Used as a stable isotope-labeled internal standard in mass spectrometry-based assays for the accurate quantification of 5-hydroxyindoleacetic acid (5-HIAA) in biological samples such as urine, plasma, and cerebrospinal fluid. Its primary application is in clinical diagnostics and research settings to measure serotonin metabolism, particularly in the detection and monitoring of neuroendocrine tumors like carcinoid tumors. The deuterated form provides high analytical precision by compensating for matrix effects and instrument variability during sample preparation and analysis.