Increased Risk of Thyrotoxicosis After COVID-19 in Adolescents and Young Adults During the Omicron-Predominant Period: A Nationwide Matched Study.

OBJECTIVE

Thyroid dysfunction has been increasingly reported after the coronavirus disease 2019 (COVID-19) pandemic. To date, no population-based study has examined this association using more than 10 million matched pairs. We compared the risk of documented thyroid outcomes after COVID-19 with that after influenza during the Omicron-predominant period.

METHODS

We conducted a nationwide retrospective cohort study using the Japanese National Database of Health Insurance Claims, including patients with COVID-19 between January 2022 and March 2023, and those with influenza between September 2017 and March 2023. Individuals with pre-existing thyroid disease were excluded. After 1:1 propensity score matching, 24,173,646 pairs were analysed. Database-recorded outcomes assessed within 90 days of documented COVID-19 infection included diagnoses of thyrotoxicosis, hypothyroidism and subacute thyroiditis, as well as initiation of antithyroid drugs (ATDs) and thyroid hormone replacement therapy (THRT).

RESULTS

Among database-recorded outcomes, COVID-19 was associated with higher risks of thyrotoxicosis diagnosis (risk ratio [RR], 1.08), hypothyroidism diagnosis (RR, 1.12), ATD initiation (RR, 1.06) and THRT initiation (RR, 1.22) than influenza, but not subacute thyroiditis diagnosis. Age-stratified analyses showed markedly higher risks in individuals aged 0-19 years for thyrotoxicosis diagnosis (RR, 1.38) and ATD initiation (RR, 1.77).

CONCLUSIONS

In this nationwide cohort, COVID-19 was associated with higher risks of database-recorded diagnoses of thyrotoxicosis and hypothyroidism than influenza during the Omicron-predominant period. The strongest association was observed for thyrotoxicosis diagnosis and ATD initiation in younger individuals, underscoring the importance of post-COVID-19 thyroid monitoring in adolescents and young adults.