Predictors of Change in Serum TSH after Iodine Fortification: An 11-Year Follow-Up to the DanThyr Study

Context: Few data are available on the effect of iodine fortification on thyroid function development in a population. Objective: Our objective was to evaluate changes in thyroid function after iodine fortification in a population and to identify predictors for changes in serum TSH. Design and Setting: A longitudinal population-based study of the DanThyr C1 cohort examined at baseline (1997–1998) and reexamined 11 yr later (2008–2010). The mandatory program for iodization of salt was initiated in 2000. Participants: A total of 2203 individuals, with no previous thyroid disease, living in two areas with different levels of iodine intake, with measurement of TSH and participation in follow-up examination were included in the analysis. Main Outcome Measure: Change in serum TSH was evaluated. Results: During the 11-yr follow-up, mean TSH increased significantly from 1.27 mU/liter [95% confidence interval (CI) = 1.23–1.30] to 1.38 mU/liter (CI = 1.34–1.43) (P < 0.001). The most pronounced increase was observed in the area with the highest iodine intake [1.30 mU/liter (CI = 1.25–1.35) to 1.49 mU/liter (CI = 1.43–1.55), P < 0.001], whereas the increase was not significant in the low-iodine-intake area [1.24 (CI = 1.19–1.29) to 1.28 (CI = 1.23–1.34), P = 0.06)]. Change in TSH was positively associated with the presence of thyroid peroxidase antibody at baseline (P < 0.001) and negatively associated with baseline thyroid enlargement (P < 0.001) and multiple nodules (P < 0.001). Conclusions: Even small differences in the level of iodine intake between otherwise comparable populations are associated with considerable differences in TSH change at the 11-yr follow-up. Multinodular goiter predicted a less pronounced TSH increase during follow-up, which may be explained by iodine-dependent activity of autonomous nodules.