Fibroblast growth factor 23 (FGF23) belongs to FGF19 subfamily, whose members function like endocrine factors, and has a phosphaturic effect, leading to hypophosphatemia associated with rickets or osteomalacia when its concentration in blood is elevated. FGF23 is involved in the pathogenesis in many forms of hypophosphatemia including the autosomal dominant and recessive types, the X-linked type and the tumor-induced type. Alpha klotho, originally discovered as an anti-aging factor, along with the FGF receptor type 1 makes a specific receptor for FGF23.