Carcinoid is a tumor of the enterochromaffin cells that normally synthesize 5-hydroxytrytophan and 5-hydroxytryptamine. The carcinoid syndrome is seen when there are significant metastases of the primary tumor in the liver. It is characterized by increased gastrointestinal motility and diarrhea, as well as by regular periodic flushing. These symptoms can be attributed to systemic release of large amounts of serotonin and can be controlled with inhibitors of tryptophan hydroxylase, such as p-chlorophenylalanine. The synthesis of 5-hydroxytryptamine in advanced carcinoid syndrome maybe so great that as much as 60% of the body's tryptophan metabolism proceeds by this pathway, compared with about 1% under normal conditions. A significant number of patients with advanced carcinoid syndrome develop clinical signs of pellagra, because of this diversion of tryptophan away from the oxidative pathway of metabolism.
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