UCP1-TG mice with ectopic expression of uncoupling protein 1 (UCP1) in skeletal muscle (SM) are a model of improved substrate metabolism and increased longevity. Analysis of myokine expression showed an induction of fibroblast growth factor 21 (FGF21) in SM, resulting in >5 fold elevated circulating FGF21 in UCP1-TG mice. Despite a reduced muscle mass, UCP1-TG mice showed no evidence for a myopathy or muscle autophagy deficiency but an activation of integrated stress response (ISR: eIF2α/ATF4) in SM. Targeting mitochondrial function in vitro by treating C2C12 myoblasts with the uncoupler FCCP resulted in a dose dependent activation of ISR, associated with increased expression of FGF21 which was also observed by treatment with respiratory chain inhibitors antimycin A and myxothiazol. The co-factor required for FGF21 action, β-klotho, was expressed in white adipose tissue (WAT) of UCP1-TG mice which showed an increased browning of WAT, resembled in altered adipocyte morphology, increased brown adipocyte markers (UCP1, CIDEA), lipolysis (HSL phosphorylation), and respiratory capacity. Importantly, treatment of primary white adipocytes with serum of UCP1-TG mice resulted in increased UCP1 expression. Additionally, UCP1-TG mice showed reduced body length through suppressed IGF1/GH axis, and decreased bone mass. We conclude that the induction of FGF21 as a myokine is coupled to disturbance of mitochondrial function and ISR activation in SM. Endocrine acting FGF21 released from SM has endocrine effects leading to increased browning of WAT and can explain the healthy metabolic phenotype of UCP1-TG mice. These results confirm muscle as an important endocrine regulator of whole body metabolism.