Tiopronin-d is intended for use as an internal standard for the quantification of tiopronin by GC- or LC-MS. Tiopronin is an antioxidant that has diverse biological activities. It reduces free radical production by murine macrophages and granulocytes in vitro in a dose-dependent manner. Tiopronin induces expression of hypoxia-inducible factor 1α (HIF-1α) and increases VEGF secretion in human colon carcinoma cells. Rectal administration (500 μl of a 10 mM solution) reduces myeloperoxidase activity and reduces pro-inflammatory cytokine production in the colon in a rat model of colitis. Tiopronin (80-320 mg/kg per day) reduces the incidence of cleft palate in fetuses born to female mice orally exposed to teratogenic methylmercury chloride. Tiopronin (100 mg/kg) reduces heme oxygenase 1 mRNA expression, lipid peroxidation, and transverse aortic constriction in a mouse model of cardiac hypertrophy. Tiopronin (20 mg/kg) is hepatoprotective, increasing activity of the antioxidant enzymes superoxide dismutase and glutathione peroxidase and reversing hepatocyte degeneration in a rat model of high-fat diet-induced non-alcoholic steatohepatitis.