False positive radioiodine uptake following thyroidectomy for differentiated thyroid cancer has been reported in some cases. A 42-year-old woman was referred for ablative radioiodine treatment six weeks after undergoing total thyroidectomy for papillary thyroid carcinoma. Posttherapeutic I-131 scintigraphy showed an intense hot spot in the left pelvis. Pelvic computed tomography (CT) revealed a benign left ovarian mass such as a functional cyst. After menstruation, ultrasonographic imaging demonstrated a shrunken ovarian mass. Therefore, we diagnosed the patient with a functional cyst that was influenced by hormonal fluctuation. It is believed that this is the first case of false-positive radioiodine uptake in a functional ovarian cyst.