Hormone
Hormone
Hormone
Vitamin D3
(cholecalciferol)
  • Originates from:
    • Dietary intake
    • Synthesis in the skin through effects of UV light on 7-dehydrocholesterol
  • Modified by liver and kidney to form active 1-25-hydroxycholecalciferol - under the influence of parathyroid hormone
  • Levels also increase in response to low phosphate levels
  • Acts to increase serum calcium levels
  • GI Tract: Increases GI absorption by upregulating number and activity of transport proteins
  • Kidneys: Weak effect to increase calcium and phosphate reabsorption
  • Plays a minimal role in calcium homeostasis, secreted only when levels rise above normal limits