FGF21 and polycystic ovary syndrome: Taken together, the results obtained from animal studies showing FGF21 not only to increase insulin sensitivity and to reduce adiposity, but also to promote BAT development and function, which has been shown to substantially improve the polycystic ovarian phenotype, IR and infertility in animal PCOS models, suggest an important beneficial effect of FGF21 on PCOS pathophysiology.