Febuxostat-d is intended for use as an internal standard for the quantification of febuxostat by GC- or LC-MS. Febuxostat is an antihyperuricemic nonpurine inhibitor of both the oxidized and reduced forms of xanthine oxidase. It inhibits bovine milk xanthine oxidase as well as mouse and rat liver xanthine oxidase/xanthine dehydrogenase (ICs = 1.4, 1.8, and 2.2 nM, respectively). It is 10-30 times more potent than the hypoxanthine analog allopurinol . Febuxostat decreases the serum level of urate in a potassium oxonate rat model of hyperuricemia (ED = 1.5 mg/kg). It reduces hepatic macrovesicular steatosis in mice fed a high-fat diet containing trans fatty acids when administered at a dose of 1 mg/kg per day. Febuxostat (0.75 mg/kg) also increases CNS expression of glutamate oxaloacetate transaminase 2 (GOT2) and improves neurological symptoms in a mouse model of secondary progressive experimental autoimmune encephalomyelitis (EAE). Formulations containing febuxostat have been used in the treatment of symptomatic hyperuricemia in patients with gout.