The dexamethasone suppression test diagnoses Cushing’s syndrome, caused by excess cortisol production. The test measures dexamethasone’s ability to suppress cortisol production, with a low-dose test used for diagnosis and a high-dose test for pinpointing the cause. The test involves blood and urine samples and has minimal risk. Abnormal cortisol levels can have different causes, so high-dose testing is done as a follow-up.
The dexamethasone suppression test is a blood test used to diagnose Cushing’s syndrome. This condition develops when the adrenal glands produce too much of a hormone called cortisol. Cushing’s syndrome is usually the result of pituitary gland dysfunction, but it can have other causes. There are two types of dexamethasone suppression tests: a high-dose test and a low-dose test. The low-dose test is used to diagnose Cushing’s syndrome, and the high-dose test helps pinpoint the cause of the increased cortisol levels.
Cortisol levels in the blood are partially regulated by the production of a pituitary hormone called adrenocorticotropic hormone, or ACTH. The relationship between ACTH and cortisol is called a negative feedback loop, because high levels of each substance reduce production of the other. This relationship is important in the dexamethasone suppression test.
Dexamethasone is a synthetic steroid hormone structurally and functionally similar to cortisol. When someone with a normally functioning pituitary gland takes dexamethasone, ACTH is reduced; this in turn reduces cortisol levels. The dexamethasone suppression test then measures the ability of dexamethasone to suppress cortisol production. In someone with a dysfunctional pituitary gland, the response to dexamethasone is abnormal. For example, if the pituitary produces too much ACTH, taking dexamethasone may have no effect on cortisol levels.
The dexamethasone suppression blood test can be done overnight or over a three-day period. In the nighttime test, a patient is given a dose of dexamethasone in the evening and a blood sample is taken for cortisol testing the next morning. In the three-day test, the patient has to collect his or her urine for three days after taking dexamethasone. In addition, he will receive additional doses of dexamethasone over the three days. The patient will receive high or low doses of dexamethasone depending on whether he undergoes the high or low dose suppression test.
Before the test, a patient may be asked to stop taking certain medications that could affect the test results. These include corticosteroids, estrogens, oral contraceptives and barbiturates. This test has very little risk, because the most invasive part of the procedure is a blood test. Possible side effects include excessive bleeding, infection, and hematoma. The risk of any of these events occurring is very low.
In most cases, a person will first be given a low-dose dexamethasone suppression test. The results of this test indicate whether a person has abnormal cortisol levels. Because abnormal cortisol levels can have different causes, high-dose testing is done as a follow-up. For example, if abnormal cortisol levels are caused by a pituitary tumor, a patient will have an abnormal reaction to a low-dose test and a normal reaction to a high-dose test. If the abnormal cortisol levels are due to an adrenal tumor, the patient will respond abnormally to both the low-dose test and the high-dose test.
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