Efficacy and safety of long-term universal salt iodization on thyroid disorders: epidemiological evidence from 31 provinces of mainland China

Yongze Li, Di Teng, Jianming Ba, Bing Chen, Jianling Du, Lanjie He, Xiaoyang Lai, Xiaochun Teng, Xiaoguang Shi, Yanbo Li, Haiyi Chi, Eryuan Liao, Chao Liu, Libin Liu, Guijun Qin, Yingfen Qin, Huibiao Quan, Bingyin Shi, Hui Sun, Xulei Tang, Nanwei Tong, Guixia Wang, Jin-An Zhang, Youmin Wang, Yuanming Xue, Li Yan, Jing Yang, Lihui Yang, Yongli Yao, Zhen Ye, Qiao Zhang, Lihui Zhang, Jun Zhu, Mei Zhu, Guang Ning, Yiming Mu, Jiajun Zhao, Zhongyan Shan, Weiping Teng
Thyroid: Official Journal of the American Thyroid Association 2020 February 19

BACKGROUND: Mandatory universal salt iodization (USI) has been implemented in China for twenty years. Although iodine deficiency disorders are effectively controlled, the risk of excess iodine have been debated.

METHODS: A nationally representative cross-sectional study with 78,470 enrolled participants, aged 18 or older, from all 31 provincial regions of mainland China was performed. 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 the adult population was 177.89 µg/L. The weighted prevalence of 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 positive thyroid antibodies (Ab), 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 risks in a population with a stable median iodine intake level of up to 300 µg/L.

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