<p>Background</p><p>Cases of deep neck abscess occurred secondary to pharyngolaryngitis caused by COVID-19 have been reported. Herein, we report a case of descending mediastinitis complicating COVID-19, accompanied by thyroid crisis, in which we suspected latent Graves’ disease.</p><p>Case</p><p>A 57-year-old woman was transported to our emergency room with the chief complaints of sore throat and chest pain after she had received a diagnosis of COVID-19. A chest CT showed formation of a deep neck abscess extending from around the right tonsil to the right lobe of the thyroid gland. The patient was admitted to the ICU following oral intubation and started on antibiotic therapy. On day 3 after admission, incisional drainage was performed owing to the abscess extending to the mediastinum. Although the infection was controlled by the drainage and antibiotic therapy, the patient developed fever, tachycardia, and gastrointestinal symptoms. We suspected underlying thyroid disease and performed blood examination, which showed a decreased blood level of TSH and increased blood levels of FT3/FT4 and TRAb. Based on the symptoms and results of the blood examination, the patient was diagnosed as having latent Grave’s disease. The thyroid hormones levels returned to normal range after the start of treatment with thiamazole, potassium iodide, and hydrocortisone. Antibiotic therapy was discontinued on day 39, and the patient was discharged from our hospital on day 88.</p><p>Conclusion</p><p>Caution is needed against the development of thyrotoxicosis in patients with infection near the thyroid gland, especially in those with thyroid disease. Also, a number of studies have suggested the association between COVID-19 infection and thyroid gland disease, so that further studies are warranted.</p>