A Review on 5-Hydroxyindoleacetic Acid (5-HIAA) in Urine: Diagnostic and Therapeutic Implications for Carcinoid Tumours

5-Hydroxyindoleacetic acid (5-HIAA), the main urinary metabolite of serotonin, is a key biomarker in diagnosing and managing carcinoid tumours. These neuroendocrine tumours, often originating from enterochromaffin cells in the gastrointestinal tract, frequently secrete excess serotonin. After hepatic metabolism, serotonin converts to 5-HIAA, which is excreted in urine. Measuring urinary 5-HIAA provides a non-invasive, cost-effective, and reliable method for detecting functional carcinoid tumours and monitoring disease progression or response to therapy. This review discusses the clinical significance of 5-HIAA, its application in routine monitoring, and factors affecting test accuracy, including diet, medications, renal function, and tumour burden. It also highlights improved detection techniques like high-performance liquid chromatography and LC-MS. Case studies illustrate 5-HIAA’s practical clinical value, while alternative biomarkers such as chromogranin A and pancreatic polypeptide are explored. The article concludes by addressing diagnostic challenges and future directions in neuroendocrine tumour biomarker research and clinical practice.