In primary hyperparathyroidism, which renal process is affected by PTH to reduce phosphate levels?

• A. Decrease phosphate reabsorption
• B. Increase phosphate reabsorption
• C. No change
• D. Increase magnesium excretion

Answer: (A)

Elevated PTH in primary hyperparathyroidism reduces phosphate reabsorption in the proximal tubule. PTH downregulates the sodium-phosphate cotransporter NaPi-IIa, so less phosphate is reabsorbed and more is excreted in urine, leading to lower serum phosphate. The other options don’t fit this mechanism: increasing phosphate reabsorption would oppose PTH’s action, no change wouldn’t explain phosphate wasting, and magnesium excretion isn’t the primary pathway involved in PTH-driven phosphate handling.