Objective To establish reference ranges for thyroid function indices such as thyroid stimulating hormone (TSH), free triiodothyronine (FT₃), free thyroxine (FT₄), and thyroglobulin (Tg) in school-aged children in Tianjin, China, and to provide a reference for the diagnosis and management of thyroid diseases and the evaluation of iodine nutrition in local children.

Methods A total of 1324 children aged 8 to 10 years were randomly selected from areas with adequate iodine intake in Tianjin using cluster sampling. The samples were analyzed using a fully automated chemiluminescence immunoassay analyzer. The 95% reference ranges for the thyroid function indices were calculated using percentiles, and the necessity of establishing reference ranges based on age, sex, and region was examined.

Results After applying the inclusion and exclusion criteria, 644 and 249 school-aged children were included to study the TSH/FT₃/FT₄ reference range and the Tg reference ragne, respectively. The reference ranges for TSH, FT₃, FT₄, and Tg were 1.07-6.17 mIU/L, 5.35-7.88 pmol/L, 13.51-21.67 pmol/L, and 3.15-34.51 μg/L, respectively. There were no significant differences in thyroid function indices between ages (all P>0.05) and the Z score was smaller than Z^*for the indices between both sexes and regions, indicating that the criteria for grouping were not satisfied. Correlation analysis showed that Tg was positively correlated with TSH (r_s=0.099, P=0.043) and negatively correlated with FT₃ (r_s=-0.131, P=0.007). Stratified analysis by sex showed that Tg was positively correlated with TSH(r_s=0.145, P=0.043) and FT₄ (r_s=0.184, P=0.010) and negatively correlated with thyroglobulin antibody (TgAb) (r_s=-0.154, P=0.031) in girls.

Conclusion The thyroid function indices in school-aged children in Tianjin differ from those in other countries and other cities of China. Therefore, it is necessary to establish region-specific reference ranges for these indices.