Post-COVID-19 Thyroid Dysfunction and Autoimmune Ultrasound Changes: A 12-Month Prospective Cohort Study.

Background and Objectives: Thyroid dysfunction has been increasingly reported during and after SARS-CoV-2 infection, yet prospective longitudinal studies integrating biochemical and imaging assessments remain limited. The aim of this study was to evaluate the evolution of thyroid function and structural changes during a 12-month follow-up after COVID-19 hospitalization. Materials and Methods: This prospective single-center cohort study included adult patients hospitalized with laboratory-confirmed COVID-19 between December 2022 and December 2024. Thyroid-stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), anti-thyroid peroxidase (anti-TPO), and anti-thyroglobulin antibodies (anti-TG) were assessed during hospitalization and at 1, 3, 6, and 12 months. Color Doppler thyroid ultrasound was performed at baseline and 6 and 12 months. Acute disease severity, vaccination status, and corticosteroid exposure were recorded. Results: Of the 71 enrolled patients, 67 completed the 12-month follow-up. Subacute thyroiditis occurred in 4/67 patients (6.0%; 95% CI: 1.7-12.6%). After excluding individuals with pre-existing disease, newly detected autoimmune thyroiditis was identified in 8/61 patients (13.1%; 95% CI: 5.8-24.2%). Most cases were mild and self-limited. Ultrasound patterns suggestive of autoimmune thyroiditis increased over time, even in patients with minimal biochemical abnormalities. Thyroid hormone trajectories showed interindividual variability, with most values remaining within reference ranges. Associations with inflammatory markers, disease severity, and vaccination status were weak. Conclusions: A subset of patients developed biochemical and structural thyroid changes during the 12 months following COVID-19 hospitalization. These findings highlight the value of targeted thyroid follow-up after COVID-19, particularly in patients with persistent symptoms, borderline TSH values, or ultrasound changes suggestive of autoimmune thyroiditis.