Carcinoid syndrome is a set of symptoms that may occur in patients who have carcinoid tumors (neuroendocrine tumors, or NET, often in the gastrointestinal [GI] tract or lungs). Not all people with carcinoid tumors have carcinoid syndrome. Carcinoid tumors may arise in many locations, but they are most commonly found in the GI tract or the lungs.

https://www.carcinoid.com/carcinoid-syndrome/disease-overview/