A syndrome connected with (mainly) serotonin secreting tumours in certain locations (mainly small intestine, lung, stomach, appendix, rectum). The key symptoms include diarrhoea, flushing of the skin (particularly the face), stomach cramping, heart problems such as palpitations, and wheezing. The syndrome is actually caused by the release of a number of hormones, in particular Serotonin, Bradykinin, Tachykinin (Substance P), Histamine, and Prostaglandins.
Carcinoid syndrome occurs when biologically active amines and peptides enter the systemic circulation escaping the first pass metabolism of the liver. Normally, these bioactive products are inactivated by the liver. However, in cases of neuroendocrine tumors with liver metastasis, either these bioactive products are directly released into the systemic circulation, or they escape inactivation due to deranged liver function.