Testosterone deficiency, also known as hypogonadism, is a condition in which the testes are unable to produce enough testosterone to fulfill the body's needs. Testosterone deficiency has many possible causes, including genetic abnormalities, injury to the testes, and being on certain medications. Normal aging also may play a role in the decline of male testosterone levels. It is also known as low testosterone.

The testes produce testosterone regulated by a complex chain of signals that begins in the brain. This chain is called the hypothalamic-pituitary-gonadal axis. The hypothalamus secretes gonadotropin-releasing hormone (GnRH) to the pituitary gland in spurts, which trigger the secretion of leutenizing hormone (LH) from the pituitary gland. This hormone stimulates the Leydig cells of the testes to produce testosterone. Normally, the testes produce 4-7 milligrams (mg) of testosterone each and every day.

After puberty, testosterone production increases rapidly, and will decrease rapidly after age 50. Recent estimates show that approximately 13 million men in the United States experience testosterone deficiency and less than 10-percent receive treatment for the condition, which is growing in cultural acceptability.

Studies also have shown that some men with obesity, diabetes, or hypertension may be twice as likely to have low testosterone levels, though as stated, low testosterone and testosterone deficiency can be caused by taking certain medications, chemotherapy, infections and other basic causes.

Signs of testosterone deficiency depend on the age of onset and the duration of hormonal deficiency. Congenital testosterone deficiency is usually characterized by underdeveloped genitalia, and sometimes even undeterminable genitalia. Acquired testosterone deficiency that develops near puberty can result in enlargement of breast tissue (gynecomastia), sparse or absent pubic and body hair, and underdeveloped penis, testes, and muscle. Adults may experience diminished libido, erectile dysfunction, muscle weakness, hair loss, depression, and other common mood disorders.