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This test is performed when adrenal disease is suspected; it can differentiate adrenal disease (altered response to ACTH) from pituitary disorders (altered production of ACTH). 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. Administering dexamethasone should reduce ACTH levels resulting in decreased cortisol levels.
How The Test Is Performed
There are basically 2 ways to give the test: the rapid method and the 6-day method.
In the rapid method, 1 mg of dexamethasone is given at 11 P.M., and the blood is drawn (venipuncture) at 8 A.M. for a cortisol measurement. If no decrease in the amount of cortisol is seen, the test is repeated with an 8-mg dose of dexamethasone.
In the 6-day method, urine is collected at 24-hour intervals over 6 days for measurement of cortisol or 17-OCHS. On day 3, a low dose (0.5 mg) of dexamethasone is given by mouth every 6 hours for 24-hours. On day 5, the procedure is repeated with a high dose (2.0 mg) of dexamethasone.
Venipuncture (blood drawing)
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. Some patients have very small veins, moving veins, or deep veins and puncturing them might be very difficult even in hands of an experienced venipuncturist and repeated attempts may be necessary.
How To Prepare For The Test
The health care provider may advise to discontinue drugs that could affect the test results. Drugs that can affect test results include barbiturates, estrogens, corticosteroids, oral contraceptives, phenytoin, spironolactone, and tetracyclines.
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