A struma ovarii (literally: goitre of the ovary) is a rare form of monodermal teratoma that contains mostly thyroid tissue, which may cause hyperthyroidism.

[2] The ultrasound features of struma ovarii are nonspecific, but a heterogeneous, predominantly solid mass may be seen.

Ultrasound demonstrates a complex appearance with multiple cystic and solid areas, findings that reflect the gross pathologic appearance of the tumor.

Magnetic resonance imaging (MRI) findings may be more characteristic: The cystic spaces demonstrate both high and low signal intensity on T1- and T2-weighted images.

Some of the cystic spaces may demonstrate low signal intensity on both T1- and T2-weighted images due to the thick, gelatinous colloid of the struma.