The vitamin D biosynthetic pathway. The steroid nucleus of 7-dehydrocholesterol is converted in skin to previtamin D₃ with exposure to UVB light and rapidly isomerized to vitamin D₃, which is found in nmol/L concentrations in the circulation. This metabolite is converted to 25 (OH)D₃ in hepatic microsomes, and is also found in nmol/L amounts in the circulation. Measurement of circulating 25 (OH)D₃ is a biomarker of total body vitamin D stores. 25 (OH)D₃ is converted in renal mitochondria to the best-known active metabolite, 1,25 (OH)₂D₃ which circulates in pg/ml concentrations in serum. Conversion of 25 (OH)D₃ to 24,25 (OH)₂D₃ also occurs in renal mitochondria.