Thyroid nodules in an eleven-year DanThyr follow-up study

Context: Limited longitudinal data are available on changes in the thyroid gland structure in a population and how this is influenced by iodine fortification (IF). Objective: To clarify how IF influenced thyroid gland structure in two regions with different iodine intake at baseline (Copenhagen, mild iodine deficiency (ID); Aalborg, moderate ID). Design and Setting: A longitudinal population-based study (DanThyr) where participants were examined before (1997) and after (2008) the Danish mandatory IF of salt (2000). Participants: We examined 2,465 adults and ultrasonography was performed by the same sonographers using the same equipment, after controlling performances. Main Outcome Measure: Change in thyroid gland structure. Results: The follow-up period saw an increased prevalence of multinodularity (9.8-13.8 %, p<0.001), especially in the previously moderate ID region of Aalborg (9.1 -15.4 %, p<0.001), whereas no change in prevalence was seen for solitary nodules (5.6- 5.1 %, p=0.34). In individual participants, changes in thyroid structure and disappearance of thyroid nodules during the 11 years was common with an overall normalization rate of 21.2 (95% CI 17.9-24.9) per 1000 person-years. Solitary nodules had a significantly higher normalization rate than multiple nodules (normalization rate ratio 0.47 (CI 0.32-0.67). Regional difference (Aalborg vs. Copenhagen) was seen between normalization rates of multiple nodules (normalization rate ratio 0.29 (CI 0.12-0.64)), but not for solitary nodules (normalization rate ratio 0.81 (CI 0.53-0.1.21)). Conclusions: Changes in the thyroid gland structure with both appearance and disappearance of thyroid nodules are common after an iodization program.