25-hydroxyvitamin D (calcifediol)
Hormone · t1/2 2-3 weeks The form measured
The main circulating and storage form of vitamin D, and the one measured to define vitamin D status. It is a prohormone with little activity of its own and must be 1-alpha-hydroxylated in the kidney before it can do anything.
Traced from the start
- Cholecalciferol from skin and diet
- Hepatic 25-hydroxylation
- 25-hydroxyvitamin D (calcifediol)
Detail
- Half-life
- 2-3 weeks bound to vitamin D-binding protein, so a single level reflects supply over months
- Australian thresholds
- Adequate at 50 nmol/L or above at the end of winter; mild deficiency 30-49, moderate 12.5-29 and severe below 12.5 nmol/L
- Activity
- Binds the vitamin D receptor only weakly; at very high concentrations it can cause hypercalcaemia by mass action, which is how vitamin D toxicity works
- Fate
- 1-alpha-hydroxylated to calcitriol in the kidney, or 24-hydroxylated by CYP24A1 to inactive 24,25-dihydroxyvitamin D
When it goes wrong
25(OH)D below 30 nmol/L
Compensatory secondary hyperparathyroidism: PTH high, calcium low-normal, phosphate low and alkaline phosphatase raised
Prolonged severe deficiency
Osteomalacia in adults with bone pain, proximal myopathy and Looser zones, and rickets in children with bowed legs and widened metaphyses
Megadose vitamin D supplements
Hypercalcaemia and hypercalciuria with a suppressed PTH and a very high 25(OH)D; calcitriol may be normal
Practise this structure
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