Post-COVID-19 Thyroid Function Changes in Primary Care: A Retrospective Observational Study

Background: Coronavirus disease 2019 (COVID-19) has been associated with thyroid dysfunction; however, evidence from primary care settings remains limited. Given that most patients with COVID-19 are managed outside hospital settings, understanding thyroid function changes in real-world primary care is clinically relevant. Methods: This retrospective study included 373 adult outpatients with confirmed COVID-19 and no prior thyroid disease, who were managed in a Family Medicine Clinic in Warsaw between January of 2022 and December of 2024. Thyroid function tests (yielding TSH, fT3, and fT4 levels) obtained within three months before and after infection were analyzed. Subacute thyroiditis (SAT) was assessed in symptomatic patients via thyroid ultrasound. Paired comparisons were performed, with p < 0.05 considered statistically significant. Results: A significant reduction in the mean TSH level was observed following COVID-19 compared with pre-infection values (mean 2.39 vs. 2.17 mIU/L; p < 0.05), while fT3 (2.93 vs. 2.98 pg/mL) and fT4 (1.31 vs. 1.30 ng/dL) concentrations remained unchanged. No significant associations were found between thyroid function changes and age or sex. Conclusions: In this real-world primary care population, lower TSH concentrations were observed following COVID-19 while peripheral thyroid hormone levels remained unchanged, suggesting predominantly modest changes in thyroid function parameters. Given the retrospective study design and the absence of a control group, these findings should be interpreted with caution. Further prospective studies are needed to clarify the clinical significance of post-COVID-19 thyroid function changes.