1,3,7-Trimethyluric acid-d9 is intended for use as an internal standard for the quantification of 1,3,7-trimethyluric acid by GC- or LC-MS. 1,3,7-Trimethyluric acid is a derivative of uric acid and a metabolite of caffeine. It is formed from caffeine by the cytochrome P450 isoform CYP3A4. 1,3,7-Trimethyluric acid scavenges hydroxyl radicals in a cell-free assay and inhibits t-butyl hydroperoxide-induced lipid peroxidation by 56.5% in isolated human erythrocyte membranes.