Subacute thyroiditis: clinical significance of diffuse 18-fluorodeoxyglucose uptake. Case report

Background. Subacute thyroiditis is a rare inflammatory thyroid disease presumably associated with viral infection, including COVID-19. It accounts for about 5% of all thyroid diseases.

Materials and methods. A clinical case of de Quervain's subacute thyroiditis in a patient with diffuse hypermetabolic 18-fluorodeoxyglucose uptake by the thyroid gland is presented.

Results. A 61-year-old patient with no history of thyroid disorders 3 weeks after COVID-19 experienced fever in the evenings (37.8–39.8°C). He had moderate pain in the thyroid gland with radiation to the jaw, severe weakness, hand tremors, shivers, 12 kg weight loss, and sweating. These symptoms lasted for 3 weeks. Physical examination showed an enlarged thyroid gland, dense and extremely tender, especially its left lobe. Ultrasound showed an enlarged thyroid gland with a heterogeneous structure and large hypoechoic areas in both lobes up to 4×7 mm with fuzzy contour. Color Doppler imaging revealed reduced blood flow. Combined positron-emission and X-ray computed tomography with 18-fluorodeoxyglucose showed an increased diffuse uptake of the radiopharmaceutical in the thyroid gland and enlargement of its lobes, SUVmax 10.55. Blood thyroid-stimulating hormone is low, free triiodothyronine and thyroxine are high, and ferritin concentration is markedly increased, consistent with thyrotoxicosis. A fine-needle aspiration biopsy was performed. Cytology confirmed de Quervain's subacute thyroiditis. Glucocorticoids and nonsteroidal anti-inflammatory drugs were administered. Four weeks after the treatment, the clinical manifestations resolved. The patient was assessed in 6 months. Ultrasound showed a thyroid gland of regular size with smooth contours; color Doppler mapping revealed normal blood flow. Thyroid-stimulating hormone, triiodothyronine, and thyroxine were within reference ranges.

Conclusion. The SARS-CoV-2 virus can be regarded as the cause of subacute thyroiditis. Combined positron-emission and X-ray computed tomography with 18-fluorodeoxyglucose showed diffuse intensive radiopharmaceutical uptake in the thyroid gland. Cytology is the basis for differential diagnostics. Glucocorticoids and nonsteroidal anti-inflammatory drugs are the first-choice agents.