Iodine is an essential component of thyroid hormones. The thyroglobulin-derived iodothyronine molecules contain 3 or 4 iodine atoms that are covalently bound during iodide organification. This process, also known as iodination, is catalyzed by thyroid peroxidase and requires that the thyroid cell concentrate iodide from plasma. Iodine is readily available from the ocean, and salt-water vertebrates, the first life forms to develop a thyroid gland, are not at risk for iodine deficiency. However, in terrestrial vertebrates, including humans, iodine availability can be limiting depending on the proximity to the ocean and the iodine content of the soil.
Iodine Landmark Paper: Pituitary Nuclear 3,5,3 -Triiodothyronine and Thyrotropin Secretion: An Explanation for the Effect of Thyroxine.
Antonio C. Bianco and Marla J. Berry. Journal of Trace Elements Experimental Medicine. February 23, 2001.