| Summary: | BackgroundHuman chorionic gonadotropin (hCG)–induced hyperthyroidism is a rare paraneoplastic syndrome observed in non-seminomatous testicular germ cell tumors, due to a cross-reaction between the β-subunit of hCG with the thyroid-stimulating hormone receptor. The precise prevalence of this paraneoplastic phenomenon is unclear as, in the majority of cases, hyperthyroidism remains subclinical.Case presentationHere, we present two cases of advanced metastatic non-seminomatous testicular germ cell tumors where patients exhibited signs and symptoms of thyrotoxicosis at primary diagnosis due to excessive serum β-hCG elevation, with complete remission of symptomatology after the start of oncological treatments and no signs of relapse at the time of publication of this report. Additionally, we provide a comprehensive review of the existing literature concerning this uncommon occurrence.ConclusionDespite being a rare event, the presence of hyperthyroidism or thyrotoxicosis without clear etiology in a young man should lead to consider less frequent causes such as testicular tumors. Even if patients typically have mild symptoms that resolve after chemotherapy, in rare cases, it can be a life-threatening condition that requires prompt recognition and specific intervention.
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