Introduction Calcium homeostasis controlled by a combination of Vitamin D pathway (this topic) PTH pathway Vitamin D Pathway Synthesis vitamin D3 synthesized in skin from sun exposure vitamin D2 ingested from plants in the diet both are prehormones that must be converted in 2 steps 25-hydroxylation in liver 1-hydroxylation in the peritubular capilliaries of the kidney only 1,25-(OH)2 vitamin D is active 24,25-(OH)2 vitamin D is an inactive form 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 Pathology rickets / osteomalacia Clinical Conditions Hypoparathyroidism Hyperparathyroidism Rickets