Study note
BiochemistryMicronutrients: Vitamins & Minerals

Vitamin D hydroxylation in the liver and kidney

THE CONTEXT
Cholecalciferol from the skin or food undergoes two hydroxylations to generate active 1,25-dihydroxyvitamin D, or calcitriol.

Hepatic conversion and circulating storage

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The liver hydroxylates cholecalciferol to the 25-hydroxy derivative calcidiol.
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Calcidiol is released into the circulation bound to vitamin D–binding globulin; this circulating bound form is the main storage form of the vitamin.

Renal pathways

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Renal 1-hydroxylation of calcidiol produces active 1,25-dihydroxyvitamin D, called calcitriol.
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Alternatively, renal 24-hydroxylation produces the probably inactive 24,25-dihydroxyvitamin D, also called 24-hydroxycalcidiol.

Other sources of active metabolites

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Ergocalciferol from fortified foods undergoes similar hydroxylation to form ercalcitriol.
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Some tissues outside those involved in calcium homeostasis take up circulating calcidiol and synthesize calcitriol that acts within the same cell.
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