Struma ovarii is a rare monodermal variant of ovarian teratoma accounting for only 2% of all mature teratomas. To be classified as a struma ovarii, teratoma must be composed predominantly of mature thyroid tissue (> 50%). This tumor is generally benign, although malignant transformation has been reported. Struma ovarii occur mostly as unilateral cases, so bilateral cases are quite rare (less than 6% of cases). Struma ovarii occur largely without symptoms or are accompanied by non-specific symptoms, such as abdominal pain, a palpable abdominal mass, and abdominal distension. The preoperative diagnosis is generally difficult. The incidence of hyperthyroidism has been reported to be 5-10% of patients with struma ovarii. Thus, cases of functional bilateral struma ovarii are very rare. We report a case of bilateral struma ovarii with subclinical thyrotoxicosis and a diffuse goiter, mimicking a malignant ovarian tumor, and include a brief review of related literature.