Dexamethasone suppression test measures the response of the adrenal glands to ACTH.

During this test, you will receive dexamethasone, and the health care provider will measure your cortisol levels.

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.

• Low-dose overnight method -- you will get 1 mg of dexamethasone at 11 p.m., and a health care provider will draw your blood at 8 a.m. for a cortisol measurement.
• Standard low-dose method -- urine is collected over 3 days (stored in 24-hour collection containers) to measure cortisol. On day 2, you will get a low dose (0.5 mg) of dexamethasone by mouth every 6 hours for 48 hours.
• High-dose overnight method -- the health care provider will measure your cortisol on the morning of the test. Then you will receive 8 mg of dexamethasone at 11 p.m. Your blood is drawn at 8 a.m. for a cortisol measurement.
• Standard high-dose test -- urine is collected over 3 days (stored in 24-hour collection containers) for measurement of cortisol. On day 2, you will receive a high dose (2 mg) of dexamethasone by mouth every 6 hours for 48 hours.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

The health care provider may advise you to stop taking drugs that may affect the test. Drugs that can affect test results include:

• Barbiturates
• Corticosteroids
• Estrogens
• Oral birth control (contraceptives)
• Phenytoin
• Spironolactone
• Tetracyclines

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.

This test is performed when the health care provider suspects that your body is producing too much cortisol.

The low-dose test can help tell whether your body is producing too much cortisol. The high-dose test can help determine whether the problem is in the pituitary gland (Cushing's disease).

The level of cortisol in the blood normally regulates the release of ACTH from the pituitary gland. As blood cortisol levels increase, ACTH release decreases. As cortisol levels decrease, ACTH increases.

Dexamethasone is a human-made (synthetic) steroid that is similar to cortisol. It reduces ACTH release in normal people. Therefore, taking dexamethasone should reduce ACTH levels and lead to decreased cortisol levels.

If your pituitary gland produces too much ACTH, you will have an abnormal response to the low-dose test, but a normal response to the high-dose test.

Cortisol levels should decrease after you receive dexamethasone.

Low dose:

• Overnight: 8 a.m. plasma cortisol < 1.8 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. Talk to your doctor about the meaning of your specific test results.

An abnormal response to the low-dose test may mean that you have abnormal release of cortisol (Cushing syndrome). This could be due to:

• Adrenal tumor that produces cortisol
• Pituitary tumor that produces ACTH
• Tumor in the body that produces ACTH

The high-dose test can help tell a pituitary cause (Cushing's disease) from other causes.

Abnormal results vary based on the condition causing the problem.

Cushing syndrome caused by an adrenal tumor:

• Low-dose test: no change
• High-dose test: no change

Cushing syndrome related to an ectopic ACTH-producing tumor:

• Low-dose test: no change
• High-dose test: no change

Cushing syndrome caused by a pituitary tumor (Cushing's disease)

• Low-dose test: no change
• High-dose test: normal suppression

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Stewart PM. The adrenal cortex. In: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR, eds. Williams Textbook of Endocrinology. 11th ed. Philadelphia, PA: Saunders Elsevier; 2008: chap 14.