Diagnosis of Graves' Disease during Treatment for Subacute Thyroiditis.

The co-occurrence of subacute thyroiditis (SAT) and Graves' disease (GD) is rare. A 62-year-old Japanese man presented with shifting neck pain and elevated thyroid hormone level. The patient tested positive for thyroid-stimulating hormone receptor antibodies. Additionally, thyroid hormone levels did not decrease during treatment with prednisolone for SAT. Consequently, concurrent GD was suspected, and diagnostic assistance was obtained by confirming increased uptake on 99mTechnetium thyroid scintigraphy. A genetic analysis of human leukocyte antigen (HLA) revealed genotypes associated with susceptibility to SAT (HLA-B*35:01) and GD (HLA-DPB1*05:01). Furthermore, the possibility of coronavirus disease 2019 as a related environmental factor cannot be ruled out in this case.