Fibroblast growth factor 21 is a protein that in mammals is encoded by the FGF21 gene The protein encoded by this gene is a member of the fibroblast growth factor FGF family and specifically a member of the endocrine subfamily which includes FGF23 and FGF15 19 FGF21 is the primary endogenous agonist of the FGF21 receptor which is composed of the co receptors FGF receptor 1 and β Klotho