n. a tumour of the *argentaffin cells in the glands of the intestine (see apudoma). Carcinoids typically occur in the tip of the appendix and are among the commonest tumours of the small intestine. They may also occur in the rectum and other parts of the digestive tract and in the bronchial tree (bronchial carcinoid adenoma). Carcinoids sometimes produce 5-hydroxytryptamine (serotonin), prostaglandins, and other physiologically active substances, which are inactivated in the liver. If a gastrointestinal tumour has spread to the liver, excess amounts of these substances are released into the systemic circulation and the carcinoid syndrome results – flushing, headache, diarrhoea, bronchial constriction causing asthma-like attacks, and in some cases damage to the right side of the heart associated with fibrosis of the tricuspid valve. Bronchial carcinoids can give rise to the syndrome without metastasizing.
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