Among T2DM men, serum irisin levels positively associated with ArIIEF-5 scores and serum testosterone (r = 0.413, p = 0.002; r = 0.936, p < 0.001, respectively) but negatively associated with glycosylated hemoglobin levels (r = -0.377, p = 0.004). Here, FNDC5 is linked to type 2 diabetes mellitus.