3,3',5,5'-Tetraiodo-L-thyronine sodium saltm is the sodium salt of levothyroxine, a synthetic levoisomer of thyroxine (T4) that is similar to the endogenous hormone produced by the thyroid gland. In peripheral tissues, levothyroxine is deiodinated by 5'-deiodinase to form triiodothyronine (T3). T3 enters the cell and binds to nuclear thyroid hormone receptors, the activated hormone-receptor complex in turn triggers gene expression and produces proteins required in the regulation of cellular respiration, thermogenesis, cellular growth and differentiation, and the metabolism of proteins, carbohydrates and lipids. T3 also exhibits cardiostimulatory effects.