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Distinguishing between pituitary Cushing and ectopic ACTH syndromes can be difficult, especially when the source of ectopic ACTH is an indolent tumor. This retrospective study from the NIH assessed metyrapone and high-dose dexamethasone testing in 170 patients with eventually confirmed pituitary Cushing syndrome and in 15 patients with ectopic ACTH syndrome (11 of whom had carcinoid tumors).
On metyrapone testing, 71 percent of pituitary Cushing cases, but no ectopic ACTH cases, had a more than 400-fold rise in plasma 11-deoxycortisol or a greater than 70 percent rise in urinary 17-hydroxysteroid. On dexamethasone testing, the criteria of suppression of urinary 17-hydroxysteroid by more than 69 percent or urinary free cortisol by more than 9…