Reverse discordant Tc-99m and I-131 imaging in a patient with a toxic thyroid adenoma

A 77-year-old woman was hospitalized with rapid atrial fibrillation and was found to be thyrotoxic. A Tc-99m pertechnetate thyroid scan performed 20 minutes after injection showed only faint uptake confined to the left thyroid bed. The patient was not taking thyroid medications and had no recent radiographic or CT studies with contrast material. A 48-hour whole-body I-131 scan revealed no ectopic source of thyroid tissue, but it did show a large hot focus in the region of the left thyroid lobe, with no uptake in the rest of the gland. An ultrasound examination showed an intact thyroid gland with a large echogenic nodule in the left thyroid lobe. The patient was treated subsequently with radioactive iodine.