FGF21 and obesity due to melanocortin 4 receptor deficiency: Although FGF21 has several beneficial effects on insulin sensitivity and glucose and lipid metabolism [6, 7], circulating FGF21 levels are paradoxically increased in hepatosteatosis, obesity, and type 2 diabetes in rodents and humans [6, 7, 8, 9, 10, 11, 12, 13].