Hyperthyroidism incidence in a large population-based study in northeastern Italy.

Objective: An improvement in iodine status in Veneto Region has been documented in the last decade. We aimed at estimating the incidence of hyperthyroidism in the Veneto Region (Italy) over the period 2013-2022. Methods: Retrospective population-based study conducted in Veneto (4.9 million people) using the population registry, an administrative health database. Between 2012 and 2022, hyperthyroidism incidence was defined thank to a health-care co-payment exemption for hyperthyroidism or any hospital diagnosis of hyperthyroidism. Incident hyperthyroidism was defined from 2013 to 2022 to exclude prevalent cases. Standardized incidence rates (IRs) were reported by age, sex, and etiology of thyroid hyperfunction too. Results: We identified 26,602 incident cases (IR of 54.38 per 100,000 person-years, 2.47-fold higher in females than in males). IR decreased from 69.87 (95% CI: 67.49, 72.25) in 2013 to 42.83 (95% CI: 40.99, 44.66) in 2022. In 2020, an out-of-trend decrease in hyperthyroidism incidence was documented, corresponding to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic outbreak, with a realignment to the trend in the subsequent years. The annual percentage change according to the cause of hyperthyroidism was as follows: -6.62% (95% CI: 8.47, 4.73) (P < 0.0001) in toxic multinodular goiter, -7.56% in toxic uninodular goiter (95% CI: 10.54, 4.48) (P < 0.001) and -4.70% (95% CI: 6.33, 3.04) in toxic diffuse goiter (Graves' disease) (P < 0.001). Conclusions: We documented a decline in the incidence of hyperthyroidism in Veneto Region, paralleling the improvement of the iodine status, thanks to a long and sustained iodine prophylaxis campaign. SARS-CoV-2 pandemic and vaccination campaign did not change the declining trend of hyperthyroidism incidence in our study region. Significance statement: An improvement in iodine status in the population residing in the Veneto region has been documented in the last decade, thanks to a nationwide voluntary iodine prophylaxis program running since 2005, but its impact on the epidemiology of thyroid disease has never been documented. This is the largest study on the incidence rates of hyperthyroidism carried out in Italy and covers the longest observation period among all regionwide population-based studies of hyperthyroidism in our country. We documented a reduction in the incidence of hyperthyroidism, which was more pronounced in nodular goiter diagnosis but involved also toxic diffuse goiter. The decline in the incidence of hyperthyroidism in Veneto Region shows the efficacy and safety of the iodine prophylaxis campaign.