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Vaccination against COVID-19 appeared safe for people treated for hypothyroidism, researchers reported.

In a population-based study in Hong Kong, vaccination with the BioNTech/Pfizer shot was not tied to an increased risk of levothyroxine dosage change, David Lui, MBBS, of the University of Hong Kong, said at ENDO 2022, the annual meeting of the Endocrine Society.

More specifically, individuals treated with levothyroxine didn't see a significant risk for dose increases (HR 0.78, 95% CI 0.52-1.17) or reductions (HR 0.97, 95% CI 0.89-1.06) after receiving the mRNA vaccine compared with unvaccinated individuals.

Similarly, people who received the CoronaVac brand -- an inactivated whole-virus vaccine -- also didn't have a significant increase (HR 0.74, 95% CI 0.48-1.06) or decrease (HR 0.97, 95% CI 0.90-1.04) in levothyroxine dose versus those who were unvaccinated.

These individuals also didn't see any significant risk for emergency department visits or unscheduled hospitalizations following vaccination with Pfizer or CoronaVac compared with unvaccinated individuals.

The researchers also performed a sensitivity analysis that likewise found no differences when patients were stratified according to age (≥65 vs <65), sex, and pre-vaccination thyroid status.

"Both inactivated and mRNA COVID-19 vaccination are not associated with unstable thyroid status or an increased risk of adverse outcomes among patients treated for hypothyroidism," Lui said during a press conference at the meeting.

"These reassuring data should encourage them to get vaccinated against COVID-19 for protection from potentially worse COVID-19-related outcomes," he added.

The analysis included 47,086 continuous users of levothyroxine for 3 or more months: 12,310 who received Pfizer; 11,353 who received CoronaVac; and 23,423 who were unvaccinated. Data were drawn from the Hong Kong Hospital Authority electronic health records linked to the Department of Health's vaccination records. People using anti-thyroid drugs or who had thyroid cancer were excluded.

The individuals were followed from the date of their first vaccine until the time to one of the outcomes, like a change in levothyroxine dose, death, or the end of the study in September 2021.

On average, unvaccinated adults tended to be older, have a greater number of comorbidities, and have higher baseline thyroid-stimulating hormone levels.

During the study period, there were only three (0.06%) deaths reported in the vaccinated individuals and only nine (0.04%) "adverse events of special interest."

In a related study at the meeting and also led by Lui, COVID-19 vaccination was not associated with any clinically significant thyroid dysfunction in adults without a history of thyroid disorders or prior COVID infection. Only 1.4% of the patients had abnormal thyroid function tests -- two cases of subclinical thyrotoxicosis and one case of isolated mildly low levels of free triiodothyronine -- none were clinically overt.

This late-breaking study of 222 individuals found there was a modest increase in anti-thyroid antibody titers after vaccination. However, there were no significant changes in anti-thyroid peroxidase (TPO)/thyroglobulin positivity.

There was a greater anti-TPO titer rise associated with the Pfizer vaccine compared with CoronaVac.

Lui's group wrote on their poster that these findings provide "important reassurance for people to receive COVID-19 vaccination."