What is the primary action of parathyroid hormone (PTH) on the kidneys?

• A. Decreases calcium reabsorption and increases phosphate reabsorption in the proximal tubule.
• B. Increases calcium reabsorption and decreases phosphate reabsorption in the distal tubule.
• C. Increases calcium reabsorption and phosphate reabsorption in the distal tubule.
• D. Has no effect on calcium or phosphate reabsorption.

Answer: (B)

PTH’s main job in the kidneys is to conserve calcium and promote phosphate excretion. It boosts calcium reabsorption in the distal tubule, so more calcium is pulled back into the bloodstream and less is lost in urine. It also reduces phosphate reabsorption, mainly in the proximal tubule, leading to more phosphate being excreted. This combination raises serum calcium and lowers serum phosphate, and it’s complemented by PTH’s stimulation of vitamin D activation, which further increases intestinal calcium absorption. So the option that describes increasing calcium reabsorption and decreasing phosphate reabsorption captures these essential renal actions of PTH.