Levothyroxine Sodium Impurity: (S)-2-amino-3-[3-chloro-4-(4-hydroxy-3,5-diiodophenoxy)-5-iodopheLevothyroxine Sodium Impurity: (S)-2-amino-3-[3-chloro-4-(4-hydroxy-3,5-diiodophenoxy)-5-iodophenyl]-propanyl]-propanoic acid (Monochlorotriiodothyronine).