Iodine Intake and Risk of Mortality: Evidence from a Nationally Representative Korean Cohort

Background/Objectives: Iodine intake influences thyroid-related and metabolic diseases that have important public health implications. However, longitudinal evidence of the association between iodine intake and mortality remains scarce and limited to Western populations. Given the markedly high iodine intake among Asians and possible ethnic or regional differences in iodine sensitivity, population-based data from Asian cohorts are needed. Methods: We analyzed 5497 adults from the Korean National Health and Nutrition Examination Survey (2013–2015) linked with mortality follow-up through 2021. Urinary iodine concentration (UIC) was quantified from spot urine samples using inductively coupled plasma mass spectrometry. Iodine intake was estimated from UIC and categorized into four groups: below the estimated average requirement, low-normal, high-normal, and above the tolerable upper level (UL). The primary outcome was all-cause mortality. Cardiovascular disease-specific and cancer-specific mortality were also assessed. Multivariable Cox proportional hazards models accounting for the complex survey design were used to estimate hazard ratios and 95% confidence intervals. Sensitivity analysis excluded participants with thyroid disease or early death, and subgroup analyses by age and sex were also conducted. Results: During a median 8.4-year follow-up, 139 deaths occurred. Compared with the low-normal intake group, excessive iodine intake (above UL) was not associated with all-cause mortality (HR 1.09, 95% CI 0.36–3.27), cardiovascular mortality (HR 1.27, 95% CI 0.21–7.61), or cancer mortality (HR 1.71, 95% CI 0.40–7.29). No significant trends were observed across intake categories (p > 0.2), and similar findings were obtained when UIC levels were analyzed. Excluding participants with thyroid disease or early death did not change the results, and no significant interaction was observed by age or sex. Conclusions: In this first population-based Asian study on iodine intake to mortality, neither estimated iodine intake nor UIC was associated with all-cause mortality. These results suggest that the relationship between iodine exposure and mortality may differ across populations, underscoring the need for further large-scale studies.