Carcinoid tumors arise from enterochromaffin cells typically located in the gastrointestinal tract or lungs. At the time of diagnosis, 20 to 30 percent of patients have disseminated disease and the carcinoid syndrome, characterized by cutaneous vasomotor flushing, secretory diarrhea, and bronchospasm.
The carcinoid syndrome is caused by the tumor's release of serotonin and other vasoactive substances. Once released, serotonin is metabolized by monoamine oxidases in the liver, lungs, and brain to 5-hydroxyindoleacetic acid (5-HIAA). When vasoactive substances are released from hepatic metastases, the right heart is exposed to high levels of these substances. The exposure is believed to result in endocardial damage, leading to thickening, retraction, and fixation of the right heart valves, valvular dysfunction, and eventually, right heart failure. Although serotonin levels are higher in patients with carcinoid tumors who have heart disease than in those without cardiac involvement it is unclear what factors are involved in the progression of the cardiac lesions. Right ventricular failure remains a major cause of morbidity and death in patients with carcinoid heart disease. Knowledge of the mechanisms involved in the progression of the cardiac lesions might lead to the development of treatments that attenuate the process. 
- Møller JE, Connolly HM, Rubin J, et al. Factors associated with progression of carcinoid heart disease. N Engl J Med. 2003 Mar 13;348(11):1005-15.