Trends of thyroid function and autoimmunity to 5 years after the introduction of mandatory iodization in Italy

Background. Mandatory iodine fortification of salt was introduced in Italy in 2005. The purpose of our study is to estimate the trend of thyroid diseases in Italy before and after mandatory iodization in Italy. Methods. 7976 patients (6802 females; 1174 males) were examined between 2003 and 2010. We divided the patients into categories according to their clinical diagnosis. Later, we evaluated for each patient if there was the positivity for antibodies anti TPO. Finally, we collected data about TSH values of all patients who did not use therapy for thyroid disease at the time of the examination. To evaluate the differences percentages for categorical variables, χ2-Pearson test was used; to evaluate the percentage differences of TSH values we used the Kruskal-Wallis test. Values were considered statistically significant at p < 0.05. Results. We considered diagnosis of "toxic adenoma / goiter", diagnosis of "Graves' disease" and diagnosis of "hyperthyroidism / thyrotoxicosis" before and after the introduction of mandatory iodization in Italy, to demonstrate the possible presence of iodine-induced hyperthyroidism. Diagnosis of toxic adenoma / goiter before 2005 were 3.7%, while after 2005 they were reduced to 3.1%. Diagnosis of Graves' disease before 2005 were 2.4%, and they remained unchanged after 2005. Finally, diagnosis of hyperthyroidism / thyrotoxicosis decreased from 2.5% to 2.1%. Comparing these results, there were no significant differences (P = 0.261) between the percentages of diagnosis of hyperthyroidism before and after the introduction of mandatory iodization. We considered diagnosis of Hashimoto's thyroiditis before and after 2005 to demonstrate a link between the administration of iodine and thyroid autoimmunity. Prevalence of Hashimoto's thyroiditis between 2003 and 2005 was 37%; prevalence between 2006 and 2010 dropped to 34.7%. This small difference has a borderline statistical significance (P = 0.049). Stratifying TSH values from year to year, we found a small increase in TSH value, which, in any case, remains in the normal range. TSH values passed from 1.37 in 2003, to 1.61 in 2010. Although this increase is modest, it is statistically significant (P values < 0.001). Conclusion. In this study there isn't a connection between iodine fortification and iodine-induced hyperthyroidism, and between iodine fortification and Hashimoto's thyroiditis. This small increase of TSH values can not find a clear explanation yet. We considered several mechanisms to explain this phenomenon: alterations of Na/I symporter reduce intracellular levels of iodine, and cause a state of hypothyroidism; excessive levels of iodine can increase apoptosis of thyrocytes, as it is demonstrated by several studies; interference from external agents (endocrine disruptors).