Introduction/Background Adnexal pathology and adnexal tumors are common accidental findings in a regular gynecological screening visit. Ovarian teratomas are germ cell tumors that derive from the three germ layers and commonly contain teeth, hair, bone, or thyroid tissue. They consist of 20% of all ovarian tumors. Ovarian teratomas which contain at least 50% thyroid tissue are known as struma ovarii. Malignant struma ovarii are found in less than 2% of mature benign teratomas.

Methodology A 22-year-old nulliparous woman, with a history of an adnexal tumor in 2020 in her annual gynecological screening. Ultrasound (US) examination showed the presence of a cystic lesion on the left ovary with a maximal diameter of 6.5 cm and mild free fluid in the pouch of Douglas. To further assess the findings, a magnetic resonance imaging (MRI) was performed during a 12-month follow-up and confirmed a moderate adnexal cystic lesion measuring 67 mm x 82 mm x 56 mm, lying in the anatomical position of the left ovary.Tumor markers and thyroid function blood tests were within normal limits. The patient underwent a laparoscopic cystoscopy after two years of observation. The intraoperative finding was an anteverted mobile uterus with a cystic lesion in the left ovary about 8 mm x 5.5 mm x 6 mm. It was removed with an endobag. The cyst was opened, and there were serous and solid elements inside. The pathology report showed a malignant struma ovarii.

Results The patient underwent a full thyroid screening with normal findings. After detailed counseling, she decided on a twice-a-year follow-up by us and the endocrinologists. She remains asymptomatic and euthyroid.

Conclusion Malignant struma ovarii is a rare ovarian tumor, which is only diagnosed by pathology reports after surgery. There is controversy regarding its management, and it should be individualized.