What is the primary function of parathyroid hormone (PTH) in calcium homeostasis, and where does it act in the kidney to influence phosphate?

What is the primary function of parathyroid hormone (PTH) in calcium homeostasis, and where does it act in the kidney to influence phosphate?

• A. Raises serum calcium by stimulating bone resorption, increases renal calcium reabsorption in the distal tubule, and decreases proximal tubular phosphate reabsorption.
• B. Lowers serum calcium by inhibiting bone resorption and reducing renal calcium reabsorption.
• C. Increases phosphate reabsorption in the proximal tubule, raising serum phosphate.
• D. Suppresses calcitriol production and has no effect on renal calcium.

Answer: (A)

PTH raises serum calcium and lowers serum phosphate by acting on bone and specific segments of the kidney. It promotes bone resorption, releasing calcium into the blood, and it increases calcium reabsorption in the distal tubule of the kidney. At the same time, it decreases phosphate reabsorption in the proximal tubule, which increases phosphate excretion. This combination raises calcium availability while preventing phosphate from binding calcium, supporting overall calcium homeostasis. The other options describe opposite or incomplete effects: PTH does not lower calcium, nor does it increase proximal phosphate reabsorption or suppress calcitriol production. It also stimulates calcitriol production, further aiding calcium (and phosphate) absorption, but the key points here are those distal-tubule calcium reabsorption and proximal-tubule phosphate reabsorption changes.

PTH raises serum calcium and lowers serum phosphate by acting on bone and specific segments of the kidney. It promotes bone resorption, releasing calcium into the blood, and it increases calcium reabsorption in the distal tubule of the kidney. At the same time, it decreases phosphate reabsorption in the proximal tubule, which increases phosphate excretion. This combination raises calcium availability while preventing phosphate from binding calcium, supporting overall calcium homeostasis. The other options describe opposite or incomplete effects: PTH does not lower calcium, nor does it increase proximal phosphate reabsorption or suppress calcitriol production. It also stimulates calcitriol production, further aiding calcium (and phosphate) absorption, but the key points here are those distal-tubule calcium reabsorption and proximal-tubule phosphate reabsorption changes.