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Struma ovarii is an uncommon type of ovarian tumor derived by germinal cells, characterized by the predominance of thyroid tissue (> 50%); 90-95% of these formations are benign and mainly affect the left ovary, while in 6% of the cases struma ovarii is bilateral. The malignant transformation is a rare condition that often occurs after 50 years. In most instances, diagnosis of malignant struma ovarii is made postoperatively during histological analysis. This tumor appears to derive by one germinal cell through loss of heterozygosity of the androgen receptor gene and of the X chromosome. Clinical symptoms comprise abdominopelvic mass, lower abdominal pain, abnormal vaginal bleeding, and ascites (the occurrence of this condition has been observed in one-third of the cases). The patients with struma ovarii generally do not manifest symptoms related to thyroid hyperfunction, reported only in 8% of the cases, and due to hyperstimulation of the thyroid by auto-antibodies. Thyroid tissue of the struma ovarii, often embedded in a teratoma, may be papillary, follicular or with mixed pattern and it can include elements of mucinous cystoadenomas, Brenner's tumor or carcinoid or melanomas cells. Here the authors report their experience with an unusual case of Hashimoto thyroiditis onset after laparoscopic removal of struma ovarii.