The number 4 represents four atoms of iodine per molecule. Thyroxine is a type of hormone produced by the thyroid gland and this hormone controls human's metabolism process. There are two hormones produced by the thyroid gland, the thyroxine and the triiodothyronine, which also control human's metabolism process. These hormones consisted of iodine atoms in each of the molecule, which human gets from eating salt.