High calcitriol has what effect on phosphate?

• A. Increases intestinal phosphate absorption
• B. Decreases intestinal phosphate absorption
• C. Has no effect on phosphate
• D. Increases renal phosphate excretion

Answer: (A)

High levels of calcitriol increase intestinal phosphate absorption. Calcitriol upregulates the intestinal phosphate transporter NaPi-IIb (and related proteins) in enterocytes, boosting the gut’s ability to take up phosphate from the diet. This is why the effect is to raise—rather than decrease—phosphate absorption from the intestine. It’s not primarily about increasing renal excretion; renal phosphate handling is more strongly influenced by other hormones (like PTH), whereas calcitriol’s gut action directly enhances absorption.