Frequency of Thyroid Antibodies at the Diagnosis of Subacute Thyroiditis

Objective: Although subacute thyroiditis (SAT) is not an autoimmune disease, the presence of antithyroid antibodies has been reported in this disease too. This study aims to determine the frequency of antithyroid antibodies at the time of diagnosis of SAT. Material and Methods: Quantitative measurements of antithyroid peroxidase antibody (anti-TPO), antithyroglobulin antibody (anti-Tg), and thyroid-stimulating hormone (TSH) receptor autoantibodies (TRAb) were made in 76 patients at the diagnosis of SAT. Cytopathological examination and iodine uptake test was performed to exclude Graves' disease and Hashimoto's disease in suspected patients. Multiple multinuclear giant cells and granulomatous formations, including epithelioid histiocytes, were the cytological findings employed to support the diagnosis of SAT in suspicious cases. Results: The median erythrocyte sedimentation rate and C-reactive protein levels were found to be 49 mL/hour (21-130) and 54 mg/L (8-179), respectively. TSH, free T4, and free T3 levels were determined to be 0.01 mIU/L (0.003-5.2), 1.98 ng/dL (0.78-6.1) and 5.51 ng/L (3.07-14), respectively. During the initial presentation, 88% of the patients were hyperthyroid, and 9% of the patients were euthyroid. Anti-TPO and anti-Tg antibody levels were detected to be above the assay-specific cut-off in 11.8% and 10.5% of SAT patients, respectively, at the time of diagnosis. Elevated TRAb was detected in 6.6% of all SAT patients. The median anti-TPO, anti-Tg, and TRAb levels of antibody-positive patients were 55 IU/mL (38-1.078), 163 IU/mL (5.5-876), 5 IU/L (1.9-23), respectively. Conclusion: Although uncommon, antibody positivity can also be observed in SAT disease. This study has proved that the previous studies claiming the absence of thyroid antibodies in SAT are flawed. SAT must be considered while assessing the differential diagnosis of Graves' and Hashimoto's disease.