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
QUESTIONS 1 of 3 1 2 3 Previous Next Sorry, this question is for PEAK Premium Subscribers only Upgrade to PEAK (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 Select Answer to see Preferred Response PREFERRED RESPONSE 3 Sorry, this question is for PEAK Premium Subscribers only Upgrade to PEAK