Non-seminomatous germ cell tumors producing human chorionic gonadotropin (HCG) were first described in 1964. HCG is capable of inducing hyperthyroidism by stimulating the TSH receptor, due to its identical alpha subunit. We report a case of HCG-mediated hyperthyroidism in the setting of a choriocarcinoma. A 28-year-old man presented with a 3-week history of unintentional weight loss, cough, hemoptysis, palpitations, shortness of breath, nausea and vomiting. Chest x-ray showed a large pleural effusion and multifocal nodular opacities on lungs measuring up to 4 cm. CT abdomen/pelvis showed a 1.4 cm left renal mass concerning for metastasis, and hepatomegaly. CT chest demonstrated a 16 cm heterogeneous enhancing mass in the mediastinum, a large pericardial effusion, moderate left pleural effusion and metastatic pulmonary nodules. Laboratory results showed elevated HCG of 947,825 mIU/mL, AFP 25 ng/ml, LDH 1,394 U/L, TSH 0.011 mcIU/mL, FT4 2.0 ng/dL, Ferritin 2680 ng/mL, CRP 118 mg/L. Testicular ultrasound was unremarkable. Biopsy of mediastinal mass was consistent with choriocarcinoma. Endocrinology was consulted for evaluation of abnormal thyroid function. He was tachycardic, without other overt symptoms of hyperthyroidism and no clinical findings of Graves’ Disease. TRAB, TSI, TPO antibodies were negative. Propylthiouracil (PTU) 100mg every 8 hours and low dose propranolol were started. Chemotherapy with Etoposide, Ifosfamide, and Cisplatin was initiated. Thyroid function improved 5 days after initiation of therapy. After 3 cycles of chemotherapy, his TSH and Free T4 levels normalized and the HCG level decreased to 1,521 mIU/mL. His most recent thyroid function tests were TSH 5.8 mcIU/ml and Free T4 level 1.1 ng/dL and the decision was made to stop PTU. HCG is a heterodimer with an alpha subunit identical to that of TSH, LH and FSH. High HCG levels can stimulate the TSH receptor and cause hyperthyroidism. Hyperthyroidism has been reported in trophoblastic tumors when HCG levels are >200,000 mIU/mL. Initiation of chemotherapy produces rapid decline in HCG levels and improvement of Free T4 levels. Diagnosis of paraneoplastic hyperthyroidism is challenging, as symptoms of hyperthyroidism may overlap with those of metastatic disease. Symptomatic patients should be started on thianomide and beta blocker which improve well-being and tolerance to chemotherapy. We illustrated the challenge of diagnosing hyperthyroidism in HCG-secreting choriocarcinoma and the importance of initiating chemotherapy and anti-thyroidal medication in symptomatic patients to improve outcomes.