Relationship Between Iodine Status and Thyroid Function in Preschool Children: From the Environmental and Development of Children (EDC) Study

Background: We investigated iodine status and its association with thyroid function among preschool children residing in iodine-sufficient area. Methods: From the Environment and Development of Children study, 477 children were evaluated for thyroid function and urine iodine concentration (UIC) at age 6 during 2015-2017. After excluding children born with multiple birth and with congenital hypothyroidism or Hashimoto thyroiditis, 439 (231 boys) were included. Subclinical hypothyroidism (SCH) was defined as thyroid stimulating hormone (TSH) levels between 4.9-10 μIU/mL with normal free T4 levels. Iodine status was evaluated by UIC and children were categorized into 4 groups: iodine deficient (UIC < 100 μg/L), adequate (UIC, 100-299 μg/L), mild excessive (UIC, 300-999 μg/L), severe excessive (UIC ≥ 1000 μg/L). Results: Goiter was palpated in 64 (14.6%) with female predominance (26.0% vs. 4.3%, P < 0.001). Serum level of free T4 and T3 was 1.2 ± 0.1 ng/dL and 148.1 ± 18.5 ng/dL, respectively. The median TSH level was 2.3 (0.53-8.59) μIU/mL and the prevalence of SCH was 4.3% without sex-difference. The median UIC level was 606.2 (19.9-16409.7) μg/L, higher in boys (684 vs. 545 μg/L, P = 0.021) than in girls. Iodine was deficient in 19 (4.3%), adequate in 96 (21.9%), mild excessive in 170 (38.7%), and severe excessive in 145 (35.1%). After excluding 19 iodine deficient children, the relationship between iodine status and thyroid function was evaluated by multiple regression analysis after adjusting for age, sex, birth weight, gestational age, body mass index Z-score, and family history. As iodine status increased from adequate, mild excessive to severe excessive group, T3 levels decreased, and TSH levels increased with marginal significance (P for trend < 0.1 for T3 and TSH). When stratified by sex, similar association was found in only girls (P for trend = 0.043 for T3, and 0.062 for TSH) but not in boys, and mild excessive group showed lower free T4 levels (β = -0.05, P = 0.013) and severe excessive group had lower T3 levels (β = -7.04, P = 0.035) than iodine adequate group in only girls, but not in boys. Conclusion: Iodine was deficient in 4.3%, adequate in 21.9%, and excessive in 73.8% among preschool children residing in South Korea. As iodine status increased from adequate to excessive group, TSH levels increased with decreasing free T4 and T3 levels in girls.