SUN-499 Extragonadal Germ Cell Tumor Induced Thyrotoxicosis

Background: Extragonadal Germ Cell Tumors (EGCTs) are extremely rare, with an incidence of around 1/1,000,000 and highly variable prognosis dependent on subclassification as seminomatous or non-seminomatous. Non-seminomatous germ cell tumors can cause significant enough elevations in hCG to induce thyrotoxicosis via structural homology allowing for cross-reactivity with the TSH-receptor. Limited cases involving EGCTs inducing thyrotoxicosis have been studied. Case: A 27-year-old male presented to the emergency department with intractable abdominal and back pain. He reported night sweats, nausea, dizziness, and a 10 lb weight loss in 1 week. He was resting comfortably and only complaining of pain. He was moderately tachycardic, tachypneic and hypertensive, with a physical exam only remarkable for tenderness to palpation of the abdomen. Abdominal CT revealed mesenteric and retroperitoneal lymphadenopathy, bilateral adrenal enlargement, a mass in the head of the pancreas, as well as gallbladder and common bile duct distention. Lymph node biopsy was conducted for a suspected lymphoma; however, pathology found a poorly differentiated carcinoma. A diagnosis of a non-seminomatous EGCT was made when ultrasound of the testes was negative for masses and labs revealed elevations in hCG (74842 mIU/ml), and LDH (1421 U/L) with normal AFP (6.98 ng/mL). Further workup showed a slightly elevated T4 Free Thyroxine (1.55 ng/dl) with normal TSH (0.555 mIU/L); thus his thyrotoxicosis was secondary to the high HCG. Treatment for thyrotoxicosis was deferred with the expectation that symptoms would resolve when the tumor burden was decreased. Our patient had numerous other complications requiring management from nephrology, GI and urology teams in addition to endocrinology and hematology-oncology. Bleomycin, Etoposide and Cisplatin (BEP) combination chemotherapy was initiated after recovery from acute complications. Further pathology evaluation suggested tumor susceptibility to the biologics nivolumab and pembrolizumab. Conclusion: Patients with thyrotoxicosis secondary to metastatic non-seminomatous germ cell tumors often present with widespread metastasis and relatively few symptoms of thyrotoxicosis that resolve as the hCG levels decrease with chemotherapy without specific antithyroid medication. This case highlights the importance of considering clinically occult thyrotoxicosis in patients who have elevated hCG secondary to germ cell tumors. Early detection of germ cell tumor and recurrence is crucial for chemotherapeutic success. Thus, patients should be closely followed for thyrotoxicosis relapse which could potentially herald a carcinoma relapse and aid in early diagnosis.