Which of the following describes the actions of FGF23 on the kidney and vitamin D axis?

• A. Increases renal phosphate reabsorption and stimulates 1,25(OH)2D production
• B. Decreases renal phosphate reabsorption and suppresses 1,25(OH)2D production
• C. Increases renal calcium reabsorption and promotes calcitriol activation
• D. Stimulates PTH secretion and increases phosphate reabsorption

Answer: (B)

FGF23 acts as a phosphatonin that coordinates phosphate and vitamin D metabolism. In the kidney, it reduces phosphate reabsorption by downregulating the proximal tubule sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc, causing phosphaturia. It also suppresses the enzyme that activates vitamin D, 1α-hydroxylase (CYP27B1), thereby lowering production of the active form 1,25(OH)2D. Additionally, FGF23 promotes breakdown of active vitamin D by upregulating CYP24A1, further reducing calcitriol levels. The net effect is decreased phosphate reabsorption and suppressed 1,25(OH)2D production, which matches the described statement.