FGF21 and obesity due to melanocortin 4 receptor deficiency: Meanwhile, GK rats in our study had FGF21 levels between 320 and 460 pg/mL, which is consistent with the elevated levels observed in individuals with obesity and diabetes, and in other animal models of diabetes (Chavez et al., 2009; Nygaard et al., 2014; Gallego-Escuredo et al., 2015; Roesch et al., 2015; Hu et al., 2016).