[3] In humans, testosterone plays a key role in the development of male reproductive tissues such as testicles and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair.

[4] In addition, testosterone in both sexes is involved in health and well-being, where it has a significant effect on overall mood, cognition, social and sexual behavior, metabolism and energy output, the cardiovascular system, and in the prevention of osteoporosis.

[5][6] Insufficient levels of testosterone in men may lead to abnormalities including frailty, accumulation of adipose fat tissue within the body, anxiety and depression, sexual performance issues, and bone loss.

[16] Testosterone can be described as having anabolic and androgenic (virilising) effects, though these categorical descriptions are somewhat arbitrary, as there is a great deal of mutual overlap between them.

[30] Pubertal effects begin to occur when androgen has been higher than normal adult female levels for months or years.

In males, these are usual late pubertal effects, and occur in women after prolonged periods of heightened levels of free testosterone in the blood.

Testosterone deficiency is associated with an increased risk of metabolic syndrome, cardiovascular disease and mortality, which are also sequelae of chronic inflammation.

[13] It can be administered as a cream or transdermal patch that is applied to the skin, by injection into a muscle, as a tablet that is placed in the cheek, or by ingestion.

[13] Serious side effects may include liver toxicity, heart disease (though a randomized trial found no evidence of major adverse cardiac events compared to placebo in men with low testosterone[58]), and behavioral changes.

[59][60] No immediate short term effects on mood or behavior were found from the administration of supraphysiologic doses of testosterone for 10 weeks on 43 healthy men.

Studies have shown small or inconsistent correlations between testosterone levels and male orgasm experience, as well as sexual assertiveness in both sexes.

[69] In accordance with sperm competition theory, testosterone levels are shown to increase as a response to previously neutral stimuli when conditioned to become sexual in male rats.

[89] This is particularly beneficial for humans since offspring are dependent on parents for extended periods of time and mothers have relatively short inter-birth intervals.

[95] Research has also found that heightened levels of testosterone and cortisol are associated with an increased risk of impulsive and violent criminal behavior.

By doing so, individuals with masculinized brains as a result of pre-natal and adult life testosterone and androgens, enhance their resource acquiring abilities to survive, attract and copulate with mates as much as possible.

The areas of binding are called hormone response elements (HREs), and influence transcriptional activity of certain genes, producing the androgen effects.

[150] Testosterone has been found to act as an antagonist of the TrkA and p75NTR, receptors for the neurotrophin nerve growth factor (NGF), with high affinity (around 5 nM).

Testosterone is also synthesized in far smaller total quantities in women by the adrenal glands, thecal cells of the ovaries, and, during pregnancy, by the placenta.

Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone-binding globulin (SHBG).

Finally, increasing levels of testosterone through a negative feedback loop act on the hypothalamus and pituitary to inhibit the release of GnRH and FSH/LH, respectively.

[200][201] Several professional medical groups have recommended that 350 ng/dL generally be considered the minimum normal level,[202] which is consistent with previous findings.

[210][211] Immunofluorescence assays exhibit considerable variability in quantifying testosterone concentrations in blood samples due to the cross-reaction of structurally similar steroids, leading to overestimating the results.

In contrast, the liquid chromatography/tandem mass spectrometry method is more desirable: it offers superior specificity and precision, making it a more suitable choice for this application.

Testosterone has been detected at variably higher and lower levels among men of various nations and from various backgrounds, explanations for the causes of this have been relatively diverse.

[216][217] People from nations of the Eurasian Steppe and Central Asia, such as Mongolia, Kyrgyzstan and Uzbekistan, have consistently been detected to have had significantly elevated levels of testosterone,[218] while people from Central European and Baltic nations such as the Czech Republic, Slovakia, Latvia and Estonia have been found to have had significantly decreased levels of testosterone.

[220] Research on the action of testosterone received a brief boost in 1889, when the Harvard professor Charles-Édouard Brown-Séquard (1817–1894), then in Paris, self-injected subcutaneously a "rejuvenating elixir" consisting of an extract of dog and guinea pig testicle.

He reported in The Lancet that his vigor and feeling of well-being were markedly restored but the effects were transient,[221] and Brown-Séquard's hopes for the compound were dashed.

In that year, Koch and his student, Lemuel McGee, derived 20 mg of a substance from a supply of 40 pounds of bovine testicles that, when administered to castrated roosters, pigs and rats, re-masculinized them.

[222] The group of Ernst Laqueur at the University of Amsterdam purified testosterone from bovine testicles in a similar manner in 1934, but the isolation of the hormone from animal tissues in amounts permitting serious study in humans was not feasible until three European pharmaceutical giants – Schering (Berlin, Germany), Organon (Oss, Netherlands) and Ciba – began full-scale steroid research and development programs in the 1930s.

[227] These independent partial syntheses of testosterone from a cholesterol base earned both Butenandt and Ruzicka the joint 1939 Nobel Prize in Chemistry.