Association Study Between Thyroid Volume And Iodine Status In 7-14 Year-old Children In Areas With Different Levels Of Water Iodine Concentration

Objective:Variation in different urinary measurements for evaluation of iodine status is of concern to clinicians and researchers. This study aimed to evaluate variations between urine iodine concentration(UIC), spot and 24-h urine sample and 24-h urine iodine excretion(24-h UIE) in repeated samples from school-aged children. Excessive iodine intake has adverse effect on thyroid, particularly on children. However, few previous studies have described safe upper iodine intake levels in children. The study aimed to determine the adverse effects of iodine excess on children by examining the associations between thyroid volume(Tvol) and total goiter rate(TGR) with daily iodine intake.Subjects and Methods: This study was performed in Dezhou and Liaocheng city, Shandong province, China, four counties with different level of iodine in drinking-water. In a multi-stage cross-sectional survey, we collected, on two nonconsecutive days, two 24-h urine samples and determined 24-h UIE. Ultrasonographic Tvol was measured for each participant to calculate TGR based on international and Chinese reference ranges for Tvol in children.Results: 1) 1927 water samples were collected and the medians of iodine content in different regions were in line with expectations and the median of all water samples was 183(69-406) μg/L. This study included 2089 children from four counties of Shandong province. The age, height, weight and BSA of boys are higher than girls(P<0.05). 2) In first collection, the medians of 24-h UIC and spot UIC were 381.1(203.5-649.4) μg/L and 479.8(217.2-821.1) μg/L, respectively, and spot UIC was higher than 24-h UIC(P<0.001). In second collection, the medians of 24-h UIC and spot UIC were 398.1(202.3-686.6) μg/L and 420.1(196.2-760.9) μg/L, respectively, and no significant difference was found(P=0.115). The medians of 24-h UIE in two collections were 275.3(139.3-480.7) μg/d and 285.3(141.3-523.7) μg/d, respectively, and the habitual iodine intake calculated from two 24-h UIEs was 298.2(186.4- 437.0) μg/d. A moderate correlation was observed between two spot UICs(rs=0.579, P<0.001); A significant correlation was found between two 24-h UICs(rs=0.718, P<0.001); A moderate correlation was observed between two24-h UIEs(rs=0.655, P<0.001). 3) Tvols were slightly higher in boys(P=0.035). The TGR was 9.7%, and did not differ between boys and girls. 4) Water iodine concentration, age, height, weight and BSA were positively correlated with habitual iodine intake. Similar with iodine intake, water iodine concentration, age, height, weight and BSA were positively correlated with Tvol, and when compared to height, weight and BSA, the correlation between age and Tvol was weaker. In addition, TGR was also positively correlated with height, weight and BSA. 5) Tvols and TGR became higher as iodine intake increased(rs=0.499,P<0.001; χ2=99.293, P<0.001). TGR exceeded 5% when iodine intake was 250-300μg/d. After grouping with age, TGR exceeded 5% when iodine intake was 200-250μg/d in 7-10 year-old children and TGR exceeded 5% when iodine intake was 250-300μg/d in 11-14 year-old children. Using logistic regression and two-piecewise linear regression, a nonlinear association was observed between Tvols, TGR and iodine intake levels, with a threshold at an iodine intake level of 110μg/d.Conclusions: 1) The 24-h UIC was more accurate and reproducible than spot UIC in evaluating iodine status in a large-scale population study. 2) Thyroid volume and the risk of TGR began to increase in children when iodine intake is >110 μg/day, and the TGR exceeded 5% with iodine intake 200 μg/d-250 μg/d in 7-10 year-old children and TGR exceeded 5% with iodine intake 250 μg/d-300 μg/d in 11-14 year-old children. 110-250μg/d appears to be an optimal iodine intake range for 7-10 year-old school-age children and 110-300 μg/d seems to be optimal iodine intake range for 11-14 year-old school-age children. 3) It is necessary to establish an ultrasonographic Tvol reference criterion based on BSA in China in order to monitor iodine status in population or individual. However, the definite optimal range depends on the further analysis.