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The metabolic pathway for vitamin D. CYP2R1 has been identified as a key vitamin D 25-hydroxylase. PTH, FGF23/klotho, and 1,25(OH)2D3 play key roles in the regulation of optimal levels of 1,25(OH)2D3. Only products of 1,25(OH)2D3 are represented for the C23 lactone pathway. See text for 25(OH)D products of the C23 lactone pathway.
