Iodine is needed to make the thyroid hormones thyroxine and triiodothyronine, which assist with the creation of proteins and enzyme activity, as well as regulating normal metabolism. Without enough iodine, these thyroid hormones do not work properly and can lead to an under-active or overactive thyroid gland, causing the medical conditions of hypothyroidism and hyperthyroidism with various negative side effects in the body.