Calcidiol, calcifediol, 25-hydroxycholecalciferol, or 25–hydroxy–vitamin D, is a prehormone which is produced by the hepatic metabolism (hydroxylation) of vitamin D3 (cholecalciferol) and which is converted by the kidneys (via 25-Hydroxyvitamin D3 1-alpha-Hydroxylase) into calcitriol (1,25-vitamin D), a steroid hormone. It can also be converted into inactive 24-hydrocalcidiol in the kidneys via 24-hydroxylation.
In medicine, blood concentration of calcidiol is the accepted compound to assess vitamin D status. Calcidiol promotes the mineralization
of bones and teeth. Increasing calcidiol levels are associated with increasing fractional absorption of calcium from the gut up to levels of 80 nmol/L (32 ng/dL). Urinary calcium excretion balances intestinal calcium absorption and does not increase with calcidiol levels up to ~400 nmol/L (160 ng/dL).
The normal range is disputed. Probably about 80–220 nmol/L (30–85 ng/dL). The upper limit is based on skin production in sun rich environments and tanning beds. The lower limit is based on intestinal calcium absorption, parathyroid hormone suppression, and muscle strength.
Calciferol which is metabolised to calcidiol and is used to treat diseases caused by vitamin D deficiency such as rickets and osteomalacia. It is effective even in azotemic patients.
- Dietary Reference intakes for Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride. Institute of Medicine, 1997
- Vitamin D Pharmacology at VitaminDCouncil.com