Updated: 4/22/2018

PTH & Vit D Physiology

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Evidence
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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
Vitamin D Physiology
  
  • 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)
      • laboratory study of choice to determine Vitamin D deficiency  
    • 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
PTH Physiology
 
  • 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
Clinical Conditions
  • Conditions related to PTH 
    •  hypoparathyrodism
    •  pseudohypoparathyroidism
    •  renal osteodystrophy
  • Conditions related to Vitamin D
    •  Rickets
 

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Questions (3)
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(OBQ13.18) Which of the following statements is correct regarding Vitamin D? Review Topic

QID: 4653
1

1,25-dihydrocholecalciferol is the best laboratory study to determine a Vitamin D deficiency

29%

(961/3350)

2

25-hydroxycholecalciferol is the active form of Vitamin D

6%

(200/3350)

3

24,25-dihydroxycholecalciferol is an inactive form of Vitamin D

52%

(1755/3350)

4

1,25-dihydrocholecalciferol is converted to 25-hydroxycholecalciferol in the kidney

8%

(254/3350)

5

The half-life of 1,25-dihydrocholecalciferol is longer than 25-hydroxycholecalciferol

5%

(157/3350)

ML 4

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PREFERRED RESPONSE 3
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