Dexamethasone suppression tests have been used for more than 30 years in the evaluation of patients with Cushing's syndrome. It was Dr. Grant Liddle of Vanderbilt who first reported on the usefulness of the classical test in 1960.

The adrenal glands produce cortisol, the major glucocorticoid hormone, in response to signals provided by the pituitary gland. Dexamethasone is a synthetic glucocorticoid hormone. In normal subjects, daily doses of Dexamethasone in excess of 0.5-0.75 mg, can suppress the coordinated functions of the hypothalamus pituitary and adrenal glands. In patients with Cushing's disease, Dexamethasone often fails to suppress the function of the hypothalamus, pituitary, and adrenal glands. These observations form the basis for the five different Dexamethasone suppression tests:

• Overnight 1 mg low dose Dexamethasone Suppression Test
• Formal low dose Dexamethasone Suppression Test
• Dexamethasone-Suppressed CRH Stimulation Test
• Overnight high dose 8 mg Dexamethasone Suppression Test
• Formal two-day high dose Dexamethasone Suppression Test

Overnight 1 mg low dose Dexamethasone Suppression Test

This test is a reasonable screening test for Cushing's syndrome. Dexamethasone 1.0 mg should be taken by mouth at 11:00 PM. A snack can be consumed with the medication. The patient then reports to the specified laboratory or physician's office at 8:00 AM the very next morning so that a sample of blood can be drawn in order to determine the serum cortisol level.

Formal low dose Dexamethasone Suppression Test

This procedure is often used in the diagnosis of Cushing's syndrome. Proper performance of the test requires proper timing and collection of urine samples as well as the ingestion of Dexamethasone at specified time intervals. Your physician may request that you collect several "basal" (or baseline) 24-hour urine samples for determination of:

• free cortisol
• 17-hydroxycorticosteroids
• creatinine

The test begins on day one with the administration of the first 8 doses of Dexamethasone. You may be asked to take the first dose at 6:00 AM. If so, subsequent doses should be at 12:00 Noon, 6:00 PM, and 12:00 Midnight on day one, and then 6:00 AM, 12:00 Noon, 6:00 PM, and 12:00 Midnight on day 2. 

You may be asked to begin the medication at some other time. If so, you will be given explicit guidelines so that you understand when to take the subsequent doses of the medication. The most important thing to remember is that the medication should be taken every 6 hours for a total of 8 doses. You should begin the 24-hour urine collection at 6:00 AM on day 2, coinciding with the 5th dose of the medication. Then complete the urine collection on 6:00 AM on day 3.

Please review the instructions for the 24-hour urine collection. Contact your physician if you have any questions.

Dexamethasone-Suppressed CRH Stimulation Test

This test is often used in the diagnosis of Cushing's syndrome. It is rather unique because it combines suppression and stimulation of the pituitary gland. It does this to gain additional information about the altered functions of the hypothalamus, pituitary, and adrenal glands in patients with probable Cushing's syndrome. 

A formal low dose Dexamethasone suppression test is performed as specified above, except that a 9th dose of Dexamethasone is taken at 6:00 AM on day 3. You should report to your physician's office by 8:00 AM for a CRH Stimulation Test on day 3. This test involves placement of an i.v., followed by injection of the hypothalamic hormone CRH, then sampling of blood for cortisol 15 minutes later.

Overnight high dose 8 mg Dexamethasone Suppression Test

This test is often used in the differential diagnosis of Cushing's syndrome. It can help distinguish between Cushing's syndrome caused by pituitary tumors and non-pituitary tumors. You should report to the specified laboratory or other location to have blood drawn. This blood will be tested to measure serum cortisol and ACTH at 8:00 AM on day 1. Then, take Dexamethasone, 8 mg., by mouth at 11:00 PM on day 1. Return to the same facility at 8:00 AM on day 2 to have another blood sample drawn for measurement of serum cortisol and ACTH.

Formal two-day high dose Dexamethasone Suppression Test

This test is often used in the differential diagnosis of Cushing's syndrome. It has largely been replaced by the high dose 8 mg overnight Dexamethasone suppression test (described above). The schedule is as follows:

• Day 1: Begin a 24-hour urine collection at 6:00 AM.
• Day 2: Finish the first urine collection at 6:00 AM and start the second 24-hour urine collection.
• Day 3: Finish the second 24-hour urine collection at 6:00 AM and start the third collection. Start Dexamethasone 0.5 mg. at 6:00 AM and take subsequent doses at 12:00 Noon, 6:00 PM, and 12:00 Midnight.
• Day 4: Finish the third 24-hour urine collection at 6:00 AM and start the fourth collection. Continue Dexamethasone 0.5 mg. at 6:00 AM, 12:00 Noon, 6:00 PM, and 12:00 Midnight.
• Day 5: Complete the fourth 24-hour urine collection at 6:00 AM and start the fifth collection. Begin Dexamethasone 2.0 mg. at 6:00 AM and take subsequent doses at 12:00 Noon, 6:00 PM, and 12:00 Midnight.
• Day 6: Finish the fifth 24-hour urine collection at 6:00 AM and begin the sixth collection. Continue Dexamethasone 2.0 mg at 6:00 AM, 12:00 Noon, 6:00 PM, and 12:00 Midnight.
• Day 7: Complete the sixth 24-hour urine collection at 6:00 AM. Transport the specimens to the designated facility for testing.

You should make sure that all collection bottles are labeled with your name and start date of the collection. As an extra precaution, it's a good idea to indicate the day of the collection (day 1, day 2, etc.)