Dexamethasone Suppression Tests

Dexamethasone suppression tests have been employed for more than 30 years in the evaluation of patients with Cushing’s syndrome. Several variations have been developed since Dr. Grant Liddle of Vanderbilt University first reported on the usefulness of the classic test in 1960.

The adrenal glands produce cortisol, the major glucocorticoid hormone in man, in response to ACTH secreted 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. Dexamethasone usually fails to suppress the function of these endocrine structures in patients with Cushing’s disease or Cushing’s syndrome. These observations form the basis for the multiple variations of the dexamethasone suppression test.

• Overnight 1 mg 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 taken with the medication. You should report to the specified laboratory or physician’s office so that a sample of blood can be obtained for determination of the serum cortisol level at 8:00 AM the very next morning.
• Formal Low-Dose Dexamethasone Suppression Test This procedure is often employed as a diagnostic test in the diagnosis of Cushing’s syndrome. The test requires precise timing and collection of urine samples as well as the ingestion of Dexamethasone at specified time intervals. We may request that you collect several "basal" 24-hour urine samples for determination of free cortisol, 17-hydroxycorticosteroids, and creatinine. The official test begins on day one with the administration of the first of 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 two. Most importantly, 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 two, coinciding with the 5th dose of the medication. The urine collection should be completed at 6:00 AM on day three. Please review the instructions for 24-hour urine collection. Contact us if you have any questions.
• Dexamethasone-suppressed CRH Stimulation Test This test is often employed in the diagnosis of Cushing’s syndrome. It is rather unique in that it combines suppression and stimulation of the pituitary gland in order to gain additional information about the altered function of the endocrine system 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 three. You will be required to report to our office by 8:00 AM for a CRH Stimulation Test on day three. This test involves placement of an i.v. followed by injection of the hypothalamic hormone CRH and then sampling of blood for cortisol 15 minutes later.
• Overnight 8 mg High-Dose Dexamethasone Suppression Test This test is often employed to figure out why someone has Cushing’s syndrome. It is designed to distinguish between Cushing’s syndrome caused by pituitary tumors and non-pituitary tumors. You should arrive at the specified laboratory or other location to have blood obtained for measurement of serum cortisol and ACTH at 8:00 AM on day one. Take Dexamethasone, 8 mg, by mouth at 11:00 PM on day one. Present to the same facility at 8:00 AM on day two to have another sample of blood drawn for measurement of serum cortisol and ACTH.