Occult ectopic secretion of corticotropin.

The clinical, biochemical, radiographic, and morphologic features of ectopic corticotropin (ACTH)-dependent Cushing's syndrome are often indistinguishable from those of Cushing's disease (pituitary-dependent Cushing's syndrome). We encountered ten patients whose ectopic ACTH-secreting neoplasms were not clinically apparent for two months to 12 years after the diagnosis of hypercortisolism or in whom the site remains unknown. Five of these patients underwent unnecessary pituitary microsurgery, and a sixth was referred for surgery. The occult ectopic ACTH syndrome occurs with equal frequency in men and women and hypokalemia is present in 60%, in contrast to the female predominance and rarity of hypokalemia in Cushing's disease. We emphasize the importance of selective venous sampling for ACTH to establish the correct diagnosis. Thirty-nine similar cases from the literature help characterize this syndrome further.