Urinary Iodine Concentration and Mortality Among US Adults

Background: Iodine deficiency has long been recognized as an important public health problem. Global approaches such as salt iodization that aim to overcome iodine deficiency have been successful. Meanwhile, they have led to excessive iodine consumption in some populations, thereby increasing the risks of iodine-induced thyroid dysfunction, as well as the comorbidities and mortality associated with hypothyroidism and hyperthyroidism. This study aimed to elucidate whether iodine intake is associated with mortality among U.S. adults. Methods: This was an observational study to estimate mortality risks according to urinary iodine concentration (UIC) utilizing a nationally representative sample of 12,264 adults aged 20-80 years enrolled in the National Health and Nutrition Examination Survey (NHANES) III. Crude and multivariable Cox proportional hazards regression models were employed to investigate the association between UIC (<50, 50-99, 100-299, 300-399, and 400g/L) and mortalities (all-cause, cardiovascular, and cancer). In sensitivity analyses, the study adjusted for total sodium intake and fat/calorie ratio in addition to other potential confounders. Stratum-specific analyses were also conducted to estimate the effects of UIC on mortality according to age, sex, race/ethnicity, and estimated glomerular filtration rate category. Results: Over a median follow-up of 19.2 years, there were 3159 deaths from all causes. Participants with excess iodine exposure (UIC 400g/L) were at higher risk for all-cause mortality compared to those with adequate iodine nutrition (hazard ratio=1.19 [confidence interval 1.04-1.37]). Elevated hazard ratios of cardiovascular and cancer mortality were also found, but the confidence interval of the effect estimates included the null value for both outcomes. Low UIC was not associated with increased mortality. Restricted cubic spline models showed similar results for all outcomes. The results did not change substantially after adjusting for total sodium intake and fat/calorie ratio. None of the potential interactions were statistically significant on a multiplicative scale. Conclusion: Higher all-cause mortality among those with excess iodine intake compared to individuals with adequate iodine intake highlights the importance of monitoring population iodine status. Further studies with longitudinal measures of iodine status are needed to validate these results and to assess the potential risks excess iodine intake may have on long-term health outcomes.