Attenuated Seasonality of Childhood Type 1 Diabetes with Increasing Autoimmune Comorbidities: A 40-year Cohort Study

BACKGROUND: Seasonal variation in the onset of Type 1 Diabetes Mellitus (T1DM) has long been recognized, yet little is known about how the severity of autoimmunity interacts with environmental triggers to influence disease manifestation. AIMS: This study aimed to elucidate the etiological interplay between seasonality, genetic predisposition, and immune dysregulation over a 40-year period. METHODS: A retrospective cohort of 2,954 patients diagnosed with T1DM between 1981 and 2024 was analyzed. Seasonal trends in disease onset and autoimmune comorbidities were evaluated acoording to age at onset, birth season, and sex. RESULTS: Distinct age and immunity-dependent seasonal patterns were identified. Early-onset T1DM showed no seasonal fluctuation, suggesting predominant genetic and immunological determinants. Conversely, late-onset T1DM demonstrated a clear autumn-winter peak and was more frequent among children born in spring-summer. The presence of multiple autoimmune disorders abolished this seasonality, implying that intense autoimmune activity may override environmental effects. CONCLUSIONS: This 40-year cohort provides novel evidence that the relative contribution of environmental and immunogenetic factors in T1DM varies with age and immune burden. Environmental seasonality appears to play a greater role in later-onset disease, whereas early-onset and multi-autoimmune forms are driven by intrinsic immune mechanisms. These findings highlight the heterogeneity of T1DM pathogenesis and support the need for age- and immunity-specific strategies for prevention and early screening.