To differentiate causes of Cushing's syndrome after initial screening, which test is commonly used?

To differentiate causes of Cushing's syndrome after initial screening, which test is commonly used?

• A. Low-Dose Dexamethasone Suppression Test.
• B. Urine Free Cortisol.
• C. High-Dose Dexamethasone Suppression Test.
• D. Serum Midnight Cortisol.

Answer: (C)

After initial screening confirms excess cortisol, the high-dose dexamethasone suppression test is used to distinguish pituitary-driven Cushing from ectopic ACTH production. The idea is that a pituitary adenoma often retains some feedback control and will suppress cortisol when exposed to a large dose of dexamethasone, whereas ectopic ACTH-secreting tumors do not show this suppression. Adrenal (ACTH-independent) sources typically don’t suppress either, but this test is most useful for telling pituitary from ectopic origins. The other tests—low-dose dexamethasone for screening, urine free cortisol for confirming hypercortisolism, and midnight serum cortisol for evaluating diurnal rhythm—do not differentiate the source in the same way.

After initial screening confirms excess cortisol, the high-dose dexamethasone suppression test is used to distinguish pituitary-driven Cushing from ectopic ACTH production. The idea is that a pituitary adenoma often retains some feedback control and will suppress cortisol when exposed to a large dose of dexamethasone, whereas ectopic ACTH-secreting tumors do not show this suppression. Adrenal (ACTH-independent) sources typically don’t suppress either, but this test is most useful for telling pituitary from ectopic origins. The other tests—low-dose dexamethasone for screening, urine free cortisol for confirming hypercortisolism, and midnight serum cortisol for evaluating diurnal rhythm—do not differentiate the source in the same way.