Use Testosterone testing is used to evaluate androgen excess or deficiency related to gonadal function, adrenal function, or tumor activity. Testosterone levels may be helpful in men for the diagnosis of hypogonadism, hypopituitarism, Klinefelter syndrome, and impotence (low values). Testosterone levels may be requested in women to investigate the cause of hirsutism, anovulation, amenorrhea, virilization, masculinizing tumors of the ovary, tumors of the adrenal cortices, and congenital adrenal hyperplasia (high values). Testosterone levels in children may be helpful to investigate issues related to puberty and development as well as the aforementioned. For testosterone measurements in females and children, use of Testosterone, Total, Women, Children, and Hypogonadal Males, LC/MS-MS (070001), which employs liquid chromatography/tandem mass spectrometry (LC/MS-MS), is recommended. Testosterone is the principal androgen in men.2,3 The production of testosterone by the male testes is stimulated by luteinizing hormone (LH), which is produced by the pituitary. LH secretion is, in turn, inhibited through a negative feedback loop by increased concentrations of testosterone and its metabolites. Most of the testosterone in males is produced by the Leydig cells of the testes and is secreted into the seminiferous tubule, where it is complexed to a protein made by the Sertoli cells. This results in the high local levels of testosterone that are required for normal sperm production.Diminished testosterone production is one of many potential causes of infertility in males.3,4 Low testosterone concentrations can be caused by testicular failure (primary hypogonadism) or inadequate stimulation by pituitary gonadotropins (secondary hypogonadism). Since men with hypogonadism often have high SHBG levels, the measurement of free or bioavailable testosterone has been advocated when total testosterone levels are normal in men with symptoms of androgen deficiency.5 Significant physiological changes occur in men as they age, in part due to a gradual decline in testosterone levels.6,7 It is generally accepted that the principal cause of this age-related decrease in testosterone production is testicular failure, although diminished gonadotropin production may play a role.5 By 75 years of age, the average male testosterone drops to 65% of average level in young adults. ?Andropause? is a term that has been used to refer to the constellation of symptoms associated with the age-related decline in testosterone production in men.5,8 The adult male reference range for testosterone was established by Bhasin and coworkers in a community-based sample of healthy young men from the Framingham Heart Study, Generation 3.9 The reference population included only men younger than 40 years of age who had no history of cardiovascular disease, diabetes, obesity, hypertension, cancer, or hypercholesterolemia.9 This study found that men of all ages who tested with testosterone levels <348 ng/dL were more likely to suffer from symptoms associated with androgen deficiency than men with higher serum testosterone levels.9 Much smaller amounts of testosterone and dihydrotestosterone are produced in women than in men.2,3 Weaker adrenal androgens and ovarian precursor molecules including androstenedione, DHEA, and DHEA sulfate can have significant androgenic effects in women. The ovary and adrenal glands produce some testosterone, but the majority of the testosterone in women is derived from the peripheral conversion of other steroids. Often, the first sign of testosterone excess in women is the development of male pattern hair growth, which is referred to as hirsutism.1,3,4,11 It should be noted that some women experience hair growth similar to that caused by increased testosterone due to racial or genetic causes and not due to excessive androgens. Measurement of the testosterone may help to distinguish racial or genetic causes of hirsutism from the abnormal pathology, particularly in women with mixed ethnic backgrounds. Women with more excessive testosterone levels may also experience virilization, with symptoms including increased muscle mass, redistribution of body fat, enlargement of the clitoris, deepening of the voice, and acne and increased perspiration. These women can also suffer from androgenic alopecia, the female equivalent of male pattern baldness.