99mTc Sestamibi Thyroid Scan in Amiodarone-Induced Thyrotoxicosis Type I

Amiodarone-induced thyrotoxicosis (AIT) type I describes inducement of clinical hyperthyroidism by excessive thyroidal iodine in the setting of latent Graves disease, and therapy differs from that used for AIT type II. A 65-year-old man previously on amiodarone for atrial fibrillation developed clinical hyperthyroidism. Diagnosis of AIT was made, but the type was not clear. Tc sestamibi thyroid scan showed diffusely increased uptake and retention in an enlarged thyroid gland, a pattern consistent with AIT type I. Methimazole was initiated and controlled the thyrotoxicosis. I iodide thyroid scan and uptake study performed later was consistent with Graves disease.