Effect of Inactivated and mRNA COVID-19 Vaccination on Thyroid Function Among Patients Treated for Hyperthyroidism

Context Reports of thyroid dysfunction following COVID-19 vaccination included cases of relapse of Graves' disease and worsening of pre-existing Graves' disease. Little is known about the thyroid-specific outcomes among patients treated for hyperthyroidism who have received COVID-19 vaccination. Objective Among patients treated for hyperthyroidism, we evaluated factors associated with not receiving the COVID-19 vaccination and whether COVID-19 vaccination was associated with thyroid function instability. Methods We included consecutive patients treated for hyperthyroidism attending the thyroid clinic at a teaching hospital between January and September 2021. They were categorized into vaccinated and unvaccinated groups. The index date was the date of first-dose vaccination for the vaccinated group, and the first date of attendance in the inclusion period for the unvaccinated group. They were followed up until March 2022 or occurrence of thyroid function instability (worsening of thyroid function/increase in antithyroid drug dosage), whichever was earlier. Results A total of 910 patients were included (mean age 51.6 years; 82.1% female). Of these, 86.2% had Graves disease and 67.3% were vaccinated (67.3% BNT162b2; 30.6% CoronaVac; 2.1% heterologous). Abnormal thyroid function and cardiovascular comorbidities were independently associated with unvaccinated status. Upon median follow-up of 5.3 months, thyroid function instability occurred in 15.9% of patients. COVID-19 vaccination did not increase risks of thyroid function instability (hazard ratio 0.78, 95% CI 0.56-1.09, P = .151); this was consistent in Graves disease, both types of vaccines, and regardless of whether baseline thyroid function was normal. Twenty-seven patients overtly thyrotoxic at the time of vaccination received COVID-19 vaccines without triggering a thyroid storm or difficulty in subsequent thyroid function control. Conclusion Among patients treated for hyperthyroidism, abnormal thyroid function was a factor predicting unvaccinated status. Our results should encourage patients treated for hyperthyroidism to receive COVID-19 vaccination to protect themselves from adverse outcomes and potential long-term sequelae of COVID-19.