SUN-361 hCG-Mediated Thyrotoxicosis in Advanced Testicular Cancer: Rapid Control With Lugol’s Iodine and Chemotherapy

Abstract Disclosure: F. Verster: None. M. Elston: None. J. Tamatea: None. V. Boyle: None. Background: Human chorionic gonadotropin (hCG)-mediated agonism of the TSH-receptor can arise due to homology between the alpha subunits of hCG and TSH. This phenomenon is well-described in transient gestational thyrotoxicosis and gestational choriocarcinoma, but is rare in male patients. Case: A 21-year-old male was diagnosed with new-onset hyperthyroidism while undergoing management for stage IV testicular cancer (choriocarcinoma sub-type) with extensive lung and liver metastases. Biochemistry included a peak hCG level of 877,000 IU/L, with a corresponding peak FT4 of 84 pmol/L (6.53 ng/dL) (ref 12 - 22 pmol/L, 0.93 - 1.71 ng/dL) and FT3 of 25.4 pmol/L (16.5 pg/ml) (ref 3.1 - 6.8 pmol/L, 2.01 - 4.42 pg/ml). TSH was fully suppressed. TSH receptor antibodies were negative. Elevations in testosterone (42.2 nmol/L (1217 ng/dL), ref 9 - 30 nmol/L, 260 - 865 ng/dL) and oestradiol (2,403 pmol/L (655pg/ml)) were also attributed to hCG-mediated cross-stimulation. The International Germ Cell Cancer Collaborative Group (IGCCCG) prognostic model for advanced testicular germ cell tumours classifies this presentation into a poor risk sub-group, based on liver metastases and elevated hCG, with an overall survival estimate of 60%. Use of anti-thyroid medication in hCG-mediated hyperthyroidism (vs focussing management solely on the underlying cause) is controversial. In this case, cycle one of chemotherapy (cisplatin, etoposide and ifosfamide with dexamethasone) was complicated by severe respiratory distress, with a concern raised for choriocarcinoma syndrome. Due to the significant clinical severity, concurrent thyrotoxicosis, and uncertainty regarding the rapidity of hCG response to chemotherapy, it was elected to include carbimazole and Lugol’s iodine in the initial management. Rapid normalisation of free T3 and T4 occurred (by days 9 and 14 respectively) following commencement of anti-thyroid therapy, in conjunction with a rapid decline in hCG concentrations (approximately 90% decline in hCG peak within one week of completing cycle 1). Neutropenia was observed, with a nadir neutrophil count of 0.11 x109/L (110 cells/μL) (ref 1.9 - 7.5 x109/L, 1,900 - 7,500 cells/μL) which was attributed to chemotherapy rather than carbimazole. At 9 months’ follow-up, the patient’s tumour markers are undetectable and he remains euthyroid off anti-thyroid medications. Conclusion: hCG-Mediated thyrotoxicosis due to testicular cancer is a rare but potentially important contributor to morbidity. In the setting of acute illness, the use of Lugol’s iodine could be considered for rapid control of thyrotoxicosis while awaiting hCG response to chemotherapy. Presentation: Sunday, July 13, 2025