Fibroblast growth factor 23 (FGF23) is a hormone mainly produced by osteocytes and regulates phosphate and vitamin D metabolism by binding to Klotho-FGF receptor complex. Most diseases previously called vitamin D-resistant rickets/osteomalacia or familial hypophosphatemic rickets/osteomalacia have been shown to be caused by excess actions of FGF23. Therefore, several methods to inhibit the actions of FGF23 are now considered as new treatment methods for these FGF23-related hypophosphatemic diseases. It has been reported that anti-FGF23 antibody increases serum phosphate in patients with FGF23-related hypophosphatemic disease.