• Calcium homeostasis controlled by a combination of  vitamin D and PTH
PTH Pathway
  • Synthesis 
    • secreted by the chief cells of parathyroid
  • Function
    • ↑ serum Ca2+ and ↓ serum phosphate in response to hypocalcemia/hypomagnesemia via
      • ↑ bone resorption of calcium and phosphate (bone is destroyed)
        • PTH receptor is on the osteoblasts secretes IL-1 to activate osteoclasts via production of M-CSF and RANK-L
        • PTH decreases osteoprotegrin (OPG), decoy receptor for RANKL, thereby resulting in an increased interaction between RANKL and osteoclasts
      • ↑ kidney resorption of calcium in distal convoluted tubule
      • ↓ kidney resorption of phosphate (increasing urine phosphate)
      • ↑ 1,25-(OH)2 vitamin D production (via 1 alpha-hydroxylase) 
Clinical Conditions
  • Hypoparathyroidism 
  • Hyperparathyroidism 

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