A rare and slow-growing form of cancer, carcinoid tumors may develop anywhere in the body where neuroendocrine (hormone-producing) cells exist. Roughly 50% of such tumors form in the gastrointestinal tract, and about 30% occur in the lungs. Only a very small percentage of the estimated 11,000 cases diagnosed each year in the U. S. grow elsewhere in the body.

Carcinoid tumors and other NETs usually originate in hormone-producing cells that line the small intestine or other cells of the digestive tract. They can also occur in the pancreas, testes, ovaries, or lungs. Carcinoid tumors can produce an excess of hormone like substances, such as serotonin, bradykinin, histamine, and prostaglandins. Excess levels of these substances can sometimes result in a diverse set of symptoms called carcinoid syndrome. Other NETs can produce other hormonal substances causing a variety of other syndromes.

When carcinoid tumors occur in the digestive tract or pancreas, the substances they produce are released into a blood vessel that flows directly to the liver (portal vein), where enzymes destroy them. Therefore, carcinoid tumors that originate in the digestive tract generally do not produce symptoms unless the tumors have spread to the liver. The hormones secreted by other NETs, particularly those in the pancreas, do not necessarily require spread to the liver to cause symptoms.

When carcinoid tumors have spread to the liver, the liver is unable to process the substances before they begin circulating throughout the body. Depending on which substances are being released by the tumors, the person will have the various symptoms of carcinoid syndrome, insulinoma syndrome, Zollinger Ellison syndrome, VIPoma syndrome, etc. Carcinoid tumors of the lungs, testes, and ovaries also cause symptoms without having spread, because the substances they produce bypass the liver and can sometimes circulate widely in the bloodstream.