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lar cells.3 Additionally, FGF23 regulates vitamin D synthesis through CYP27B1 inhibition and CYP24A1 stimulation, which will lead to suppression of vitamin D activity and a decrease in the levels of phosphorus and calcium. In the parathyroid glands, FGF23 directly inhibits the synthesis and secretion of PTH.4 The importance of FGF23 has been documented in an experimental study utilizing a rat model of chronic kidney disease (CKD). Circulating FGF23 activity was lowered by using anti-FGF23 antibody and there was a significant increase in serum phosphate while the mean PTH level remained unchanged.5 These findings indicate that FGF23 is more direct and strong regulator of phosphorus than PTH, at least in CKD. Actions of Phosphorus Homeostasis-Maintaining Hormones Fibroblast growth factor 23 (FGF23) and Klotho are recently discovered hormones that have a close relationship with parathyroid hormone (PTH) in maintaining phosphorus and vitamin D metabolism.1 FGF23 is most highly expressed in osteocytes, but because FGF23 is the most soluble form of the subfamily of FGFs, it can be also found in circulating serum.2 Although this enables FGF23 to exert systemic actions, the 2 main organs of action are the kidneys and parathyroid glands (Figure).1,2 In the kidneys, FGF23 augments urinary phosphate exertion by promoting endocytosis of sodium-phosphate cotransporters (NPT2a and NPT2c) from renal proximal tubu-

Is Circulating Fibroblast Growth Factor 23 a Surrogate Marker or Active Mediator for the Construction of Atrial Fibrillation Substrate?