The cluster of symptoms—flushing, diarrhea, wheezing—caused by a neuroendocrine tumor releasing too much serotonin into the bloodstream.Carcinoid syndrome occurs when neuroendocrine tumors (typically with hepatic metastases) secrete serotonin, histamine, tachykinins, and prostaglandins into systemic circulation, bypassing hepatic first-pass metabolism. Symptoms include episodic flushing, secretory diarrhea, bronchospasm, and carcinoid heart disease (endocardial fibrosis). Somatostatin analogs (octreotide, lanreotide) are first-line treatment, suppressing hormone secretion via SSTR2 activation on tumor cells. Rescue octreotide injections manage acute carcinoid crises.