Cushing's disease occurs sufferers have high levels of cortisol because they have a pituitary adenoma. An adenoma or a benign tumor, has structures which look like a gland. This pituitary adenoma is located in the anterior pituitary gland. It does not invade any adjoining structures and usually proves so small, that it does not press on other tissues either.
Overnight Dexamethasone Suppression
The first test for Cushing's disease includes the overnight dexamethasone suppression test. In Cushing's disease, the pituitary adenoma releases high levels of the adrenocorticotropic hormone, or ACTH. The levels become high because the adenoma releases the hormone in an uncontrolled manner. ACTH tells the adrenal gland to release cortisol. This explains why high levels of cortisol occur in Cushing's disease, says Paul Fitzgerald, M.D., Clinical Professor of Medicine at the University of California in "Current Medical Diagnosis & Treatment." In the dexamethasone suppression test, people drink 1 mg of dexamethasone at 11 p.m. Physicians take a sample of their blood at 8 a.m. the next day to check the cortisol level. Dexamethasone is the same type of hormone as cortisol. Normally, taking dexamethasone will tell the anterior pituitary to stop releasing ACTH. The adrenal gland will then release less levels of cortisol. If the test results have cortisol levels below a certain amount, this is a normal result. If the levels prove higher than the cutoff, another test usually proves necessary.
24-Hour Urinary Free Cortisol or UFC
In this test, people must collect their urine for 24 hours. The lab will then measure how much cortisol exists in the urine sample. A normal test result has cortisol levels within a certain range. If the levels of cortisol in the sample prove higher than the cutoff, this is an abnormal result. The person now has the diagnosis of Cushing's syndrome. Nevertheless, Cushing's syndrome is not the same as Cushing's disease. Cushing's syndrome describes any disorder where there high levels of cortisol exist in the bloodstream, according to Elizabeth Corwin, Ph.D. in "Handbook of Pathophysiology." The next step in the test for Cushing's disease includes determining why cortisol levels become high. Reasons for this include a pituitary adenoma or Cushing's disease or another type of disorder.
High-Dose Dexamethasone Suppression
The overnight dexamethasone suppression test is a low-dose test. In the high-dose test, people drink 4 mg or 8 mg of dexamethasone at 11 p.m. At 8 a.m. the next day, a blood sample will check the cortisol level. If the levels fall below a certain amount, then the person has Cushing's disease. Ashley Grossman, M.D., Professor of Neuroendocrinology at the London School of Medicine writes in "The Merck Manual for Healthcare Professionals," that the ACTH levels are also checked.
CT or MRI
The CT or MRI imaging tests view the pituitary adenoma, according to Dr. Grossman in "The Merck Manual for Healthcare Professionals." The tumor usually proves so small in children, it cannot than an MRI cannot see it. Physicians use an MRI in pregnant women, so the fetus does not become exposed to radiation.
References
- "Current Medical Diagnosis & Treatment 2010"; Stephen McPhee, M.D., Maxine Papadakis, M.D.; 2010
- "Handbook of Pathophysiology"; Elizabeth Corwin, MSN, Ph.D., FNP; 2000
- The Merck Manual for Healthcare Professionals: Cushing's Syndrome
