Vitamin D deficiency: rickets and osteomalacia
Effect · 25(OH)D low ALP high
Too little substrate reaches the 1-alpha-hydroxylase, so gut calcium absorption falls and PTH rises to hold serum calcium up, wasting phosphate as it does so. Osteoid is laid down but not mineralised: rickets at the growth plate in children, osteomalacia in adults.
Traced from the start
- Fall in ionised calciumRise in ionised calcium
- Calcium-sensing receptor on parathyroid chief cells
- Phosphate load and hyperphosphataemia
- Osteocyte phosphate sensing
- Cholecalciferol from skin and diet
- Hepatic 25-hydroxylation
- Parathyroid hormoneFibroblast growth factor 2325-hydroxyvitamin D (calcifediol)
- Thyroid parafollicular (C) cells
- Calcitonin
- Renal proximal tubule: 1-alpha-hydroxylaseBone: osteoblast-osteoclast coupling
- Vitamin D deficiency: rickets and osteomalacia
Detail
- Biochemistry
- 25(OH)D low, calcium low or low-normal, phosphate low, PTH high, alkaline phosphatase high; calcitriol is often normal because the high PTH drives what substrate remains
- Rickets in children
- Bowed legs, widened wrists and costochondral junctions (the rachitic rosary), craniotabes, delayed fontanelle closure, delayed dentition and hypocalcaemic seizures
- Osteomalacia in adults
- Diffuse bone pain and tenderness, proximal myopathy with a waddling gait and difficulty rising from a chair, and Looser zones (pseudofractures) on x-ray
- Treatment
- Cholecalciferol, a loading dose then maintenance, with calcium if dietary intake is poor; recheck calcium afterwards in case coexisting primary hyperparathyroidism is unmasked
When it goes wrong
Low 25(OH)D with a high PTH, low phosphate and raised alkaline phosphatase
Vitamin D deficiency with secondary hyperparathyroidism: the pattern to separate from primary hyperparathyroidism, where the calcium is high rather than low
Hypophosphataemic rickets that does not respond to cholecalciferol
Think X-linked hypophosphataemia from FGF23 excess, or vitamin D-dependent rickets, rather than simple deficiency
Practise this structure
1 question in the bank tagged Vitamin D.