Radioiodine Therapy-Induced Conversion of Toxic Adenoma to Graves' Disease

We present a 50-year-old female who was evaluated for the symptoms of thyrotoxicosis. She had low thyroid stimulating hormone (TSH) 0.02 with normal free thyroxine (FT4) 1.00 (0.61-1.76 ng/dL) and normal total triiodothyronine (TT3) 1.0 (0.60-2.20 ng/mL) levels. Her thyrotropin receptor antibody (TRAbs) and thyroid peroxidase antibody (TPOAb) titers were negative. Thyroid ultrasound revealed an ill-defined, heterogeneous, 1.8 cm x 0.8 cm x 0.7 cm nodule in the left lower lobe. 123-radioiodine (RAI) thyroid scan revealed 38.5% uptake, which was concentrated in the lower left thyroid lobe, a finding consistent with a solitary toxic adenoma of the thyroid. The patient became clinically and biochemically euthyroid on methimazole (MMI). She then underwent 131-RAI therapy with 12 mCi, which cured her hyperthyroidism with normalization of TSH levels for four months. She then developed overt thyrotoxicosis with low TSH of 0.02, elevated TT3 of 3.2, and normal FT4 of 0.91. Repeat TRAbs and TPOAb were elevated along with diffusely increased uptake on the I-123 RAI thyroid uptake scan, consistent with Graves' disease (GD). The patient was then placed on MMI again to bridge to definitive treatment with total thyroidectomy. Our case is a rare case where the patient with solitary toxic adenoma with negative TPOAb serology developed GD following I-131 RAI treatment.