Optimal and safe upper limits of iodine intake for early pregnancy in iodine-sufficient regions: a cross-sectional study of 7190 pregnant women in China

Xiaoguang Shi, Cheng Han, Chenyan Li, Jinyuan Mao, Weiwei Wang, Xiaochen Xie, Chenyang Li, Bin Xu, Tao Meng, Jianling Du, Shaowei Zhang, Zhengnan Gao, Xiaomei Zhang, Chenling Fan, Zhongyan Shan, Weiping Teng
Journal of Clinical Endocrinology and Metabolism 2015, 100 (4): 1630-8

CONTEXT: The WHO Technical Consultation recommends urinary iodine concentrations (UIC) from 250 to 499 μg/L as more-than-adequate iodine intake and UIC ≥ 500 μg/L as excessive iodine for pregnant and lactating women, but scientific evidence for this is weak.

OBJECTIVE: We investigated optimal and safe ranges of iodine intake during early pregnancy in an iodine-sufficient region of China.

METHOD: Seven thousand one hundred ninety pregnant women at 4-8 weeks gestation were investigated and their UIC, serum thyroid stimulating hormone (TSH), free thyroxine (FT4), thyroid-peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), and thyroglobulin (Tg) were measured.

RESULTS: The prevalence of overt hypothyroidism was lowest in the group with UIC 150-249 μg/L, which corresponded to the lowest serum Tg concentration (10.18 μg/L). Prevalences of subclinical hypothyroidism (2.4%) and isolated hypothyroxinemia (1.7%) were lower in the group with UIC 150-249 μg/L. Multivariate logistic regression indicated that more-than-adequate iodine intake (UIC 250-499 μg/L) and excessive iodine intake (UIC ≥ 500 μg/L) were associated with a 1.72-fold and a 2.17-fold increased risk of subclinical hypothyroidism, respectively. Meanwhile, excessive iodine intake was associated with a 2.85-fold increased risk of isolated hypothyroxinemia. Moreover, the prevalence of TPOAb positivity and TgAb positivity presented a U-shaped curve, ranging from mild iodine deficiency to iodine excess.

CONCLUSION: The upper limit of iodine intake during early pregnancy in an iodine-sufficient region should not exceed UIC 250 μg/L, because this is associated with a significantly high risk of subclinical hypothyroidism, and a UIC of 500 μg/L should not be exceeded, as it is associated with a significantly high risk of isolated hypothyroxinemia.

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