What is the action of PTH in the gut?

• A. Decreases absorption of calcium
• B. Inhibits gut calcium transport
• C. Increases permeability of gut cells to calcium
• D. Stimulates phosphate absorption

Answer: (C)

PTH raises intestinal calcium absorption indirectly by boosting the production of calcitriol (the active form of vitamin D) in the kidney. Calcitriol then acts on the intestinal lining to upregulate calcium transport proteins and channels, increasing the gut’s capacity to move calcium from the lumen into the bloodstream. In other words, the gut becomes more permeable to calcium due to calcitriol’s effect, which is driven by PTH. This reflects why the action in the gut is described as increased calcium permeability/absorption. PTH does not directly decrease absorption or inhibit gut transport, and its effect on phosphate is mediated through calcitriol rather than a direct gut action.