Publication date: Jan 30, 2025

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 post-vaccination monitoring. This study evaluated the risk of thyroid dysfunction in COVID-19 vaccinated individuals compared to unvaccinated individuals using a large cohort. 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. The risk of subacute thyroiditis remained unchanged (95% confidence intervals [CIs] included 1). A significant reduction in hyperthyroidism risk was observed from 3 to 9 months post-vaccination (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 post-vaccination (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). 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.

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