Patient Education - Endocrine Encyclopedia
Endocrine Surgery Encyclopedia
Definition:
Dexamethasone suppression test measures the response of the adrenal glands to ACTH. 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 an 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 3 days (stored in 24-hour collection
containers) 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, then 8 mg of dexamethasone is given at 11 p.m. Blood is drawn at 8 a.m. for a cortisol
measurement. For the standard high-dose test, urine is collected over 3 days (stored in 24-hour
collection containers) 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 stop taking drugs that may affect the test. Drugs
that can affect test results include barbiturates, estrogens, corticosteroids, oral contraceptives,
phenytoin, spironolactone, and tetracyclines.
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 blood plasma. ACTH stimulates the adrenal cortex to produce cortisol. As plasma cortisol
levels increase, ACTH secretion is suppressed. As cortisol levels decrease, ACTH increases.
Dexamethasone is a synthetic steroid similar to cortisol, which suppresses ACTH secretion
in normal people. Therefore, 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
Normal value ranges may vary slightly among different laboratories.
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
Special considerations:
There are none.
Review Date: 3/8/2006
Reviewed By: Robert Hurd, M.D., Department of Biology, College of Arts and Sciences, Xavier
University, Cincinnati, OH. Review provided by VeriMed Healthcare Network.
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