Iodine Modifies the Susceptibility of Thyroid to Fluoride Exposure in School-age Children: a Cross-sectional Study in Yellow River Basin, Henan, China

Excessive fluoride exposure has detrimental effects on the thyroid gland, which may be modified by iodine. However, the role of iodine in it remains unclear. This study aims to evaluate the role of iodine in thyroid abnormalities caused by fluoride exposure in school-age children. A total of 446 children aged 7-12 years were recruited from Tongxu County, Henan province, in 2017 (ZZUIRB 2017-018). We obtained demographic information through questionnaire surveys. The concentrations of urinary fluoride (UF) and urinary iodine (UI) were determined by the ion-selective electrode method and the catalytic spectrophotometric method, respectively. The radiation immunoassay was used to determine the serum concentrations of total triiodothyronine (TT3), total thyroxine (TT4), and thyroid-stimulating hormone (TSH). The B-mode ultrasound was performed to assess thyroid volumes (Tvols). The associations between fluoride exposure and thyroid-related indicators were tested by linear regression models. We found that Tvols increased by 0.22 (95% CI: 0.14, 0.31) cm(3) with each standard deviation increment of UF. Moreover, Tvols in boys were more susceptible to fluoride exposure than those in girls, and the Tvols of children with high urinary iodine are less susceptible to fluoride exposure (P for interaction < 0.05). We also observed that TT3 levels were negatively related to UF concentrations at moderate urinary iodine levels (<= 300 mu g/l). Fluoride exposure can elevate the Tvols of school-age children, especially in boys, and high levels of iodine may alleviate this effect to some extent.