Testosterone deficiency or male hypogonadism is the body's insufficient production of the sex hormone testosterone.

Diagnosis of the condition is based on symptoms as well as results of blood tests measuring the patient's testosterone levels. If the diagnosis is confirmed, the patient undergoes further testing to determine its cause. Such tests may include hormone testing, semen analysis, pituitary imaging, genetic studies, and testicular biopsy. Testing the patient's testosterone levels can help determine the appropriated medication dosage.

The condition is treated depending on its cause and when it occurs. Testosterone replacement therapy is used in some types of hypogonadism in which fertility is not an issue. If the cause is pituitary problem, pituitary hormones are used to stimulate sperm production and restore fertility. Patients with a pituitary tumor may undergo surgery to remove the tumor, medication, radiation, or the replacement of other hormones. Assisted reproduction may also help fertility.

The signs and symptoms of the condition depend on its development. If the condition occurs during fetal development, a genetically male child may be born with female genitals, ambiguous genitals, and underdeveloped male genitals. If it occurs during puberty, a child's growth and development is affected causing decreased development of muscle mass, lack of a deep voice, impaired growth of body hair, impaired growth of the penis and testicles, excessive growth of the arms and legs in relation to the trunk of the body, and development of breast tissue. If the condition develops during adulthood, certain masculine physical characteristics may be altered and normal reproductive function is impaired. Adults with the condition may have erectile dysfunction, infertility, increased in body fat, a developed breast tissue, loss of bone mass. It may also result in mental and emotional changes such as those experienced by women during menopause.

The condition is categorized as primary or secondary. Either category may be congenitally inherited or acquired later in life through an injury or an infection. Primary hypogonadism is primarily caused by Klinefelter 's syndrome, Undescended testicles, Mumps orchitis, Hemochromatosis, injury to the testicles, cancer treatment, and aging. Causes of secondary hypogonadism may include Kallmann syndrome, pituitary disorders, inflammatory disease, HIV/AIDS, medications, and obesity.

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