Updated: 6/16/2021

PTH & Vit D Physiology

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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-hydroxyvitamin 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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(OBQ13.18) Which of the following statements is correct regarding Vitamin D?

QID: 4653
1

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

28%

(1088/3892)

2

25-hydroxycholecalciferol is the active form of Vitamin D

6%

(244/3892)

3

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

53%

(2054/3892)

4

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

8%

(295/3892)

5

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

5%

(182/3892)

L 4 B

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