POMC and Cushing syndrome: In addition, sometimes MTC can secrete other peptides such as serotonin (which can produce a carcinoid syndrome), corticotropin-releasing hormone CRH, adrenocorticotropic hormone ACTH (causing Cushing's syndrome), histamine, neurotensin, somatostatin, and various gastrointestinal hormones such as chromogranin A. It also produces carcinoembryonic antigen (CEA) which, although less sensitive and specific than calcitonin, is also useful in the follow-up of MTC.