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Updated: Aug 5 2021

Parathyroid Hormone (PTH)

Images d.jpg pathway.jpg
  • Introduction
    • Calcium homeostasis controlled by a combination of vitamin D and PTH
  • PTH Pathway
    • Synthesis
      • secreted by the chief cells of parathyroid
      • responds to ionized calcium levels (physiologically-active), not total calcium levels (which includes physiologically-inactive calcium bound to protein)
        • in low pH settings, an excess of hydrogen ions bind to protein, displacing calcium ions leading to an increase in ionized calcium and vice versa in high pH settings
    • Function
      • ↑ serum free 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
    • Familial hypocalciuric hypercalcemia
      • defective calcium receptor that can not detect elevated calcium level
        • loss of negative feedback causes hypercalcemia (via increased PTH, 1-alpha OH)
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