We report a 14 year-old girl with a huge adenomatous goiter in the left lobe in the thyroid gland reaching the aortic arch with elevated thyroglobulin. The tumor grew rapidly for two years since the initial diagnosis as an euthyroid hemilateral goiter. Ultrasonography revealed multinodular tumors and magnetic resonance imaging made clear the lower expanding site of the tumor. We recommend careful follow up of hemilateral diffuse goiters by ultrasonography for earlier detection of nodular changes as seen in our case.