Your doctor will conduct a physical exam during which he or she will note whether your sexual development, such as your pubic hair, muscle mass and size of your testes, is consistent with your age. Your doctor may test your blood level of testosterone if you have any of the signs or symptoms of hypogonadism.

Early detection in boys can help prevent problems from delayed puberty. Early diagnosis and treatment in men offer better protection against osteoporosis and other related conditions.

Doctors base a diagnosis of hypogonadism on symptoms and results of blood tests that measure testosterone levels. Because testosterone levels vary and are generally highest in the morning, blood testing is usually done early in the day, near 8 a.m.

If tests confirm you have low testosterone, further testing can determine if a testicular disorder or a pituitary abnormality is the cause. Based on specific signs and symptoms, additional studies can pinpoint the cause. These studies may include:

Testosterone testing also plays an important role in managing hypogonadism. This helps your doctor determine the right dosage of medication, both initially and over time.

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Excerpt from:
Male hypogonadism Tests and diagnosis - Mayo Clinic