To assess iodine intake levels and their impact on thyroid volume in children aged 8–10 years in a district of Beijing city from 2022 to 2024.

From 2022 to 2024, 200 non-boarding students aged 8–10 years were randomly selected annually from five townships in the district, totaling 600 participants over the three years. Urine samples, household salt samples, and ultrasound-measured thyroid volumes were collected. Generalized estimating equations (GEE) models were used to analyze annual trends and conduct sensitivity analyses by sex.

The median urinary iodine concentrations (UICs) in children aged 8–10 years from 2022 to 2024 were 170.96, 157.35, and 163.10 µg/L, respectively, indicating overall stability in UICs. However, a slight increasing trend in UICs was observed in boys over the years (P = 0.032). The mean salt iodine content (SIC) from 2022 to 2024 was (17.76 ± 7.96), (16.78 ± 11.48), and (16.27 ± 9.08) mg/kg, respectively, showing slight variations across the years. GEE model results showed that, after controlling for other factors, boys had larger thyroid volumes than girls (β = 0.32, 95%CI: 0.10–0.49); increasing age had a positive and significant effect on thyroid volume (β = 0.56, 95%CI: 0.46–0.64). Compared to girls, boys had higher mean UICs (β = 5.68, 95%CI: 3.10–7.79); SIC had a significant positive effect on UICs (β = 0.08, 95%CI: 0.02–0.14). Sensitivity analyses by sex revealed that the effect of SIC on UICs was significant and positive in both boys (β = 0.56, 95%CI: 0.27–0.86) and girls (β = 1.20, 95%CI: 0.01–0.80), while the direct effect of SIC on thyroid volume was not significant in either boys (β = 0.17, 95%CI: –0.40–0.60) or girls (β = 0.19, 95%CI: –0.50–0.80).

In iodine-sufficient environments, thyroid volume is primarily influenced by physiological factors such as age and sex, with salt iodine intake having no significant effect on thyroid volume. However, SIC is positively correlated with UIC. Iodine monitoring can focus on UIC measurements, as changes in thyroid volume are relatively small under iodine-sufficient conditions.