Longitudinal Trends in Iodine Nutritional Status and Thyroid Morphology Among School-Age Children in Shenzhen,China(2021–2025)

Universal Salt Iodization(USI)has effectively eliminated iodine deficiency disorders(IDD)in China.However,the iodine landscape is shifting due to rapid urbanization,changing dietary habits,and salt reduction efforts.This study examined longitudinal trends in iodine status and thyroid morphology among 9,000 school-age children in Shenzhen from 2021 to 2025 to understand these dynamics in a post-elimination setting.Using a five-year repeated cross-sectional design,we assessed urinary iodine concentration(UIC),salt iodine content(SIC),and thyroid ultrasound( n  = 3,400).The overall median UIC was 216.2µg/L,classifying as adequate.Despite general sufficiency,a clear downward trend appeared:median levels fell from 246.6µg/L in 2021 to 188.0µg/L in 2025( p  < 0.001),even though household salt iodine remained stable and compliance was high(92.7%).Girls had lower urinary iodine than boys(209.6 vs.224.0µg/L)yet faced a significantly higher risk of thyroid enlargement(Adjusted OR = 1.95,95%CI:1.19–3.31).These findings suggest that relying solely on household salt monitoring may no longer fully capture total iodine intake,likely due to the increasing influence of non-household dietary sources such as processed foods and dining out.