doctor will swab the site with antiseptic. They may wrap an elastic band around the top of your arm to cause the vein to swell with blood, making it more visible. Your doctor will then insert a fine needle into the vein and collect a blood sample into a tube attached to the needle. The band is removed and gauze is applied to the site to prevent further bleeding.
Low-dose overnight test
- Your doctor will give you 1 milligram of dexamethasone at 11 p.m.
- They’ll draw a blood sample at 8 a.m. the following morning to test your cortisol levels.
Standard low-dose test
- You’ll collect urine samples over three days and store them in 24-hour collection bottles.
- On the second day, your doctor will give you 0.5 milligrams of oral dexamethasone every six hours for 48 hours.
High-dose overnight test
- Your doctor will measure your cortisol levels on the morning of the test.
- You’ll be given 8 milligrams of dexamethasone at 11 p.m.
- Your doctor will take a blood sample at 8 a.m. to measure your cortisol levels.
Standard high-dose test
- You’ll collect samples of urine over three days and store them in 24-hour containers.
- On the second day, your doctor will give you 2 milligrams of oral dexamethasone every 6 hours for 48 hours.
An abnormal low-dose test result may indicate that you’re experiencing an excessive release of cortisol. This is known as Cushing syndrome. This disorder could be caused by an adrenal tumor, a pituitary tumor, or a tumor elsewhere in your body that’s producing ACTH. The results of the high-dose test can help isolate the cause of Cushing syndrome.
High cortisol levels may also be caused by a number of other conditions, such as:
- a heart attack
- heart failure
- a poor diet
- an overactive thyroid gland
- anorexia nervosa
- untreated diabetes
As with any blood draw, there’s a minimal risk of minor bruising at the needle site. In rare cases, the vein may become swollen after blood is drawn. This condition, known as phlebitis, can be treated with a warm compress several times a day. Ongoing bleeding could be a problem if you have a bleeding disorder or you’re taking blood thinners such as warfarin (Coumadin) or aspirin.
Even with an abnormally high result, your doctor may recommend further tests to diagnose Cushing syndrome. If this disorder is diagnosed, you’ll be given appropriate medications to control your high cortisol levels.
If cancer is causing your high cortisol levels, your doctor will recommend further tests to determine the type of cancer and the appropriate treatment.
If your high cortisol levels are caused by other disorders, your doctor may recommend another course of treatment.