In this article, a case of cystic struma ovarii with macrocystic change is presented. Cysts derived from struma ovarii may mimic a mucinous or serous cystadenoma. A careful examination for any thyroid microfollicles within fibrous septa or areas of solid stroma is key. In problematic cases, immunohistochemical staining with thyroglobulin and thyroid transcription factor-1 may be required to establish the diagnosis.
