Vitamin D

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