RESUMEN
Malignant transformation of benign monodermal teratoma is a rare presentation of struma ovarii. A 41-year-old Filipina presented with 1-year history of recurrent lower abdominal pain, abdominal swelling and vomiting. Ultrasound of the abdomen revealed a large cystic abdomino-pelvic mass densely adherent to the intestinal and uterine wall with moderate ascitis. The patient underwent total abdominal hysterectomy and bilateral salphingo-oophorectomy and histopathology showed papillary thyroid-type carcinoma arising from the left struma ovarii. She underwent total thyroidectomy, postoperative radioactive iodine ablation and maintained on levothyroxine suppressive therapy. She is now on regular follow-up for monitoring of possible tumour recurrence and metastasis.