Background: Malignant transformation of mature cystic teratomas (MCTs) is a rare phenomenon. The most common histology of a malignant transformation is squamous cell carcinoma, and there are limited reports of multiple malignancies arising in a single MCT. Further data are necessary to guide management of these atypical cases.

Case: We present the case of a 48-year-old with MCT containing a malignant papillary thyroid carcinoma (PTC) arising in the context of struma ovarii and a carcinoid tumour.

Conclusion: Malignant transformations of MCTs are exceedingly rare with no guidelines on management. We use this case to demonstrate an approach for the workup and management of malignantly transformed MCTs.