Which factor suppresses 1α-hydroxylase, reducing calcitriol production?

Which factor suppresses 1α-hydroxylase, reducing calcitriol production?

• A. FGF23
• B. PTH
• C. Calcitonin
• D. Vitamin D receptor

Answer: (A)

Regulation of renal 1α-hydroxylase controls calcitriol production. FGF23, produced by osteocytes in response to phosphate load and calcitriol, acts on proximal tubule cells through the FGFR1–Klotho complex to downregulate CYP27B1 (the enzyme that makes calcitriol) and upregulate CYP24A1 (which degrades calcitriol). This combination lowers calcitriol levels, reducing intestinal calcium and phosphate absorption and promoting phosphate excretion. In contrast, PTH stimulates 1α-hydroxylase, increasing calcitriol production, so it does not suppress it. Calcitonin does not directly regulate this enzyme, and the vitamin D receptor mediates calcitriol’s actions rather than its synthesis.

Regulation of renal 1α-hydroxylase controls calcitriol production. FGF23, produced by osteocytes in response to phosphate load and calcitriol, acts on proximal tubule cells through the FGFR1–Klotho complex to downregulate CYP27B1 (the enzyme that makes calcitriol) and upregulate CYP24A1 (which degrades calcitriol). This combination lowers calcitriol levels, reducing intestinal calcium and phosphate absorption and promoting phosphate excretion.

In contrast, PTH stimulates 1α-hydroxylase, increasing calcitriol production, so it does not suppress it. Calcitonin does not directly regulate this enzyme, and the vitamin D receptor mediates calcitriol’s actions rather than its synthesis.