[Rickets/Osteomalacia. Action of FGF23.]

FGF23 is an "endocrine FGF" mainly secreted by osteocytes. FGF23 exerts its action through binding to an FGF receptor isoform, FGFR1c, using α-Klotho as a co-receptor. The main physiological function of FGF23 is to suppress phosphate reabsorption and active vitamin D production in the proximal tubule of the kidney, thereby lowering serum concentration of inorganic phosphate. In recent years, however, FGF23 has been shown to contribute to various pathological processes including cardiac hypertrophy via a distinct mode of action in a Klotho-independent manner. This review summarizes such pleiotropic actions of FGF23.