N-Formyl-O-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodo-L-tyrosine Levothyroxine N-Formyl Impurity,2-formamido-3-(4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl)propanoic acid,Thyroxine derivative which showed less thyroid activity than Thyroxine,Levothyroxine Intermediate