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This test is performed when the health care provider suspects that your body is producing too much cortisol.
The low-dose test can help tell whether your body is producing too much cortisol. The high-dose test can help determine whether the problem is in the pituitary gland (Cushing's Disease).
The level of cortisol in the blood normally regulates the release of ACTH from the pituitary gland. As blood cortisol levels increase, ACTH release decreases. As cortisol levels decrease, ACTH increases.
Dexamethasone is a man-made (synthetic) steroid that is similar to cortisol. It reduces ACTH release in normal people. Therefore, taking dexamethasone should reduce ACTH levels and lead to decreased cortisol levels.
If your pituitary gland produces too much ACTH, you will have an abnormal response to the low-dose test, but a normal response to the high-dose test.
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