Malignant transformation of struma ovarii is a rare event observed in about 5% of cases. We present here an unusual case that closely simulated highly differentiated follicular carcinoma, an entity with a relatively similar appearance to malignant struma ovarii. In our case, peritoneal and systemic dissemination occurred 18 y after excision of struma ovarii, which had been reported as histopathologically benign. There were 3 noteworthy features in the case: the highly functioning nature of the lesions (evidenced by a low level of thyroid-stimulating hormone even without thyroxine suppression); the low to minimal ¹⁸FFDG avidity of the foci, which reiterates the well-differentiated nature of these lesions, as expected in highly differentiated follicular carcinoma; and prominent involvement of rare sites such as spleen and liver, in addition to usual sites such as lungs, peritoneum, and bilateral adnexae.