Carcinoid syndrome occurs due to release of serotonin (5-hydroxytryptamine) and other vasoactive peptides into the systemic circulation from a carcinoid tumour. Twenty percent to 30% of midgut neuroendocrine tumours (NETs), 5% of bronchial carcinoid tumours, and approximately 1% of pancreatic NETs secrete 5-hydroxytryptamine and other peptides.

