dexamethasone suppression test
The dexamethasone suppression test checks whether the body's cortisol thermostat still works. Dexamethasone is a potent synthetic glucocorticoid; to the brain it looks just like cortisol, so giving a dose should trick the hypothalamus and pituitary into thinking cortisol is plentiful and shutting down their command to the adrenal glands. In a healthy person, the next morning's cortisol should be low.
If cortisol stays high despite the dexamethasone, the feedback brake has failed, which is the central clue to Cushing's syndrome — a state of chronic cortisol excess. The simplest version is the overnight 1 mg test: 1 milligram of dexamethasone taken at about 11 p.m., with cortisol measured at 8 a.m. the next day. A value above the cutoff is abnormal and prompts further work-up.
Higher-dose versions help locate the source. A pituitary tumor secreting ACTH usually retains partial sensitivity and is suppressed by a large dose, whereas cortisol from an adrenal tumor or from ACTH made by a tumor elsewhere in the body typically is not. The test is sensitive but not perfect: depression, alcohol, obesity, estrogen, and drugs that speed dexamethasone metabolism can all cause false results.
A 45-year-old with weight gain, purple stretch marks, and high blood pressure takes 1 mg dexamethasone at night; her 8 a.m. cortisol remains elevated, supporting a diagnosis of Cushing's syndrome.
Failure of cortisol to suppress after dexamethasone is the hallmark of Cushing's syndrome.