A postmenopausal female presented with an enlarging multinodular goiter. Microcalcifications within the largest thyroid nodule found by ultrasound prompted her to elect a total thyroidectomy. Histopathologic evaluation led to the diagnosis of confined papillary thyroid carcinoma (follicular variant). Elevated serum thyroglobulin levels were noted on postoperative laboratory workup, with the differential diagnosis of residual thyroid tissue, substernal extension of an adenomatoid multinodular goiter, and/or metastatic thyroid cancer. The patient then underwent thyrogen-stimulated I-131 ablation therapy, with postablation scans detecting a solitary focus of intensely increased radiotracer accumulation in the midline pelvis. Ultrasound of the pelvis revealed a corresponding right ovarian mass with mixed solid and cystic components. These combined findings were highly suggestive of struma ovarii. An exploratory laparotomy/bilateral salpingo-oophorectomy was performed, and pathologic examination confirmed a mature teratoma with predominant benign thyroid component consistent with struma ovarii.