Long-Term Thyroid Outcomes After COVID-19 Vaccination

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.