Ectopic ACTH syndrome is seen most commonly with

* Ectopic ACTH production is predominantly caused by occult carcinoid tumors, most frequently in the lung, but also in thymus or pancreas. * Advanced small cell lung cancer can cause ectopic ACTH production. In rare cases, ectopic ACTH production has been found to originate from medullary thyroid carcinoma or pheochromocytoma, the latter co-secreting catecholamines and ACTH.