what is carcinoid syndrome

Carcinoid syndrome is a clinical condition that arises due to the systemic effects of bioactive substances, particularly serotonin and other vasoactive amines, secreted by neuroendocrine tumors (NETs), primarily those originating from the small intestine but occasionally from other sites such as the lungs PubMed. This syndrome manifests with symptoms including flushing, diarrhea, tachycardia, bronchial obstruction, and sweating, which result from the hormonal hypersecretion by functioning NETs that release these substances directly into the systemic circulation PubMed Oleinikov et al. 2019.

The syndrome typically occurs when NETs metastasize to the liver, allowing secretion of vasoactive substances into the systemic circulation without first being metabolized by the liver PubMed. About 30%–40% of patients with small intestinal NETs present with carcinoid syndrome, where carcinoid-related symptoms significantly worsen quality of life and may lead to complications such as carcinoid heart disease (CHD), a serious and potentially fatal fibrotic cardiac valve pathology affecting mainly the right heart valves PubMed Arbit B 2025.

Carcinoid syndrome symptoms include episodic flushing (visible erythema), profuse sweating, diarrhea, tachycardia or paroxysmal tachyarrhythmias, bronchospasm, and dyspnea PubMed Oleinikov et al. 2019. Although classically associated with small intestinal NETs, carcinoid syndrome is rare in pulmonary carcinoids and typically occurs only when hepatic metastases are present; exceptions exist where pulmonary carcinoids induce carcinoid syndrome without liver metastases, albeit rarely and sometimes despite negative biochemical markers PubMed Chauhan et al. 2026.

The diagnosis of carcinoid syndrome relies on clinical assessment supported by biochemical testing for serotonin metabolites, primarily 24-hour urinary 5-hydroxyindoleacetic acid (5-HIAA), which reflects serotonin metabolism, and measurement of serum chromogranin A and other markers; however, biochemical tests can be negative in symptomatic patients due to factors such as tumor secretion of vasoactive substances other than serotonin or analytical limitations PubMed Chauhan et al. 2026. Therefore, clinical suspicion remains paramount especially in the presence of typical symptoms. Functional imaging and biopsy confirm diagnosis and guide management PubMed Sato et al. 2026.

The pathophysiology underlying carcinoid syndrome involves hypersecretion of serotonin and other substances like tachykinins, prostaglandins, and histamine that bypass normal hepatic degradation and cause systemic effects PubMed. These bioactive compounds contribute not only to symptoms but also to the development of CHD through serotonin-induced fibrotic plaque formation on cardiac valves, principally the tricuspid and pulmonary valves in the right heart PubMed Arbit B 2025.

Treatment focuses on controlling hormonal hypersecretion and tumor growth. Somatostatin analogues (SSA) such as octreotide or lanreotide are the mainstay of therapy, effectively reducing symptoms and biochemical markers by inhibiting secretion of vasoactive substances PubMed Oleinikov et al. 2019. Surgical resection of localized tumors can sometimes control symptoms PubMed Chauhan et al. 2026. In metastatic or progressive disease, additional therapies including peptide receptor radionuclide therapy (PRRT) and targeted agents may be employed PubMed Sato et al. 2026.