Thyroid Nodules in an 11-Year DanThyr Follow-Up Study

Abstract

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: Our objective was to clarify how IF influenced thyroid gland structure in 2 regions with different iodine intake at baseline (Copenhagen, mild iodine deficiency [ID]; Aalborg, moderate ID). Design and Setting: We conducted 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 2465 adults, and ultrasonography was performed by the same sonographers using the same equipment, after controlling performances. Main Outcome Measure: Change in thyroid gland structure was evaluated. Results: The follow-up period saw an increased prevalence of multinodularity (9.8%–13.8 %, P < .001), especially in the previously moderate ID region of Aalborg (9.1%–15.4%, P < .001), whereas no change in prevalence was seen for solitary nodules (5.6%–5.1%, P = .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% confidence interval [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 [95% CI = 0.32–0.67]). A regional difference (Aalborg vs Copenhagen) was seen between normalization rates of multiple nodules (normalization rate ratio 0.29 [95% CI = 0.12–0.64]), but not for solitary nodules (normalization rate ratio 0.81 [95% CI = 0.53–1.21]). Conclusions: Changes in the thyroid gland structure with both appearance and disappearance of thyroid nodules are common after an iodization program.