Posted in Web Carcinoid Syndrome – Endocrine and Metabolic Disorders … January 13, 2021 Carcinoid syndrome is usually due to endocrinologically active malignant tumors that develop from neuroendocrine cells (mostly in the ileum—see Small Bowel Tumors) and produce serotonin. https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/carcinoid-tumors/carcinoid-syndrome