A 55-year-old woman post-thyroidectomy develops perioral tingling and carpopedal spasm (positive Trousseau sign). Serum Ca2+ is 6.8 mg/dL, PTH undetectable. The physiological action of PTH on the kidney that would restore calcium if present is:

• A PTH increases renal calcium excretion in the proximal tubule, reducing serum calcium back to normal
• B PTH directly stimulates intestinal calcium absorption without vitamin D involvement
• C PTH stimulates Ca2+ reabsorption in the thick ascending limb and distal convoluted tubule via TRPV5 channels; also stimulates 25-hydroxyvitamin D 1-alpha-hydroxylase in the proximal tubule to increase calcitriol synthesis ✓
• D PTH activates osteoclasts via direct receptor binding on osteoclast RANK receptors

Explanation

PTH (parathyroid hormone) exerts multiple renal actions to raise serum calcium: (1) Increases Ca2+ reabsorption in the distal convoluted tubule (DCT) by upregulating apical TRPV5 (transient receptor potential vanilloid 5) calcium channels, calbindin D28k, and basolateral PMCA/NCX; similar action in thick ascending limb. (2) Stimulates 1-alpha-hydroxylase (CYP27B1) enzyme in proximal tubule converting 25-OH vitamin D to 1,25-dihydroxyvitamin D3 (calcitriol), which increases intestinal Ca2+ absorption (via TRPV6, calbindin D9k). (3) Reduces renal phosphate reabsorption (phosphaturic effect via downregulating NaPi-IIa). PTH also stimulates osteoclastic bone resorption, but indirectly—PTH receptors on osteoblasts stimulate RANKL expression, which activates osteoclasts via RANK (osteoclasts themselves lack PTH receptors).

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

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Written and medically reviewed by the StethoPrep medical team.