What is the effect of PTH on serum phosphate?

What is the effect of PTH on serum phosphate?

• A. Increases serum phosphate.
• B. Decreases serum phosphate.
• C. No effect.
• D. Increases only when calcium is high.

Answer: (B)

PTH lowers serum phosphate by increasing its excretion in the kidneys. It acts on the proximal tubule to inhibit phosphate reabsorption via downregulation of the NaPi-IIa transporter, causing phosphate to be lost in urine (phosphaturia). While PTH also stimulates bone resorption, releasing phosphate into the bloodstream, the renal excretion effect dominates, so the net result is a decrease in serum phosphate. The rise in active vitamin D from PTH increases intestinal absorption of phosphate, but this does not usually counterbalance the renal phosphate loss, so the overall effect remains a decrease.

PTH lowers serum phosphate by increasing its excretion in the kidneys. It acts on the proximal tubule to inhibit phosphate reabsorption via downregulation of the NaPi-IIa transporter, causing phosphate to be lost in urine (phosphaturia). While PTH also stimulates bone resorption, releasing phosphate into the bloodstream, the renal excretion effect dominates, so the net result is a decrease in serum phosphate. The rise in active vitamin D from PTH increases intestinal absorption of phosphate, but this does not usually counterbalance the renal phosphate loss, so the overall effect remains a decrease.