Complex sentence: Thyroxine is produced by attaching iodine atoms to the ring structures of this protein's tyrosine residues; thyroxine (T 4) contains four iodine atoms, while triiodothyronine (T 3), otherwise identical to T 4, has one less iodine atom per molecule.
Split into simpler sentences: Thyroxine is produced by attaching iodine atoms to the ring structures of tyrosine molecules. Triiodothyronine (T 3) is identical to T 4, but it has one less iodine atom per molecule.