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Dexamethasone suppression test
Definition:
A test that measures the response of the adrenal glands to ACTH, in which dexamethasone is given and levels of cortisol are measured; cortisol levels should decrease in response to the administration of dexamethasone.

Alternative Names:
DST; ACTH suppression test; Cortisol suppression test

How the test is performed:

There are two different types of dexamethasone suppression tests: the low dose test and the high dose test. Each type can either be done in a overnight or standard (3 day) way.

For the low dose, overnight method, 1 mg of dexamethasone is given at 11 P.M., and the blood is drawn at 8 A.M. for a cortisol measurement (see venipuncture).
In the standard, low dose method, urine is collected at 24-hour intervals over 3 days for measurement of cortisol. On day 2, a low dose (0.5 mg) of dexamethasone is given by mouth every 6 hours for 48-hours.

For the high dose overnight method, a baseline cortisol is measured on the morning of the test and then 8 mg of dexamethasone is given at 11 P.M.. Blood is drawn at 8 A.M. for a cortisol measurement (see venipuncture). For the standard high dose test, urine is collected at 24-hour intervals over 3 days for measurement of cortisol. On day 2, a high dose (2 mg) of dexamethasone is given by mouth every 6 hours for 48-hours.
How to prepare for the test:
The health care provider may advise you to discontinue drugs that may affect the test. Drugs that can affect test results include barbiturates, estrogens, corticosteroids, oral contraceptives, phenytoin, spironolactone, and tetracyclines.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:

This test is performed when overproduction of cortisol is suspected. The low dose test can help differentiate healthy people from those who produce too much cortisol. The high dose test can help determine if the abnormality is in the pituitary gland (Cushing's Disease).

The secretion of ACTH from the pituitary gland is normally regulated by the level of cortisol in the plasma. ACTH stimulates the adrenal cortex to produce cortisol. As plasma cortisol levels increase, ACTH secretion is suppressed, as cortisol levels decrease and ACTH increases. Dexamethasone is a synthetic steroid similar to cortisol, which suppresses ACTH secretion in normal people; giving dexamethasone should reduce ACTH levels resulting in decreased cortisol levels. People with pituitary glands which produce too much ACTH will have an abnormal response to the low dose test, but a normal response to the high dose.
Normal Values:

Low Dose:

Overnight: 8 a.m. plasma cortisol< 5 mcg/dl

Standard: Urinary free cortisol on day 3 < 10 mcg/day

High Dose:

Overnight: > 50 % reduction in plasma cortisol

Standard: > 90% reduction in urinary free cortisol.
What abnormal results mean:

If there is not a normal response on the low dose test, abnormal secretion of cortisol is likely (Cushing's Syndrome). This could be a result of a cortisol producing adrenal tumor, a pituitary tumor that produces ACTH, or a tumor in the body that inappropriately produces ACTH. The high dose test can help distinguish a pituitary cause (Cushing's Disease) from the others.

Cushing's syndrome caused by adrenal tumor
  • low dose: no change
  • high dose: no change
Cushing's syndrome related to ectopic ACTH-producing tumor
  • low dose: no change
  • high dose: no change

Cushing's Syndrome caused by pituitary tumor (Cushing's Disease)

  • low dose: no change

  • high dose: normal suppression
What the risks are:
  • excessive bleeding
  • fainting or feeling lightheaded
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures to locate veins
Review Date: 3/14/2002
Reviewed By: Todd T. Brown, M.D., Division of Endocrinology and Metabolism, Johns Hopkins Hospital, Baltimore, MD. Review provided by VeriMed Healthcare Network.
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