Cushing's Syndrome Testing
Testing can include the dexamethasone suppression test. This test helps to distinguish patients with excess production of ACTH (adrenocorticotropin) due to pituitary adenomas from those with ectopic ACTH-producing tumors. It may also be used as an initial screening test in place of the 24-hour urinary free cortisol test.
For this test, patients are given dexamethasone, a synthetic glucocorticoid, by mouth every six hours for four days. For the first two days, low doses of dexamethasone are given, and for the last two days, higher doses are administered. Twenty-four-hour urine collections are made before dexamethasone is administered and again on each day of the test.
Since cortisol and other glucocorticoids signal the pituitary to lower the secretion of ACTH, the normal response after taking dexamethasone is a drop in blood and urine cortisol levels. Different responses of cortisol to dexamethasone are obtained, depending on whether the cause of Cushing's syndrome is a pituitary adenoma or an ectopic ACTH-producing tumor (see Cushing's Syndrome Causes).
The dexamethasone suppression test can produce false-positive results in patients with depression, alcohol abuse, high estrogen levels, acute illness, and stress. Conversely, drugs such as phenytoin and phenobarbital may cause false-negative results in response to dexamethasone suppression. For this reason, patients are usually advised by their physicians to stop taking these drugs at least one week before the test.
Cushing's syndrome testing can include the CRH stimulation test. This test helps to distinguish between patients with pituitary adenomas and those with ectopic ACTH syndrome or cortisol-secreting adrenal tumors.
For this test, patients are given an injection of CRH, the corticotropin-releasing hormone that causes the pituitary to secrete ACTH. Patients with pituitary adenomas usually experience a rise in blood levels of ACTH and cortisol. This response is rarely seen in patients with ectopic ACTH syndrome, and practically never in patients with cortisol-secreting adrenal tumors.