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Introduction
  • Calcium homeostasis controlled by a combination of
    • Vitamin D pathway (this topic)
    •  PTH pathway
Vitamin D Pathway

  • Synthesis
    • vitamin D3
      • synthesized in skin from sun exposure
    • vitamin D2
      • ingested from plants in the diet
    • both are prehormones that must be converted in 2 steps
      • 25-hydroxylation in liver
      • 1-hydroxylation in the peritubular capilliaries of the kidney
    • only 1,25-(OH)2 vitamin D is active
    • 24,25-(OH)2 vitamin D is an inactive form
  • Function
    • ↑ serum Ca2+ and phosphate via
      • ↑ absorption of calcium and phosphate from the intestine
      • ↑ bone resorption of Ca2+ and phosphate
    • recall PTH functions to ↑ serum Ca2+ but ↓ serum phosphate
  • Regulation
    • PTH stimulates 1,25-(OH)2 vitamin D production 
    • hypocalcemia/hypophoshatemia stimulates 1,25-(OH)2 vitamin D production
    • 1,25-(OH)2 vitamin D feedback negatively on itself
  • Pathology
    • rickets / osteomalacia
Clinical Conditions
  • Hypoparathyroidism 
  • Hyperparathyroidism
  • Rickets
 

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