Thyroid nodules in an 11-year DanThyr follow-up study.

Abstract

Limited longitudinal data are available on changes in the thyroid gland structure in a population and how this is influenced by iodine fortification (IF). 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). We conducted a longitudinal population-based study (DanThyr) where participants were examined before (1997) and after (2008) the Danish mandatory IF of salt (2000). We examined 2465 adults, and ultrasonography was performed by the same sonographers using the same equipment, after controlling performances. Change in thyroid gland structure was evaluated. 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]). Changes in the thyroid gland structure with both appearance and disappearance of thyroid nodules are common after an iodization program.