Efficacy and Safety of Long-Term Universal Salt Iodization on Thyroid Disorders: An Epidemiological Evidence from 31 Provinces of Mainland China

Background: The mandatory universal salt iodization (USI) program has been implemented in China for twenty years. Although iodine deficiency disorders are effectively controlled, the dangers of excess iodine have been debated. Methods: A nationally representative cross-sectional study of 78,470 participants, aged 18 or older, were enrolled from all 31 provincial regions of mainland China. The participants were given a questionnaire and B-mode ultrasonography on the thyroid. Serum concentrations of thyroid hormones, thyroid antibodies and urine iodine concentration (UIC) were measured. Findings: The median UIC of school-age children was 199·75µg/L. The weighted prevalence of the thyroid disorders in adults were as follows: 0·78% of overt hyperthyroidism, 0·44 % of subclinical hyperthyroidism, 0·53% of Graves’ disease, 1·02% of overt hypothyroidism, 12·93% of subclinical hypothyroidism, 14·19% of autoimmune thyroiditis, 10·19% of positive TPOAb, 9·70% of positive TgAb, 1·17% of goiter and 20·43 % of thyroid nodules. Iodine excess was only associated with higher odds of overt hyperthyroidism and subclinical hypothyroidism, while iodine deficiency was significantly associated with higher odds of most thyroid disorders. In addition, increased iodine intake was significantly associated with elevated serum TSH levels, but was inversely associated with thyroid antibodies and thyroid nodule. Interpretation: The long-term mandatory USI program is not only effective for iodine deficient disorders but also safe for prevalence of thyroid disorders based on a sufficient iodine status of pre-USI. The risk of iodine deficiency outweighs that of iodine excess. Funding Statement: Research Fund for Public Welfare from National Health and Family Planning Commission of China. Declaration of Interests: All authors declare no competing interests. Ethics Approval Statement: The research protocols were approved by the Medical Ethics Committee of China Medical University. All subjects were provided written informed consent following a thorough explanation of the research procedures.