Testosterone (17β-OH-4-androstene-3-one) is a steroid hormone which is part of the androgens group and is mainly produced by the Leydig cells located in the testes and, minimally, by the ovaries and the adrenal cortex. It is also present in women who, compared to men, have a greater tendency to convert this into oestrogen.
In postpubertal males, testosterone is secreted primarily by the testes with only a small amount derived from peripheral conversion of androstenedione. In humans it is involved in the development of the sexual organs (differentiation of the testis and the whole genital apparatus) and of secondary sexual characteristics, such as beard, hair distribution, the timbre of the voice and muscles. During puberty, testosterone is also involved in skeletal development, limiting the elongation of the long bones and, in so doing, avoiding disproportionate growth of the limbs. In adult humans, the levels of testosterone have a very important role as regards the sexuality, the musculoskeletal system, the vitality and good health (mainly understood as protection from metabolic diseases such as hypertension and diabetes mellitus); helps to ensure fertility, as it stimulates the maturation of sperm in the testes. Daily production of testosterone in men varies from 5 to 7 milligrams, but after 30 years of age, typically tends to decrease annually by 1%.
In adult women, over 50% of serum testosterone is derived from peripheral conversion of androstenedione secreted by the adrenal and ovary, with the remainder from direct secretion of testosterone by these glands.
The majority of circulating testosterone is bound by SHBG and a smaller portion is bound by albumin. Only a small percentage (< 1%) exists in circulation as unbound or free testosterone. Testosterone effects can be classified as virilising and anabolic effects, although the distinction is somewhat artificial, as many of the effects can be considered both. Anabolic effects include growth of muscle mass and strength, increased bone density and strength and stimulation of linear growth and bone maturation. Virilising effects include maturation of the sex organs and after birth, usually at puberty, a deepening of the voice, growth of the beard and axillary hair (male secondary sex characteristics).
Testosterone levels decline gradually with age in men (andropause). The signs and symptoms are nonspecific and are generally associated with aging such as loss of muscle mass and bone density, decreased physical endurance, decreased memory ability and loss of libido. In females of all ages, elevated testosterone levels can be associated with a variety of virilising conditions, including adrenal tumours and polycystic ovarian disease.