Association Between Household Iodized Salt Trajectory and High Milk–Enriched Diet with Thyroid Nodules and Goiter in Children: A Prospective Cohort Study

Background: Universal salt iodization (USI) policies have achieved remarkable results in preventing iodine deficiency disorders globally. However, concerns regarding its safety have emerged along with rising detection rates of thyroid disorders and shifts in dietary structures. The association between iodized salt consumption and thyroid nodule development remains unclear, and the modifying role of dietary structures is not well understood. This study aimed to examine associations between iodized salt consumption and the incidence of thyroid nodules and goiter, accounting for dietary structures. Methods: This prospective cohort study enrolled 8464 school-aged children across 79 schools from 2021 to 2023. Salt exposure and dietary structures were assessed via household sampling and questionnaires. Thyroid nodules and goiter were identified through ultrasonography. Latent class trajectory modeling identified iodized and noniodized salt trajectories, alongside high milk–enriched and low milk–enriched dietary structures. Cox proportional hazards model and logistic regression model were applied to analyze the associations between salt trajectories and dietary structures with thyroid nodules and goiter separately. Results: Two thousand seven hundred and seven (32.0%) participants developed thyroid nodules and 164 (1.9%) had goiter. Two salt trajectories were identified: iodized (7892; 93.2%) and noniodized (572; 6.8%). Iodized salt was associated with lower risks of nodules (hazard ratio [HR], 0.806 [confidence interval or CI: 0.659–0.986]) and goiter (odds ratio, 0.458 [CI: 0.299–0.669]). However, the association with nodules did not persist in region-stratified analyses (HR, 1.114 [CI: 0.745–1.667]). A high milk–enriched diet was consistently associated with reduced nodule risk (HR, 0.867 [CI: 0.787–0.956]) and remained significant in all sensitivity analyses. A significant interaction was observed between dietary structure and salt trajectory (adjusted p for interaction = 0.024). A U-shaped relationship was observed between salt iodine concentration and nodule risk, with an inflection point at 21.3 mg/kg. Conclusions: Iodized salt was associated with lower goiter risk, but its inverse association with thyroid nodules was not observed after region restriction. A high milk–enriched diet was inversely associated with thyroid nodules. These findings support continued USI for goiter prevention and highlight the importance of regional iodine status and dietary patterns when interpreting household salt use and thyroid outcomes in children.