Iodine deficiency influences thyroid autoimmunity in old age--a comparative population-based study.

Objective: To assess thyroid autoimmunity among elderly people living in an area with low iodine intake compared to the sustained recommended iodine intake from a natural source, and to estimate the importance of migration.

Design And Setting: Iodine content of drinking water is highly different in the Danish towns Randers and Skagen. We collected blood and spot urine samples from 430 long-term Randers and Skagen dwellers aged 75-80 years, who filled in a questionnaire. We measured thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TGAb) in serum and iodine and creatinine in urine.

Results: Participation rate was 47% (n=212 (men/women 82/130) in Randers; 218 (84/134) in Skagen). Iodine deficiency prevailed in Randers while Skagen dwellers were iodine replete (median urinary iodine 74 μg/24h vs. 184 μg/24h, p<0.001). Thyroid antibodies were more frequent in Randers than in Skagen residents (42% vs. 32%; p=0.006) and more likely with iodine excretion <50 μg/24h (OR, 95%CI: 1.9, 1.1-3.4). Differences between towns increased with longer duration of residence as trends in the occurrence of TGAb and TPOAb were opposite (p<0.001; p=0.007).

Conclusions: Thyroid autoantibodies were common in old age, influenced by the iodine intake level, and the lowest frequency was found at the recommended iodine intake level.