In a cross-sectional study of 142 T2DM patients, the serum FGF-21 concentrations were markedly elevated in the DR group (103.5 pg/mL) compared to those without DR (99 pg/mL), although no association was found regarding the severity of DR as the FGF21 levels were comparable between the NPDR and PDR groups [67]. Here, FGF21 is linked to type 2 diabetes mellitus.