TG and heart failure: These effects were reversible with thyroid hormone replacement.36 Low T3 syndrome could be caused by type 3 deiodinase activation, which was characterized by decreased T3 and elevated rT3 levels.37 Type 3 deiodinase can specifically reduce the thyroid hormone signal transduction in the heart tissue, thereby regulating thyroid function and accelerating cardiac dysfunction progression.37 Even if thyroid stimulating hormone was maintained at normal levels, low T3 may increase mortality in heart failure patients.38,39