Testosterone is the main sex hormone (androgen) in men. It is responsible for male physical characteristics. Although it is considered to be a "male" sex hormone, it is present in the blood of both men and women. This test measures the level of testosterone in the blood.

Testosterone is mainly produced by special endocrine tissue (the Leydig cells) in the male testicles. It is also produced by the adrenal glands in both males and females and, in small amounts, by the ovaries in females.

In males, testosterone stimulates development of secondary sex characteristics, including enlargement of the penis, growth of body hair, muscle development, and a deepening voice. It is present in large amounts in males during puberty and in adult males to regulate the sex drive and maintain muscle mass. In women, testosterone is converted to estradiol, the main sex hormone in females.

Testosterone production is stimulated and controlled by luteinizing hormone (LH), which is manufactured by the pituitary gland. Testosterone works within a negative feedback mechanism: as the testosterone level increases, LH production decreases, which slows testosterone production; decreased testosterone causes increased production of LH, which in turn stimulates testosterone production.

Testosterone levels are diurnal, peaking in the early morning hours (about 4:00 to 8:00 am), with the lowest levels in the evening (about 4:00 to 8:00 pm). Levels also increase after exercise and also decrease with age.

About two-thirds of testosterone circulates in the blood bound to sex-hormone binding globulin (SHBG) and slightly less than one-third bound to albumin. A small percent (less than 4%) circulates as free testosterone. The free plus the albumin-bound testosterone is the bioavailable fraction, which can act on target tissues.