Metastatic malignant struma ovarii is rare and there is a lack of agreement on the criteria of diagnosis and the lines of management. Here we describe a patient with struma ovarii that was initially diagnosed as benign and presented 10 years later with distant metastases. At this time, a pathological review of the initial lesion found that it contained invasive well-differentiated follicular carcinoma. The case was associated with a number of unusual features and challenging management issues, such as a delayed diagnosis of recurrence, functioning metastases with treatment consequences, tumour lysis-induced thyrotoxicosis and cerebrospinal fluid rhinorrhea. The diagnosis and management of struma ovarii should be led by an expert multidisciplinary team. Radioactive iodine should be considered in the management of metastatic disease.