Autoimmune thyroiditis incidence in a large population-based study in northeastern Italy.

Purpose: An improvement in iodine status in the Veneto region (Italy) in the last decade has been documented. Our aim was to estimate the incidence of autoimmune thyroiditis (AT) in this region over the period 2012-2022.

Methods: A retrospective population-based study conducted in Veneto using the population registry and administrative health databases. We documented incident hyperthyroidism from 2013 to 2022 to exclude prevalent cases and calculated standardised incidence rates (IR) per 10,000 person-years by age and sex.

Results: We identified 65,379 incident cases (IR: 13.38), 5.44-fold higher in females than in males. IR decreased from 15.86 (95% CI: 15.50, 16.21) in 2013 to 12.35 (95% CI: 12.04, 12.67) in 2022. The decline was evident only in females, with a documented reduction in IR from 27.26 (95% CI: 26.61, 27.91) in 2013 to 20.49 (95% CI: 19.92, 21.07) in 2022 (P = 0.002). The decrease was sharper in females aged 15-54 years (IR from 37.86 (95%CI: 36.79, 38.94) in 2013 to 27.40 (95% CI: 26.44, 28.36) in 2022; P < 0.001) than in those aged ≥55 years (IR from 20.06 (95% CI: 19.13, 20.99) in 2013 to 16.56 (95% CI: 15.78, 17.35) in 2022; P = 0.034). In 2020, an out-of-trend decrease in AT incidence was documented, corresponding with the SARS-CoV-2 pandemic, with a realignment to the trend in the subsequent years.

Conclusions: A decline in AT was documented in the Veneto region in the last decade, paralleling improvement in the iodine status. The reduction was significant only among females, particularly in reproductive age.