Parathyroid hormone (PTH) acts on the kidney to restore serum calcium levels in three ways. Which of the following is NOT a direct renal action of PTH?

• A Increases Ca2+ reabsorption in the thick ascending limb and distal tubule
• B Decreases phosphate reabsorption in the proximal tubule (phosphaturia)
• C Activates 1-alpha-hydroxylase to convert 25(OH)D to 1,25(OH)2D (calcitriol)
• D Stimulates osteoclast differentiation directly in bone ✓

Explanation

PTH acts directly on osteoblasts (not osteoclasts) to stimulate RANKL expression, which then drives osteoclast differentiation and bone resorption. The direct renal actions of PTH include: (1) increasing Ca2+ reabsorption in the thick ascending limb and distal convoluted tubule, (2) inhibiting phosphate reabsorption in the proximal tubule (causing phosphaturia to prevent Ca-P precipitation), and (3) activating renal 1-alpha-hydroxylase to produce active calcitriol (1,25-dihydroxyvitamin D3), which increases intestinal Ca2+ absorption.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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