The overnight dexamethasone suppression test checks to see how taking a corticosteroid medicine (called dexamethasone) changes the levels of the hormone cortisol in the blood. This test checks for a condition in which large amounts of cortisol are produced by the adrenal glands (Cushing's syndrome).
Normally, when the pituitary glands make less adrenocorticotropic hormone (ACTH), the adrenal glands make less cortisol.
Dexamethasone, which is like cortisol, decreases the amount of ACTH released by the pituitary gland, which in turn decreases the amount of cortisol released by the adrenal glands.
After taking a dose of dexamethasone, cortisol levels often stay abnormally high in people who have Cushing's syndrome. Sometimes other conditions (such as major depression, alcoholism, stress, obesity, kidney failure, pregnancy, or uncontrolled diabetes) can keep cortisol levels from going down after taking a dose of dexamethasone.
The night before the blood test, you will take a pill containing dexamethasone. The next morning, the cortisol level in your blood will be measured. If your cortisol level remains high, Cushing's syndrome may be the cause.
An ACTH test is sometimes done at the same time as the cortisol test.
Why It Is Done
An overnight dexamethasone suppression test is done to check for a condition in which large amounts of cortisol are produced by the adrenal glands (Cushing's syndrome).
How To Prepare
You will not be able to eat or drink anything for 10 to 12 hours before the morning blood test.
Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. You may be asked to stop taking some medicines (such as birth control pills, aspirin, morphine, methadone, lithium, monoamine oxidase inhibitors [MAOIs], and diuretics) for 24 to 48 hours before your blood is drawn.
Talk to your doctor about any concerns you have about the need for the test, its risks, how it will be done, or what the results will mean. To help you learn about this test and how important it is, fill out the medical test information form.
How It Is Done
The night before your blood is drawn (usually at 11:00 p.m.), you will swallow a pill containing 1 milligram (mg) of dexamethasone. The next morning (usually at 8:00 a.m.), a health professional will draw a sample of your blood. Take the pill with milk or an antacid to help prevent an upset stomach or heartburn.
The health professional drawing blood will:
Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
Clean the needle site with alcohol.
Put the needle into the vein. More than one needle stick may be needed.
Attach a tube to the needle to fill it with blood.
Remove the band from your arm when enough blood is collected.
Put a gauze pad or cotton ball over the needle site as the needle is removed.
Put pressure to the site and then put on a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
Risks
Risks of a blood test
There is very little chance of a problem from having blood sample taken from a vein.
You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
Bruising may be more likely in people with high ACTH and cortisol levels.
Results
The overnight dexamethasone suppression test involves taking a dose of a corticosteroid medicine called dexamethasone to see how it affects the level of a hormone called cortisol in the blood. This test screens for Cushing's syndrome, a condition in which excess amounts of cortisol are being produced by the adrenal glands. Test results are usually available in 2 to 3 days.
The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Normal
Overnight dexamethasone suppression test Normal:
Cortisol level is less than 5 micrograms per deciliter (mcg/dL) or less than 138 nanomoles per liter (nmol/L).
High values
High cortisol levels may be caused by:
Cushing's syndrome.
Other health problems, such as a heart attack or heart failure, fever, poor diet, an overactive thyroid gland (hyperthyroidism), depression, anorexia nervosa, uncontrolled diabetes, or alcoholism.
Cancers that make ACTH, such as lung cancer.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
Pregnancy or extreme obesity.
Severe weight loss, dehydration, or acute alcohol withdrawal.
Severe injury.
Diabetes.
You take medicines, such as barbiturates, phenytoin (Dilantin), birth control pills, aspirin, morphine, methadone, lithium, monoamine oxidase inhibitors (MAOIs), spironolactone (Aldactone), and diuretics.
Some people may quickly process (metabolize) the dose of dexamethasone. In these people, cortisol levels will not drop unless a higher dose of the medicine is given.
What To Think About
An overnight dexamethasone suppression test is used to check for Cushing's syndrome. An abnormal test result may mean that further testing is needed to identify Cushing's syndrome. Likewise, a normal test result means that you do not have Cushing's syndrome. Cushing's syndrome can be hard to diagnose, so an endocrinologist should be consulted if test results are uncertain or if the test results do not help explain your symptoms.
Sometimes a more extensive dexamethasone suppression test may be done. For this test, you will take up to 8 dexamethasone pills over 2 days and then cortisol levels in your blood and urine will be measured.
Some doctors think that a 24-hour urine free cortisol test is more accurate than an overnight dexamethasone suppression test. Like an overnight dexamethasone suppression test, a 24-hour urine free cortisol test is used to look for Cushing's syndrome. For more information, see the topic Cortisol in Urine.
An adrenocorticotropic hormone (ACTH) test may be done at the same time as the cortisol test. For more information, see the topic Adrenocorticotropic Hormone.
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