Paraneoplastic Endocrine Changes in Gastrointestinal Tumors: A Clinical and Mechanistic Review

Paraneoplastic endocrine syndromes (PESs) are hormonal disturbances associated with malignancies that result from tumor-related production of hormone-like substances, immune-mediated mechanisms, or dysregulated signaling pathways. While they are well recognized in lung and neuroendocrine cancers, their relevance in gastrointestinal tumors remains less clearly defined. This narrative review synthesizes current knowledge on paraneoplastic endocrine manifestations in gastrointestinal malignancies, based on a structured search of the literature in major databases, including PubMed, Scopus, and Web of Science. The analysis focuses on clinically relevant syndromes such as hypercalcemia, Cushing-like manifestations, disorders of water balance, hypoglycemia, and acromegaly, with emphasis on underlying mechanisms, associated tumor types, diagnostic approaches, and therapeutic considerations. Available evidence indicates that gastrointestinal tumors can produce a range of biologically active substances, leading to diverse endocrine manifestations that may precede tumor detection and influence disease course. Among these, hypercalcemia and Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) are among the most frequently reported, while other syndromes, such as ectopic Cushing syndrome or tumor-related hypoglycemia, are less common but often associated with more severe clinical outcomes. Recognition of these manifestations has direct clinical implications, as they may support earlier diagnosis, contribute to prognostic assessment, and guide therapeutic management. Improved awareness and a multidisciplinary approach remain essential for optimizing outcomes in patients with gastrointestinal malignancies.