FGF21 and obesity due to melanocortin 4 receptor deficiency: On the other hand, increased concentrations of FGF21 can be observed in several pathological states such as insulin resistance, obesity, hypertension, diabetes, and metabolic syndrome, which may indicate that increased FGF21 is a response to unfavorable metabolic conditions (Woo et al., 2013[15]; Cheung and Deng, 2014[4]).