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

How 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 at 8 A.M. for a cortisol measurement (see venipuncture). 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.

How To Prepare: 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:

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

How It Feels: 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.

Risks:

• 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

Why Performed: 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; giving dexamethasone should reduce ACTH levels resulting in decreased cortisol levels.

Normal Values: low dose: less than 50% reduction of plasma cortisol and 17-hydroxycorticosteroid (17-OCHS)

high dose: more than 50% reduction of plasma cortisol and 17-OCHS

Abnormal Results: In Cushing’s syndrome related to bilateral adrenal hyperplasia, the pituitary gland is reset upward and responds only to high plasma levels of cortisol (or dexamethasone). In Cushing’s syndrome related to adrenal adenoma (tumor) or cancer (which acts autonomously), cortisol secretion continues in spite of a decrease in ACTH. A similar situation is seen when Cushing’s syndrome is caused by an ectopic ACTH-producing tumor.

Cushing’s syndrome related to bilateral adrenal hyperplasia

• low dose: no change
• high dose: > 50% reduction of plasma cortisol and 17-OCHS

• low dose: no change
• high dose: no change

• low dose: no change
• high dose: no change

• ectopic Cushing’s syndrome
• pituitary Cushing’s (Cushing’s disease)
• thyroid cancer

Cost:

Special Considerations: Stress can cause ACTH release, which can reduce the accuracy of this test.