A A century ago, A large part of North America was known as the thyroid gland belt. In some areas, between 26% to 70% of school -age children suffer from distinctive swelling in the neck, or thyroid enlargement, caused by thyroid gland. After experiments showed that iodine supplements could prevent thyroid enlargement, iodine -treated salt in 1924, starting with grocery stores in Michigan. By the 1940s, the thyroid gland gland belt had disappeared.
