FGF21 and mitochondrial disease: Similar to other studies using PPAR agonists, we observed dose‐dependent increases in serum GDF‐15 and FGF‐21 (Inagaki et al, 2007; Yatsuga & Suomalainen, 2012), which have been proposed as biomarkers of mitochondrial disease (Lehtonen et al, 2016).