Rare complications of complete hydatidiform molar pregnancy: the 'hook effect' and thyrotoxicosis.

A multiparous woman in her 40s presented with a positive pregnancy test, vaginal bleeding, abdominal distention and shortness of breath, 8 weeks after her last menstrual period. A serum human chorionic gonadotrophin (hCG) was reported 900 mIU/ml, and a transvaginal ultrasound (TVUS) diagnosed a pregnancy of unknown location. The patient deteriorated re-presenting 6 days later with features of thyrotoxicosis. TVUS demonstrated a 198×110×165-mm intrauterine mass with features of a complete hydatidiform mole (CHM), and a serum hCG was reported as 440 mIU/ml. Due to the discrepancy between hCG level and suspicion of CHM, the hCG was thought to be falsely low secondary to the 'hook effect'. Following appropriate dilution, the hCG was reported as 4 573 344 mIU/mL. She underwent an uncomplicated surgical evacuation of molar pregnancy and was discharged 3 days postoperatively with resolution of her symptoms and follow-up in a regional gestational trophoblastic disease centre.