What Is a Dexamethasone Suppression Test?
A dexamethasone suppression test is primarily used to help diagnose Cushing’s syndrome. Cushing’s 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.
A dexamethasone suppression test basically does two things: First, it measures the amount of dexamethasone being produced in your body. Dexamethasone is a corticosteroid similar to one produced naturally by your adrenal glands. It affects the levels of cortisol in your blood and is prescribed to replace the natural chemical if your body is not producing enough of it. It may also be prescribed as an anti-inflammatory agent used to treat arthritis and blood, kidney, and eye disorders.
Your adrenal glands are located above your kidneys. They produce steroid hormones such as androgens, as well as cortisol, epinephrine, and norepinephrine.
The test 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’s syndrome.
What the Test Addresses
If you are currently taking the corticosteroid medicine dexamethasone, your doctor may recommend a dexamethasone suppression test to determine how it is affecting the levels of cortisol in your blood.
In a healthy person, as the pituitary glands make less ACTH, the adrenal glands make less cortisol.
Dexamethasone is prescribed to relieve inflammation related to arthritis, severe allergies, and other conditions. When you take dexamethasone, which is very similar to cortisol, it should decrease the amount of ACTH released into your blood. However, if your cortisol level is high after taking a dose of dexamethasone, this indicates an abnormal condition.
Preparation for the Test
Before the test, your doctor may tell you to stop taking certain prescription medications that may affect the test. These include:
- oral contraceptives
- phenytoin, used to treat seizures
- spironolactone, used to treat patients with congestive heart failure, cirrhosis, or kidney problems
- tetracyclines, a family of antibiotics
How Is the Test Administered?
There are two variations of the dexamethasone suppression tests: a “low-dose” test and a “high-dose” test. Each form of the test can be done either overnight, or over the course of a 3-day period (referred to as a “standard” test).
During both forms of the tests, your doctor will give you some 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 the doctor will swab the site with antiseptic.He or she may then wrap an elastic band around the top of your arm to cause the vein to swell with blood. 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 any further bleeding.
Low-Dose Overnight Test
- Your doctor will give you 1 mg of dexamethasone at 11 p.m.
- He or she will draw a blood sample at 8 a.m. the following morning to test your cortisol levels.
Standard Low-Dose Test
- You will 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 mg 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 will be given 8 mg 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 will collect samples of urine over three days and store them in 24-hour containers.
- On the second day, your doctor will give you 2 mg of oral dexamethasoneevery 6 hours for 48 hours.
Understanding the Results
An abnormal result to the low-dose test may indicate that you are suffering from an excessive release of cortisol, known as Cushing’s syndrome. This disorder could be caused by an adrenal tumor, a pituitary tumor, or a tumor elsewhere in the body that is producing ACTH. The results of a high-dose test can help to isolate the cause of Cushing’s syndrome to a pituitary tumor.
High cortisol levels may also be caused by a number of other conditions including:
- heart attack
- heart failure
- poor diet
- overactive thyroid gland
- anorexia nervosa
- untreated diabetes
What Are the Risks of the Test?
As with any blood draw, there is the minimal risk of experiencing 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 each day. Ongoing bleeding could be a problem if you suffer from a bleeding disorder or you are taking blood-thinning medication such as warfarin (Coumadin) or aspirin.
Following Up After the Test
Even with an abnormally high result, your doctor my recommend further tests to diagnose Cushing’s syndrome. If this disorder is diagnosed, you will be given appropriate medications to control your high cortisol levels.
If your high cortisol levels are being caused by a form of cancer, your doctor will recommend further tests to determine the type of tumor and the treatment required.
If your high cortisol levels are caused by other disorders, your doctor may recommend another course of treatment.