Over 90% of carcinoid tumors originate in the gastrointestinal tract. The most frequent sites are the appendix, terminal ileum and rectum. In general, non- metastasized carcinoid tumors are asymptomatic. The carcinoid syndrome is associated only with carcinoid tumors that have metastasized extensively to the liver or are extraintestinal (e.g., lung tumors). Metastasis is infrequent in carcinoids of the appendix, but is common in extra-appendiceal carcinoids.

Although carcinoid tumors differ in their ability to produce and store 5- hydroxytryptamine (5-HT), the excessive production of this substance and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) remains their most characteristic chemical abnormality. Production of this hormone (as well as histamine, catecholamines, kinase and prostaglandins) causes the majority of symptoms. The symptom complex comprises diarrhea, flushing, wheezing, cluster headache, valvular heart disease (particularly pulmonary stenosis) and a pellagra-like skin rash. The carcinoid syndrome can be suspected clinically and confirmed biochemically by the demonstration of increased urinary 5-HIAA or platelet 5-HT.

Once the carcinoid syndrome is apparent, cure is usually impossible, since the tumor has metastasized by this time. Nevertheless, the intestinal origin of the tumor should be removed if it is causing obstruction. Serotonin antagonists (e.g., methysergide and cyproheptadine) can sometimes reduce symptoms. The somatostatin analogue octreotide may prove to be very effective in reducing the patient's symptoms; interferon may also prove to be useful. It is prudent to delay initiation of chemotherapy or radiation in the early metastatic stage of the disease, since the course is often indolent and patients survive many years with diffuse metastatic disease.