Pathogenesis

Chronic renal failure causes the following:

• reduced 1-alpha hydroxylation of 25(0H) vitamin D:
  • calcium absorption from the gut is reduced
  • plasma calcium concentrations fall resulting in defective bone mineralization
  • secondary hyperparathyroidism results in increased bone resorption
• metabolic acidosis:
  • buffering of H+ by calcium salts in bone also contributes to demineralisation
• phosphate retention:
  • the [calcium] x [phosphate] solubility constant is exceeded resulting in metastatic calcification (osteosclerosis)

It is rare for the hypocalcaemia of renal osteodystrophy to cause tetany as both the acidosis and hypoalbuminaemia that accompany renal failure decrease the proportion of the total calcium which is protein-bound.