Nintey-five percent of the total testosterone in males is synthesized in the Leydig cells of the testis. Defects, whether acquired or congenital, that interefere with interactions in the hypothalamic-pituitary-testicular axis can cause male hypogonadism It is essential to distinguish between primary hypogonadism (which originates in the testes) and secondary hypogonadism (which originates in the hypothalamus or pituitary). Symptoms highly suggestive of hypogonadism include decreased spontaneous erections, decreased nocturnal penile tumescence, decreased libido, and reduced testicular volume. The normal range for early morning testosterone in a male is between 300 ng/dL to 1000 ng/dL. Hypogonadism is diagnosed when the morning serum testosterone level is less than 300 ng/dL. However, clinical judgment can be exercised in the diagnosis of hypogonadism for patients with persistent symptoms of testosterone deficiency despite having testosterone levels are in the normal range . Of note, total testosterone less than 405.9 ng/dL is below the fifth percentile. Elderly males should aim for testosterone levels between 500 and 800 ng/dL while young adults should aim for testosterone levels between 600 and 900 ng/dL.
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