Carcinoid syndrome refers to a characteristic set of signs and symptoms including:
Other possible features include fibrosis and scarring of the pleura (spaces around the lungs), and peritoneal and retroperitoneal spaces of the gastrointestinal tract.
Pellagra, a condition caused by a deficiency in vitamin B3 or otherwise known characterised by dermatitis, diarrhoea and depression may also occur. This is because the carcinoid cells convert the essential amino acid tryptophan into a variety of hormones (see section 'What is a carcinoid tumour?'). By the conversion of trypophan into a variety of hormones, this greatly reduces the conversion of tryptophan into niacin, causing the deficiency.
The syndrome is caused by carcinoid tumours arising in the liver (normally as secondary lesions) or outside the gut, that secrete chemicals into the bloodstream, producing such symptoms.
Carcinoid tumours affect approximately 2/100,000 of the population. Only 10% of these actually cause carcinoid syndrome. They tend to present from middle age onwards.