A Case of Euthyroid Graves Disease Which Developed into Graves Disease with Thyrotoxicosis after Subacute Thyroiditis

The presence of stimulating TSH receptor antibody (TRAb) results in Graves disease with thyrotoxicosis. However, some patients with Graves disease are euthyroid despite the presence of this antibody. Patient: A 42-year-old Japanese woman had suffered front diplopia. Thyroid function, however, was normal. Serum TRAb levels were elevated at 1.7 U/L (normal range: under 1.0 U/L). The working diagnosis was euthyroid Graves disease. Eighteen months later, she noticed pain and tumor in the anterior region of the neck. Physical examination revealed 3.2-cm diameter, firm tumor in the left lobe of thyroid accompanied by exquisite tenderness. The serum free plasma free thyroxin, free triiodethyronine, and TSH was 1.60 ng/dL, 3.62 pg/mL, and below 0.01 mU/L, respectively. The erythrocyte sedimentation rate was 64 mm/h, TRAb values had increased to 3.52 U/L. Three months later, the serum levels of FT4 and TRAb increased and the diagnosis of Graves disease with thyrotoxicosis was made. To our best knowledge, this is the first case report of euthyroid Graves disease being connected to hyperthyroid Graves disease by subacute thyroiditis.