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.
Results: 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. Conclusions: The long-term mandatory USI program with timely adjustments is successful in preventing iodine deficiency disorders and it appears to be safe. The benefits outweigh the risk in a population with a stable a median iodine intake