Why does iodine deficiency causes the thyroid to swell and goiter?

Gland needs iodine: Iodine deficiency can be seen in underdeveloped countries in the world, it is rare in developed countries such as the us. In us there is too much iodine and a higher incidence of autoimmune thyroid disease. Goiters in the us are familial or autoimmune related. Iodine deficiency causes goiter because the thyroid requires iodine to make thyroid hormone. Without it, thyroid enlarges due to high tsh.