This case report describes a case of hyperthyroidism as manifestation of an embryonal carcinoma, and illustrates the causes that led to it. The case describes a 33-year-old male patient who complained of chest pain, palpitations, mild dyspnoea, and weight loss. Blood analysis reveals high levels of human chorionic gonadotropin (833818 mlU/ml), T3 (16.90 pg/ml), and T4 (7.77 ng/dl), as well as a fall of TSH (0.01 ulU/ml). Physical examination and imaging procedures confirm the occurrence of a left testicular tumour associated with numerous lung, hepatic and retroperitoneal metastases. Treatment with carbimazol and propanolol is established to manage hyperthyroidism, and an urgent orchiectomy is performed; the histologic diagnosis confirms an embryonal carcinoma (organoid type), but the patient died unexpectedly 24 hours later after having suffered sudden dyspnoea, tachypnoea, and tachyarrhythmia. Hyperthyroidism is a rare manifestation of a testicular tumour that should be borne in mind with regard to the patient's symptomatology and HCG levels.