Iodine, an essential component of the hormones produced by the thyroid gland, is widely but unevenly distributed in the earth's environment. Great difference exists in the iodine nutritional status of populations residing in different region. Both iodine deficiency and iodine excess can injure the thyroid gland. Iodine deficiency tigers endemic goiter, cretinism, and hyperthyroidism, while iodine excess can result in high iodine goiter, chronic lymphocytic thyroiditis, iodine-induced hyperthyroidism, and hypothyroidism; also, iodine deficiency or excess may affect the histological type of thyroid cancer. In 1996, China began to implement the universal salt iodization policy, which has basically eliminated the iodine deficiency disorders nationwide; however, it also caused the changes in the spectra of other thyroid diseases including iodine-induced hyperthyroidism, autoimmune thyroid disease, and papillary thyroid carcinoma. Individualized iodine nutritional status assessment for the populations, particularly those with thyroid diseases, will be beneficial.