A 26-year-old pregnant woman presenting with repeated episodes of vaginal bleeding, weight loss, and shortness of breath was diagnosed with choriocarcinoma with metastases to both lungs. Chorionic gonadotropin levels (hCG) were >2.5 x 10(6)mU/mL. Consistent with hCG-induced subclinical hyperthyroidism, she had a suppressed TSH of 0.037 mU/L (0.49 - 4.67), a T4 of 18.1 microg/dL (4.9 - 10.7), and T3 of 136 ng/dL (45 - 137). Chemotherapy with a combined regimen with etoposide, methotrexate, and dactinomycine was started. The initial course was complicated by urosepsis with respiratory distress requiring endotracheal intubation for 3 days. She then improved rapidly, and her thyroid function tests were within normal limits by day 12. Six months later, after ten cycles of chemotherapy, the patient was in remission without signs of residual tumor or hCG-induced paraneoplastic activity.