Struma ovarii is a rare form of ovary tumour defined as the presence of ectopic thyroid tissue in the ovarian structures. It usually presents with a benign course, although in some cases carcinoma or other malignant tumours can be found in the context of the ectopic tissue. Herein we report the case of a young patient affected by struma ovarii visualized by means of 124I-NaPET/CT. Thanks to the excellent target-to-background ratio of the tracer and the high resolution of the method, we could well identify the presence of some minimal tumour at the level of the left ovary. To our knowledge, this is the first report of its kind.