What is the role of PTH in calcium homeostasis and which organs are involved in its actions?

• A. PTH raises serum calcium by increasing bone resorption, increasing renal calcium reabsorption, decreasing renal phosphate reabsorption, and stimulating active vitamin D production in the kidney
• B. PTH lowers serum calcium
• C. PTH acts only on bone
• D. PTH increases phosphate reabsorption in kidney

Answer: (A)

PTH raises serum calcium through coordinated actions on bone, kidney, and, indirectly, the intestine via vitamin D activation. In bone, PTH stimulates osteoblasts to increase RANKL, which activates osteoclasts and promotes bone resorption, releasing calcium (and phosphate) into the bloodstream. In the kidney, PTH increases calcium reabsorption in the distal tubule, helping to conserve calcium, and it decreases phosphate reabsorption in the proximal tubule, increasing phosphate excretion. It also stimulates 1-alpha-hydroxylase in the kidney, converting 25-hydroxy vitamin D to the active form, calcitriol, which raises intestinal absorption of calcium (and phosphate). The net effect is an increase in serum calcium, with a tendency to lower serum phosphate due to renal excretion, and enhanced calcium uptake from the gut via calcitriol. This combination of actions explains why the described mechanism best fits PTH’s role in calcium homeostasis and why the organs involved are bone, kidney, and the intestine (through calcitriol).