An Unusual Case of Malignant Struma Ovarii Causing Thyrotoxicosis

Struma ovarii (SO) is a specialized monodermal teratoma predominantly composed of mature thyroid tissue, accounting for approximately 5% of all ovarian teratomas. Thyrotoxicosis is seen in about 8% of patients with SO. Most SO cases are benign with only 5-10% being malignant, and malignant SO causing thyrotoxicosis is very uncommon.

A 64-year-old woman had been diagnosed with thyrotoxicosis 2 years previously. The thyroid gland was palpable with a micronodular texture, and the patient was euthyroid under carbimazole. She presented with abdominal pain and progressive enlargement of the abdomen over a 2-month period. An abdominal ultrasonography revealed a pelvic mass and a large fluid collection. Additional imaging confirmed the presence of a complex right ovarian mass measuring 13 cm. The patient underwent a total abdominal hysterectomy and bilateral salpingo-oophorectomy, omentectomy and appendectomy. The histological examination revealed the presence of 'follicular thyroid-type carcinoma arising in an SO of the right ovary, with metastatic infiltration in the tissue fragments from the pouch of Douglas'. Antithyroid treatment was discontinued 1 month after surgery in light of the pathology result. During the 4-year follow-up, the patient remained euthyroid.

There has been controversy about the management of malignant SO, which is a rare entity. Malignant SO causing thyrotoxicosis is even more uncommon. As clinical signs are nonspecific, other causes of thyrotoxicosis must be considered for a differential diagnosis. Our case is one of the very few cases ever reported.