Animals with thyroid glands need iodine. Iodine deficiency makes a person seriously stupid. Youngsters suffer irreversible mental retardation from lack of iodine. For this reason, iodine is added to table salt throughout much of the world.
Iodine is naturally found in seawater as a water-soluble ion. Unsurprisingly, seaweed and seafood are good sources of iodine. Iodine is also found in soybeans.