Testosterone is the primary androgen in the male. In the adult, a variety of symptoms have been associated with low testosterone. These include sexual symptoms, such as low libido, erectile dysfunction, decreased intensity of orgasm, as well as nonsexual signs and symptoms, such as hot flashes, osteoporosis, and loss of body hair. In addition, many nonspecific symptoms, including lethargy, depression, difficulty concentrating, sleep disturbances, anemia, loss of muscle bulk and strength, increased body fat, and increased body mass index, are commonly present in men with low testosterone levels. Testosterone production declines as men age at the rate of ∼0.5%–1% per year. In general, aging men have lower testosterone levels than young men. Whether this age-associated decline in testosterone is pathologic and needs treatment or is part of the normal aging process has become controversial.
Testosterone is produced in the testes in response to stimulation from LH produced in the pituitary gland. Primary hypogonadism is defined as an inability of the testes to produce sufficient testosterone despite adequate LH stimulation. Secondary hypogonadism exists when there is inadequate LH stimulation. Causes of primary hypogonadism include testicular damage due to chemotherapy, mumps orchitis, and genetic (e.g., Klinefelter syndrome) or congenital conditions. Classic causes of secondary hypogonadism include Kallman syndrome and pituitary insufficiency due to pituitary tumors, surgery, or radiation.