Carcinoid syndrome refers to a characteristic set of signs and symptoms including:
Other possible features include fibrosis (thickening and scarring) of the pleural, peritoneal and retroperitoneal spaces (potential spaces around the lungs, the guts and behind the gut respectively).
Pellagra, a condition 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 'What is a carcinoid tumour?'), greatly reducing the conversion of tryptophan into the compound niacin. Niacin is a molecule used in wide variety of metabolic reactions and deficiency in it causes pellagra.
The syndrome is caused by carcinoid tumours (see 'What is a carcinoid tumour?') 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.