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| Introduction |
- Vitamin D and PTH play an important role in calcium homeostasis
- skin, liver, parathyroid gland, kidney, bone, and small intestine all play a role
- increased PTH and Vitamin D leads to increase serum calcium levels
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| Vitamin D Physiology |
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- Overview
 
- Vitamin D and PTH play an important role in calcium homeostasis
- skin, liver, parathyroid gland, kidney, bone, and small intestine all play a role
- Increased PTH and Vitamin D leads to increase serum calcium levels
- Synthesis
- 7-Dehydrocholesterol
- precursor to calcitriol is stored in the skin where UV exposure converts it to previtamin D3.
- cholecalciferol (Vitamin D3)
- Previtamin D3 is then bound to vitamin-D binding protein (DBP) where it is carried to the liver and metabolized to 25-hydroxyvitamin D3.
- 25-hyrdoxyvitamin D3
- when calcium is low, parathyroid hormone (PTH) levels become elevated which activates 1-alpha-hydroxylase in the kidney.
- 1-alpha-hydroxylase converts 25-hydroxyvitamin D to the active Vitamin D (calcitriol).
- 1,25-dihydroxyvitamin D3 (Vitamin D, calcitriol)
- active form that controls calcium homeostasis in body by targeting intestines and bones (see function below)
- 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
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| PTH Physiology |
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- 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 which secretes IL-1 to activated osteoclasts
- ↑ kidney resorption of calcium in distal convoluted tubule
- ↓ kidney resorption of phosphate
- ↑ 1,25-(OH)2 vitamin D production
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| Clinical Conditions |
- Conditions related to PTH
 hypoparathyrodism- pseudohypoparathyroidism
- renal osteodystrophy
- Conditions related to Vitamin D
 Rickets
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