Long-Term Thyroid Outcomes After COVID-19 Vaccination: A Cohort Study of 2 333 496 Patients From the TriNetX Network

Abstract Context Reports on long-term thyroid dysfunction following COVID-19 vaccination are limited. Understanding the risk of subacute thyroiditis, hyperthyroidism, and hypothyroidism in vaccinated individuals is crucial for postvaccination monitoring. Objective This study evaluated the risk of thyroid dysfunction in individuals vaccinated against COVID-19 compared to unvaccinated individuals, using a large cohort. Methods We conducted a retrospective cohort study from January 1, 2022, to December 31, 2023, using the TriNetX database, including 1 166 748 vaccinated and 1 166 748 unvaccinated individuals. Propensity score matching was used to balance baseline characteristics. The primary outcomes were new diagnoses of subacute thyroiditis, hyperthyroidism, and hypothyroidism. Results The risk of subacute thyroiditis remained unchanged (95% CIs included 1). A significant reduction in hyperthyroidism risk was observed from 3 to 9 months postvaccination (hazard ratios [HRs]: 0.65-0.89, all 95% CIs below 1), but this trend was not significant at 12 months (HR: 0.99; 95% CI: 0.92-1.06). In contrast, the risk of hypothyroidism significantly increased from 6 to 12 months postvaccination (HR: 1.14-1.30, all 95% CIs above 1). Among mRNA vaccine recipients, the risk of both hyperthyroidism and hypothyroidism was significantly elevated at 12 months (HR: 1.16-2.13). Conclusion COVID-19 vaccination was associated with a reduced risk of hyperthyroidism and an increased risk of hypothyroidism, highlighting the need for ongoing thyroid function monitoring.