Fibroblast growth factor 21 （FGF-21） is a member of the FGF gene family. According to its structure, it is further classified as a member of the FGF19 subfamily.?The subfamily includes FGF-19,-21 and-23.?FGF-21 is an effective adipocyte glucose uptake activator that protects animals from diet-induced obesity when overexpressed in transgenic mice and reduces blood glucose and triglyceride levels during treatment in diabetic rodents.?Some studies have shown that FGF-21 is a biomarker for the diagnosis of mitochondrial diseases.?FGF-21 can cross the blood-brain barrier and regulate the phosphorylation cascade and gene expression in the whole hypothalamus.?Intracerebroventricular injection of FGF21 can increase metabolic rate and insulin sensitivity in rats, but the site of action is not clear.