Cushing's Disease Diagnosis
Dexamethasone Suppression Test
Cushing's disease testing can include the dexamethasone suppression test. This test helps to distinguish patients with excess production of ACTH due to pituitary adenomas from those with ectopic ACTH-producing tumors.
For this Cushing's disease 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 given. 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 reduce secretion of ACTH, the normal response after taking dexamethasone is a drop in blood and urine cortisol levels.
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.
CRH Stimulation Test
Cushing's disease testing can also include the CRH stimulation test. CRH stands for corticotropin-releasing hormone. 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, which causes the pituitary to secrete ACTH. Patients with pituitary adenomas usually experience a rise in blood levels of ACTH and cortisol.
Imaging tests (such as CT and MRI scans) can reveal the size and shape of the pituitary and adrenal glands, and help determine if a tumor is present. A magnetic resonance imaging (MRI) scan is the most sensitive test for detecting pituitary tumors and determining their size. MRI scans may be repeated periodically to assess tumor progression and the effects of pituitary tumor treatment. Computed tomography (CT) scans also provide an image of the pituitary, but they are less sensitive than the MRI.