The Status of Iodine Nutrition and Thyroid Disorders in China: Two Decades After the Universal Salt Iodization

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 deficiency was significantly associated with higher odds of most thyroid disorders, while excessive iodine was only significantly associated with higher odds of overt hyperthyroidism and subclinical hypothyroidism. In addition, excessive iodine was negatively associated with thyroid nodules. No elevated prevalence of all thyroid disorders was found in UIC 200-299µg/L group compared with 100-199µg/L group. INTERPRETATION: The prevalence of overt thyroid dysfunction has maintained at a stable level as a result of twenty years mandatory USI program except for subclinical hypothyroidism and thyroid nodule. The risk of iodine deficiency outweighs that of iodine excess. Definite advantages were implied to combine the two categories of UIC values of 100-199µg/L and 200-299µg/L as one single range of recommended iodine intake for general population. 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.