Testosterone

Testosterone production declines naturally with age. Testosterone deficiency (TD) may result from disease or damage to the hypothalamus, pituitary gland, or testicles that inhibits hormone secretion and testosterone production, and is also known as hypogonadism. Depending on age, insufficient testosterone production can lead to abnormalities in muscle and bone development, underdeveloped genitalia, and diminished virility.

Testosterone is the androgenic hormone primarily responsible for normal growth and development of male sex and reproductive organs, including the penis, testicles, scrotum, prostate, and seminal vesicles. It facilitates the development of secondary male sex characteristics such as musculature, bone mass, fat distribution, hair patterns, laryngeal enlargement, and vocal chord thickening. Additionally, normal testosterone levels maintain energy level, healthy mood, fertility, and sexual desire.

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 carefully timed pulses (bursts), which triggers the secretion of leutenizing hormone (LH) from the pituitary gland. Leutenizing hormone stimulates the Leydig cells of the testes to produce testosterone. Normally, the testes produce 4–7 milligrams (mg) of testosterone daily.

Click here to complete the Androgen Deficiency in Aging Males (ADAM) questionnaire.

Incidence and Prevalence Testosterone production increases rapidly at the onset of puberty and decreases rapidly after age 50 (to 20% to 50% of peak level by age 80). Approximately 8 million men in the United States experience testosterone deficiency; approximately 600,000 receive treatment.

Testosterone deficiency (hypogonadism) may be present at birth (congenital) or may develop later (acquired). It is classified by the location of its cause along the hypothalamic-pituitary-gonadal axis:

Primary, disruption in the testicles Secondary, disruption in the pituitary Tertiary, disruption in the hypothalamus The most common congenital cause is Klinefelter's syndrome. This condition, which is caused by an extra X chromosome, results in infertility, sparse facial and body hair, abnormal breast enlargement (gynecomastia), and small testes. Congenital hormonal disorders such as leutenizing hormone-releasing hormone (LHRH) deficiency and gonadotropin-releasing hormone (GnRH) deficiency (e.g., Kallmann's syndrome) also may cause testosterone deficiency.

Other congenital causes include absence of the testes (anorchism; also may be acquired) and failure of the testicles to descend into the scrotum (cryptorchidism).

Acquired causes of testosterone deficiency include the following: Chemotherapy Damage occurring during surgery involving the pituitary gland, hypothalamus, or testes Glandular malformation Head trauma that affects the hypothalamus Infection (e.g., meningitis, syphilis, mumps) Isolated LH deficiency (e.g., fertile eunuch syndrome) Radiation Testicular trauma Tumors of the pituitary gland, hypothalamus, or testicles Signs and Symptoms

Signs depend on the age of onset and the duration of hormonal deficiency. Congenital testosterone deficiency is generally characterized by underdeveloped genitalia (testes that do not descend into the scrotum) and, occasionally, 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. Adult men may experience diminished libido, erectile dysfunction, muscle weakness, loss of body hair, depression, and other mood disorders.