Metastatic ovarian strumosis in an in-vitro fertilization patient

Malignant struma ovarii is a very rare tumour, with considerable controversy concerning the necessary histologic features for malignancy. Still more infrequent is the condition termed 'metastatic ovarian strumosis' or simply 'benign strumosis or strumatosis' and characterized by the presence of peritoneal implants of mature thyroid tissue occurring in struma ovarii. 'Strumosis' should not be confused with malignancy. Presented is a case of 'metastatic ovarian strumosis' in a 36-year-old woman with primary infertility who underwent three in-vitro fertilization (IVF) cycles with ovarian stimulation. She received hormonal treatment for 6 months after her last IVF because of 'persistent enlarged ovarian follicles' which were in fact 'thyroid follicles'.