A 14-year-old female with a history of papillary thyroid cancer underwent radioactive iodine surveillance imaging. Recurrent tumor identified in the neck was treated with 125 mCi I-131. Post-therapy scan revealed a large focus of I-131 uptake in the pelvis, not seen on the pretherapy I-123 scan. Postvoid imaging was unchanged, and there were no symptoms of urinary tract infection. No renal or ureteral activity was present to suggest delayed excretion or obstruction. Further history revealed the patient had not yet reached menarche. For concerns of an imperforate hymen possibly trapping radioactive blood, a pelvic ultrasound was performed and was compatible with hematocolpos. After the discovery of a transverse vaginal septum, a large amount of clotted blood was evacuated following incision. Hematocolpos as a cause of I-131 pelvic uptake and a potential false positive scan for metastatic thyroid cancer is probably rare. Findings concerning a genital tract outflow obstruction in a young female are generally treated without delay to prevent future reproductive problems. In this case, continued radiation exposure to the patient's ovaries heightened the urgency for surgical management.