Thyroid hormones exert important effects on heart remodeling through mir-208. The process may have a role in myocardial changes in chronic kidney disease where thyroid abnormalities are common. In this study the effect of T4 supplementation on left ventricle (LV) remodeling in 5/6 nephrectomized rats (5/6Nx) was analyzed. 5/6Nx rats and 5/6Nx under T4 supplementation (5/6Nx + T4) were compared with control (C) and thyroidectomized (Tx) rats. After 8 weeks of follow-up, LV was analyzed for α-MHC, β-MHC, TGF-β, and mir-208 expression, hydroxyproline content, and myocardial fibrosis. Serum collagenase activity was also analyzed. Heart weight increased in 5/6Nx rats compared to C, which was prevented with T4 supplementation (C, 1.5 ± 0.04; 5/6Nx, 1.8 ± 0.09; 5/6Nx + T4, 1.6 ± 0.07 g, p