Testosterone is a male hormone (androgen) that affects sexual features and development. It is produced in large amounts by the testicles. Before puberty, the testosterone level in boys is low. An increase in testosterone during puberty causes sex organs to mature, sperm to be produced, sexual features to develop, including facial and body hair, enlarged muscles, and a deep voice. The level of testosterone continues to rise during adulthood until it peaks around age 40, then it gradually decreases. Free and weakly bound testosterone (FWBT), also referred to as bioavailable testosterone, is thought to reflect an individual's biologically active, circulating testosterone. FWBT includes free testosterone and testosterone that is bound to albumin. FWBT does not include sex hormone binding globulin-bound testosterone, which is biologically inactive. Elevated levels of FWBT are observed in female hirsutism. In males, decreased serum concentrations are associated with hypogonadism. Total testosterone levels in women decrease by approximately 30% after menopause. Administration of exogenous estrogens has the physiologic effect of increasing SHBG concentrations and suppressing the production of androgens by the ovary. This results in a net decrease in FWBT. Decreased FWBT levels have been associated with diminished libido and loss of bone density. FWBT levels in males fall with age at a rate that exceeds that of total testosterone and parallels the drop in DHEA sulfate. This decrease is thought to be caused by diminished testicular production and not due to hypothalamic/pituitary insufficiency. Since SHBG has been found to increase with age, the FWBT level may be a more reliable indicator of testosterone production than total testosterone. The release of testosterone is controlled by a hormone called luteinizing hormone, or LH, which is produced by the pituitary gland. When the testosterone level is low, the pituitary gland releases LH, which increases the amount of testosterone produced by the testicles. In women and men, testosterone is also produced in small amounts by the adrenal glands, and, in women, by the ovaries.