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

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.