Background: Environmental factors play a role in the pathogenesis of autoimmune conditions. The incidence of type 1 diabetes is higher in winter. It is unclear whether autoimmune thyroid disease is similarly affected by seasonal variation. We aimed to study the variation in anti thyroid peroxidase (TPO) antibodies in relation to calendar month of sampling.
Methods: We obtained TPO-antibody results for the 12 months (October 2009 till September 2010). Individuals with known thyroid disease were excluded. TPO-antibody levels were measured quantitatively by ELISA and levels >50 IU/ml were classed as high. Geometric mean TPO-antibody concentrations were compared against month of sampling after correcting for multiple comparisons. Mean maximum temperatures and sunshine hours for the north east of England for the same time period were obtained from the Met Office. Stepwise linear regression analyses were performed to determine factors predicting TPO levels.
Results: The mean age (S.D.) of the cohort (n=841) was 51.2 (21.7) years and majority were female (73.1%). The age and gender distribution of the cohort was similar in each month of sampling. Geometric mean (range) TPO-antibody levels were lower in winter than in summer with the lowest level in December of 15.25 (3540) IU/ml and highest in August of 32.6 (93000) IU/ml (P<0.001). The proportion of individuals with high TPO levels was higher in August (32.9%) than in December (18.6%), P<0.01. Age, gender and maximum temperature on the day of sampling explained 6% of the variation in TPO levels with temperature alone accounting for 3% of variation.
Conclusions: Thyroid autoimmunity as measured by TPO-antibody levels has a seasonal variation with lower concentrations in winter and higher in summer which is partly explained by the effect of temperature. This apparent association of thyroid autoimmunity with temperature may have implications for the future, especially in the context of global warming.