Fibroblast growth factor 23 (FGF23) is a member of the FGF family, within a subfamily that also includes FGF19 and FGF21. FGF proteins contain a 120 amino acid (aa) core FGF domain that exhibits a betatrefoil structure. β-Klotho has been identified as co-factor required for FGF-19, 21, 23 signaling. It can obviously increase ligand-receptor affinity. Klotho has a restricted distribution that limits FGF23 activity. FGF23 is produced by osteocytes and osteoblasts in response to high circulating phosphate levels, elevated parathyroid hormone, and circulatory volume loading. It functions as an endocrine phosphatonin by suppressing circulating phosphate levels. FGF23 interaction with renal proximal tubular epithelium decreases the renal resorption of phosphate by downregulating phosphate transporters and by suppressing vitamin D production. It also decreases the intestinal absorption of phosphate.