Testosterone is the primary male sex hormone and an anabolic steroid. In male humans, it is secreted by the testes and it plays a key role in the development of male reproductive tissues, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair.
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Testosterone is produced by the gonads (by the Leydig cells in testes in men and by the ovaries in women), although small quantities are also produced by the adrenal glands in both sexes. It is an androgen, meaning that it stimulates the development of male characteristics.
Testosterone is expressed at much higher levels in men than women and it initiates the development of the male internal and external reproductive organs during fetal development and is essential for the production of sperm in adult life. It is important for bone strength and the development of lean muscle mass and for enhancing libido both in men and women.
The regulation of testosterone production is tightly controlled to maintain normal levels in blood. Excess testosterone can lead to precocious puberty in children, and in women, high blood levels of testosterone may be an indicator of polycystic ovary syndrome, androgen resistance, congenital adrenal hyperplasia and ovarian cancer.
In males, a testosterone assay is used to detect hypogonadism, testicular failure, infertility, hypopituitarism and hyperprolactinemia. In females, it can be used to diagnose polycystic ovary syndrome, adrenal hyperplasia, infertility, hirsutism, amenorrhea, obesity and virilization.
Typically, a test for total testosterone is used – this test measures testosterone that is bound to proteins in the blood (e.g., albumin and sex-hormone binding globulin [SHBG]) as well as testosterone that is not bound (free testosterone). About two-thirds of testosterone circulates in the blood bound to SHBG and slightly less than one-third bound to albumin. A small percent (less than 4%) circulates as free testosterone. Free testosterone plus the testosterone bound to albumin is the bioavailable testosterone, which can act on target tissues. In many cases, the total testosterone test provides adequate information. However, in certain cases, for example when the level of SHBG is abnormal, a test for free or bioavailable testosterone may be performed as it may more accurately reflect the presence of a medical condition. Testosterone immunoassays are typically MAb-based competitive ELISAs.
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