FGF21 and obesity due to melanocortin 4 receptor deficiency: FGF21 is synthesizedand secreted into the circulation mainly by the liver (Fisheret al., 2011) in response to metabolic stresses such as fooddeprivation (Zhang Y. et al., 2012; Bazhan et al., 2019a), coldexposure (Dutchak et al., 2012), and obesity (Chukijrungroatet al., 2017; Bazhan et al., 2019b).