Strong association of relatively low and extremely excessive iodine intakes with thyroid cancer in an iodine-replete area

Purpose The relationship between iodine intake and development of thyroid diseases shows a U-shaped curve with an increase of risk in both deficient and excessive iodine intakes. Our aim was to investigate the relationship between iodine intake and thyroid cancer in patients with thyroid nodules in an iodine-replete area. Methods Retrospective analysis of 1170 patients with thyroid nodules was performed. Urinary iodine concentration (UIC) was measured by inductively-coupled plasma mass spectrometry. Predictive factors for thyroid cancer were evaluated using multivariate logistic regression models. Results The median UIC in all patients with thyroid nodules was 360 mu g/L (range from 4 to 9631 mu g/L). More than half of the patients (650/1170, 56 %) belonged to the category of excessive iodine intake (UIC = 300 mu g/L) according to WHO iodine recommendations. Patients with thyroid cancer were more likely to be distributed in UIC < 300 mu g/L and in UIC = 2500 mu g/L than those with benign thyroid nodules. Male gender (OR 1.528, p = 0.028) and UIC were independent predictors for thyroid cancer. The multivariate-adjusted OR (95 % CI) in the relatively low (UIC < 300 mu g/L) and extremely excessive (UIC = 2500 mu g/L) iodine intake groups for thyroid cancer were 1.519 (1.099-2.098) and 1.874 (1.094-3.208), respectively, compared to the other iodine intake group (300-2499 mu g/L). Conclusion Male gender and UIC were independent predictors of thyroid cancer in patients with thyroid nodules. This study suggests that relatively low and extremely excessive iodine intakes are associated with thyroid cancer in an iodine-replete area.