Physiology
- Skin converts 7-dehydrocholesterol to cholecalciferol (D3) under UVB light
- Dietary sources supply D2 and D3
- Liver 25-hydroxylation forms 25-hydroxyvitamin D, the storage form measured in blood
- Kidney 1-alpha hydroxylase forms 1,25-dihydroxyvitamin D (calcitriol), the active hormone
- PTH and low phosphate stimulate 1-alpha hydroxylase
- Vitamin D binding protein carries the metabolites in blood
- Fibroblast growth factor 23 suppresses 1-alpha hydroxylase
Actions
- Increases intestinal calcium and phosphate absorption
- Increases renal calcium reabsorption
- Promotes osteoclast activity through RANKL to mobilise calcium
- Needed for mineralisation of osteoid
Deficiency
- Rickets in children and osteomalacia in adults
- Secondary hyperparathyroidism
- Muscle weakness and falls
- Australian target 25-hydroxyvitamin D of 50 nmol/L or more
- Risk factors are dark skin, covered clothing, institutional care, obesity and malabsorption
- Levels under 30 nmol/L indicate moderate to severe deficiency
- Raised alkaline phosphatase and low phosphate in osteomalacia
- Looser zones are pseudofractures on radiographs
Management
- Cholecalciferol 1000 to 2000 IU daily for most adults
- Higher loading doses for severe deficiency
- Calcitriol for renal failure or 1-alpha hydroxylase deficiency
- See Osteoporosis
- Ensure adequate calcium intake of about 1000 to 1300 mg daily
- Recheck levels after about 3 months
- Annual high dose oral bolus increased falls and fractures in older women (Sanders, JAMA 2010)
- Routine supplementation without deficiency does not prevent fractures (VITAL)
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.