Is iodine found in sea salt?

Goiter was first discovered when people moved away from the seacoast and stopped having salt water fish in their diet. The fish were the source of iodine for the thyroid. We add iodine to table salt so everyone gets enough iodine. Sea salt has a label that states no iodine added, and not enough to prevent goiter. If the fish have iodine, why does not sea salt?