We report the case of a 61-year-old woman with persistent thyrotoxicosis for 7 years despite low thyroidal radioiodine uptake and methimazole treatment. Her initial 123I whole-body scan (WBS) was read as negative. Upon evaluation in our institution, she remained hyperthyroid after discontinuation of methimazole. Repeat WBS with SPECT/CT revealed low 24-hour thyroidal uptake (RAIU = 2%) and intensely focal radioiodine uptake in a large heterogeneous left pelvic mass, consistent with left adnexal struma ovarii. Resection of this mass confirmed benign struma ovarii. This case illustrates the advantage of fusion SPECT/CT imaging with planar 123I-WBS for diagnosis of extrathyroidal thyrotoxicosis.