Carcinoid Tumor grow slowly and rarely cause any signs or symptoms until late in the disease. Even under a microscope a carcinoid tumor can appear to be noncancerous (benign), even though it may have already spread. Carcinoid tumors can also produce and release hormones into your body that cause signs and symptoms, such as diarrhea or skin flushing.

The prevalence of carcinoid tumors is difficult to determine since many carcinoid tumors are not detected because they produce no symptoms. In one autopsy series, carcinoid tumors were estimated to have a prevalence of 8/100,000 people annually, but 90% of the carcinoids found at autopsy were not the cause of death.

The carcinoid syndrome is very rare. This is because many carcinoid tumors do not produce hormones. Another reason that the syndrome is rare is that hormones released by carcinoid tumors, particularly from tumors within the abdomen, often are destroyed by the liver before they reach the general circulation to cause symptoms. For example, hormones produced by small intestinal carcinoid tumors are released into the portal vein. The portal vein blood passes through the liver before reaching the heart and the general circulation. As the portal vein blood passes through the liver, the hormones are destroyed by the liver.

Only those carcinoid tumors that can release hormones directly into the general circulation (and not into the portal veins which pass through the liver) can produce the carcinoid syndrome. Thus, the most common cause of carcinoid syndrome is small intestinal carcinoid tumors that have metastasized to the liver. The metastases in the liver can release hormones directly into the circulation. Another rare example is carcinoid tumors of the bronchial airways. Carcinoid tumors in the bronchial airways can release hormones directly into the general circulation via the pulmonary veins without passing through the liver.