Iodine: An element in the diet that is essential for the manufacture of hormones by the thyroid gland. The thyroid gland uses iodine to make thyroxine (T4), which has four iodine molecules attached to its structure, and triiodothyronine (T3), which has three iodine molecules attached. Iodine is found in seafood, bread, iodized salt, and seaweed.
