Here, we report a case of a 54-year-old woman affected by thyrotoxicosis, with scintigraphic evidence of a diffuse hyperfunctioning goiter and a large ectopic thyroid tissue in the thyroglossal duct remnant. The latter was apparently more active than the two lobes at 99m Tc-pertechnetate scintigraphy, mimicking a condition of preexisting increased sensitivity to thyroid-stimulating hormone stimulation. On the other hand, single-photon emission computed tomography/computed tomography has proven to be a very useful tool in demonstrating this activity to be similar to the thyroid lobes and in defining extension and anatomical relationships of the mass.