Abstract
BACKGROUND: Season of birth, an exogenous indicator of early life environment, has been linked with a higher risk of adverse health outcomes such as autoimmune thyroiditis, multiple sclerosis and schizophrenia later in life. Whether the development and cause of hyperthyroidism is influenced by season of birth is unclarified. We aimed, at a nationwide level, to investigate whether season of birth influences the risk of hyperthyroidism due to Graves' disease (GD) and/or toxic nodular goitre (TNG). METHOD: Register-based nationwide cohort study. By record-linkage between Danish health registers, 36,087 and 20,537 patients with GD and TNG, respectively, were identified. Each case was matched with four controls without thyroid disease, according to age and sex. Differences in month of birth across the year were evaluated by the Walter-Elwood test. Hazard ratios, for the risk of GD and TNG in individuals born in a certain month or season of the year, were calculated using Cox regression models. RESULTS: Neither for GD nor for TNG could we demonstrate a significant difference in birth rate across months or seasons of the year (Walter-Elwood's test; X2 = 5.92 and X2 = 1.27, P = 0.052 and P = 0.53, respectively). CONCLUSION: Irrespective of its cause, our findings do not support the hypothesis that season of birth is significantly related to the development of hyperthyroidism.