FGF21 and mitochondrial disease: FGF-21 (fibroblast growth factor 21) and GDF-15 (growth and differentiation factor 15) are serum cytokines produced in liver and muscle tissue that were found to be raised in mitochondrial diseases, particularly in muscle manifesting mitochondrial diseases, when compared to previously used biomarkers (lactate, pyruvate, lactate to pyruvate ratio and creatine kinase).