FGF21 and obesity due to melanocortin 4 receptor deficiency: Furthermore, fibroblast growth factor 21 (FGF21), a metabolic hormone that is expressed in the liver and regulates glucose metabolism and lipid homeostasis is closely associated with metabolism-related diseases, such as obesity, diabetes mellitus, and nonalcoholic fatty liver disease (NAFLD)(Geng et al. 2020).