A 29-year-old man comes to the office due to increased sweating, heat intolerance, insomnia, and unintentional weight loss over the past 4 weeks. The patient has also noticed that his right testis feels bigger than the left but has had no scrotal pain. He has no prior medical problems and takes no medications. The patient does not use tobacco, alcohol, or illicit drugs. Blood pressure is 121/71 mm Hg and pulse is 108/min. On examination, the thyroid is mildly enlarged. Testicular examination reveals an enlarged, nontender right testicle. Laboratory testing shows elevated serum thyroxine and triiodothyronine levels. Scrotal ultrasonography demonstrates a hypoechoic mass within the right testicle. Elevated levels of which of the following substances would most likely explain this patient's symptoms?

Question

A 29-year-old man comes to the office due to increased sweating, heat intolerance, insomnia, and unintentional weight loss over the past 4 weeks.  The patient has also noticed that his right testis feels bigger than the left but has had no scrotal pain.  He has no prior medical problems and takes no medications.  The patient does not use tobacco, alcohol, or illicit drugs.  Blood pressure is 121/71 mm Hg and pulse is 108/min.  On examination, the thyroid is mildly enlarged.  Testicular examination reveals an enlarged, nontender right testicle.  Laboratory testing shows elevated serum thyroxine and triiodothyronine levels.  Scrotal ultrasonography demonstrates a hypoechoic mass within the right testicle.  Elevated levels of which of the following substances would most likely explain this patient's symptoms?

Quizplus · Accepted Answer

11ec160b_39a4_901e_9249_3bdc5f6da392_MD0001_00 The human glycoprotein hormone family includes 4 hormones: hCG, TSH, FSH, and LH. Each is a heterodimeric structure consisting of an identical alpha subunit noncovalently associated with a unique but homologous beta subunit, which confers its specific biologic properties. The beta subunits of hCG and TSH share significant sequence homology; this structural similarity allows hCG to bind and activate the TSH receptor, albeit with much lower affinity than TSH. hCG is normally produced in the placenta but can also be released by a number of malignancies, especially choriocarcinoma and nonseminomatous germ cell tumors. This patient likely has a testicular germ cell tumor producing large quantities of hCG. Activation of TSH receptors on the thyroid gland by high levels of hCG can cause paraneoplastic hyperthyroidism, presenting with weight loss, sweating, and heat intolerance. (Choice A) Alpha-fetoprotein can be used as a tumor marker for a number of malignancies, including hepatocellular carcinoma and nonseminomatous germ cell tumors of the testes. However, elevated levels would not result in symptoms of hyperthyroidism. (Choice B) FSH is produced by gonadotrophs in the anterior pituitary cells. It is not a marker for testicular tumors and has no affinity for thyroid receptors. (Choice D) Lactate dehydrogenase is an enzyme involved in anaerobic glycolysis. Although increased levels can occur with both seminomatous and nonseminomatous tumors of the testes, this enzyme does not interact with TSH receptors. (Choice E) The majority of circulating alkaline phosphatase comes from bone, liver, gastrointestinal tract, and placenta. Placenta-like alkaline phosphatase is a nonspecific tumor marker that can be increased in testicular seminoma and other malignancies but has no homology with TSH. Educational objective: Human chorionic gonadotropin (hCG) has a structure similar to TSH. Patients with testicular germ cell tumors or gestational trophoblastic disease may develop very high serum hCG concentrations, which can stimulate TSH receptors and cause paraneoplastic hyperthyroidism. __________ References: Paraneoplastic hyperthyroidism. (http://www.ncbi.nlm.nih.gov/pubmed/22688478) Thyrotropic action of human chorionic gonadotropin. (http://www.ncbi.nlm.nih.gov/pubmed/8563483)

A 29-Year-Old Man Comes to the Office Due to Increased