FGF21 and Mitochondrial myopathy: Because deletion of FGF21 in OPA1‐deficient mice (a model of mitochondrial myopathy) resulted in partial attenuation of muscle wasting and prolonged survival [10], and the findings that FGF21 increased circulating glucocorticoid levels and suppressed physical activity [22], we investigated whether GDF15 neutralization reduced FGF21 levels, leading to improvements of disease conditions as well as reductions in circulating corticosterone levels.