Context: Autoimmune thyroid disease develops in genetic susceptible subjects, provoked by environmental factors. Little is known of the environment in the early stages of autoimmunity.
Objective: We evaluated environmental factors contributing to de novo occurrence of thyroid antibodies.
Design: We conducted a prospective cohort study of 521 euthyroid women without thyroid antibodies in serum who were relatives of autoimmune thyroid disease patients. Follow-up was 5 yr. Baseline characteristics were related to the occurrence of thyroid peroxidase (TPO) and/or thyroglobulin (Tg) antibodies. Exposure to environmental factors in the year prior to the occurrence of antibodies was investigated in a nested case-control study.
Results: The 5-yr probability for conversion to TPO antibodies (TPO-Ab) and/or Tg antibodies (Tg-Ab) was 20.1%, and for TPO-Ab alone the probability was 14.5%. None of the baseline characteristics except TSH contributed to the risk of seroconversion. Each case (occurrence of antibodies) was matched for age and duration of follow-up with two controls (no seroconversion). Exposure to environmental stimuli was similar between cases and controls except for smoking. At study entrance, current smokers among cases and controls were 31.3 and 35.5%, respectively (nonsignificant). Current smoking decreased in cases during follow-up. Consequently, the odds ratios (OR) of smoking for developing TPO-Ab and/or Tg-Ab were 0.62 [95% confidence interval (CI), 0.37-1.04] 1 yr before seroconversion and 0.59 (95% CI, 0.35-0.99) at seroconversion; for conversion to TPO-Ab, these figures are 0.58 (95% CI, 0.31-1.09) and 0.54 (95% CI, 0.29-1.02), respectively.
Conclusion: Discontinuation of smoking is associated with an increased risk for occurrence of TPO-Ab and/or Tg-Ab in serum. The observation is in line with the decreased risk of hypothyroidism in smokers.