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Background: Longitudinal data assessing the impact of iodine deficiency (ID) on mortality are scarce. We aimed to study the association between the state of iodine nutrition and the risk of total and cause-specific mortality in a representative sample of the Spanish adult population. Methods: We performed a longitudinal observational study to estimate mortality risk according to urinary iodine (UI) concentrations using a sample of 4370 subjects >18 years representative of the Spanish adult population participating in the nationwide study Di@bet.es (2008–2010). We used Cox regression to assess the association between UI at the start of the study (<50, 50–99, 100–199, 200–299, and ≥300 μg/L) and mortality during follow-up (National death registry—end of follow-up December 2016) in raw models, and adjusted for possible confounding variables: age, sex, educational level, hypertension, diabetes, obesity, chronic kidney disease, smoking, hypercholesterolemia, thyroid dysfunction, diagnosis of cardiovascular disease or cancer, area of residence, physical activity, adherence to Mediterranean diet, dairy and iodinated salt intake. Results: A total of 254 deaths were recorded during an average follow-up period of 7.3 years. The causes of death were cardiovascular 71 (28%); cancer 85 (33.5%); and other causes 98 (38.5%). Compared with the reference category with adequate iodine nutrition (UI 100–300 μg/L), the hazard ratios (HRs) of all-cause mortality in the category with UI ≥300 μg/L were 1.04 (95% confidence interval [CI 0.54–1.98]); however, in the categories with 50–99 UI and <50 μg/L, the HRs were 1.29 [CI 0.97–1.70] and 1.71 [1.18–2.48], respectively (p for trend 0.004). Multivariate adjustment did not significantly modify the results. Conclusions: Our data indicate an excess mortality in individuals with moderate-severe ID adjusted for other possible confounding factors. [ABSTRACT FROM AUTHOR]