High PTH causes hypophosphatemia by which mechanism?

High PTH causes hypophosphatemia by which mechanism?

• A. Decreasing intestinal phosphate absorption and promoting renal excretion
• B. Increasing intestinal phosphate absorption
• C. Decreasing renal excretion of phosphate
• D. Calcium phosphate precipitation

Answer: (A)

High PTH lowers serum phosphate mainly by causing phosphate to be lost in the urine. In the kidney, PTH inhibits the proximal tubule Na+/phosphate cotransporter (NaPi-IIa), reducing phosphate reabsorption and increasing phosphate excretion (phosphaturia). This renal phosphate wasting drives down serum phosphate levels. PTH also raises calcitriol, which raises intestinal phosphate absorption, but the net effect on serum phosphate is a drop due to increased renal excretion. Therefore, the best description is that high PTH promotes renal phosphate excretion as the key mechanism for hypophosphatemia.

High PTH lowers serum phosphate mainly by causing phosphate to be lost in the urine. In the kidney, PTH inhibits the proximal tubule Na+/phosphate cotransporter (NaPi-IIa), reducing phosphate reabsorption and increasing phosphate excretion (phosphaturia). This renal phosphate wasting drives down serum phosphate levels. PTH also raises calcitriol, which raises intestinal phosphate absorption, but the net effect on serum phosphate is a drop due to increased renal excretion. Therefore, the best description is that high PTH promotes renal phosphate excretion as the key mechanism for hypophosphatemia.