Overnight Dexamethasone Suppression Test
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 a few days.
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.
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.
Overnight dexamethasone suppression test
Cortisol level is less than 5 micrograms per deciliter (mcg/dL) or less than 138 nanomoles per liter (nmol/L).
High cortisol levels may be caused by:
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.
- You take medicines, such as barbiturates, phenytoin (Dilantin), birth control pills, aspirin, morphine, methadone, lithium, monoamine oxidase inhibitors (MAOIs), spironolactone (Aldactone), or 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.