FGF23 and chronic kidney disease: Since we recently reported that serum FGF-23 acts physiologically to regulate serum Pi levels from the standpoint of vitamin D metabolism in Japanese non-CKD subjects with eGFR ≥ 60 mL/min/1.73 m2 5, serum FGF-23 has potential as a clinically useful surrogate marker of the Pi load in the circulation in non-CKD subjects.