Effect of COVID-19 Vaccination on Thyroid Disease in 7 Million Adult and 0.2 Million Adolescent Vaccine Recipients

Context Emerging reports have raised concerns regarding the potential association between COVID-19 vaccination and thyroid dysfunction, specifically thyroiditis.Objective This study aimed to investigate the potential association between COVID-19 vaccination and thyroid diseases using data from a nationwide cohort of South Korea.Methods This study included 7 579 210 adult and 241 063 adolescent vaccine recipients. A self-controlled case series design was applied to estimate the incidence rate ratio (IRR) of thyroid disease during the 55-day postvaccination period (first and second dose) compared to the baseline period (nonexposure period) using conditional Poisson regression.Results In the adult population, IRRs for hyperthyroidism, hypothyroidism, subacute thyroiditis, and thyroid eye disease were 0.98 (95% CI, 0.89-1.09), 0.88 (95% CI, 0.84-0.93), 1.27 (95% CI, 0.94-1.72), and 0.68 (95% CI, 0.50-0.93) after the first vaccine dose and 0.96 (95% CI, 0.87-1.06), 0.96 (95% CI, 0.92-1.02), 1.32 (95% CI, 0.96-1.80), and 1.06 (95% CI, 0.81-1.38) after the second vaccine dose. While the risk of hyperthyroidism recurrence did not increase after vaccination, we observed an increased risk of hypothyroidism exacerbation. Subgroup analyses based on age, sex, and vaccine type revealed no significant differences in the incidence of thyroid diseases. In adolescents aged 12 to 17 years, no increase in the risk of thyroid disease was observed after vaccination.Conclusion This extensive, national self-controlled case series analysis found an increased risk of exacerbation of hypothyroidism following COVID-19 vaccination. However, no association was observed between COVID-19 vaccination and an increased risk of most other thyroid diseases. These findings contribute to increasing evidence supporting the safety of COVID-19 vaccination in relation to thyroid health.