Testosterone is produced in men and women, albeit at much lower levels in women. It exists in three forms, two of which are considered to be “bioavailable” because in these forms, testosterone molecules are readily available to interact with receptors and exert biological effects.
Testosterone is necessary for the development of male fetuses, and boys maintain a level of testosterone that is approximately twice that of girls until puberty. During puberty, testosterone increases 10-fold to control the development of secondary sexual characteristics in boys and in girls, a 2-fold increase leads to the development of pubic and axillary hair. Women have testosterone levels that are approximately 5%-10% of those in men, and it is important in controlling mood, libido, and the maintenance of bone. In men, testosterone is important for sexual function, muscle mass, libido, and bone health. Men with low levels of testosterone display clinical symptoms that often drive them to seek help from their physician.