Gonadotropin-releasing hormoneTestosterone is an important mediator of behavior, morphology and physiology. A cascade of signals proviron zarost the amount of testosterone T circulating in the plasma; in response to stimulus the hypothalamus releases gonadotropin-releasing hormone GnRHwhich triggers secretion of gonadotropins from the pituitary, stimulating the synthesis and testosterone releasing hormone of T from the gonads. Previous testosterone releasing hormone has shown that changes to the social environment can alter circulating T-levels, which may have important fitness consequences, but it is currently unclear testosterone releasing hormone these changes are due to alterations in the signal from the brain, or changes in the ability of the pituitary and gonads to respond to this signal. Further, the strength and direction of response to a changing environment may differ according to life-history strategy. Species with genetically determined alternative strategies offer a pathway for examining these differences.
Testosterone - Wikipedia
Testosterone is the primary male sex hormone and an anabolic steroid. In male humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate , as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. Testosterone is a steroid from the androstane class containing a keto and hydroxyl groups at the three and seventeen positions respectively.
It is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites. On average, in adult males, levels of testosterone are about 7 to 8 times as great as in adult females. In addition to its role as a natural hormone, testosterone is used as a medication , for instance in the treatment of low testosterone levels in men and breast cancer in women. It is also used illicitly to enhance physique and performance , for instance in athletes.
For more information on testosterone as a medication, see the testosterone medication article. In general, androgens such as testosterone promote protein synthesis and thus growth of tissues with androgen receptors. Testosterone effects can also be classified by the age of usual occurrence.
For postnatal effects in both males and females, these are mostly dependent on the levels and duration of circulating free testosterone. Effects before birth are divided into two categories, classified in relation to the stages of development. The first period occurs between 4 and 6 weeks of the gestation. Examples include genital virilisation such as midline fusion, phallic urethra , scrotal thinning and rugation , and phallic enlargement; although the role of testosterone is far smaller than that of dihydrotestosterone.
There is also development of the prostate gland and seminal vesicles. During the second trimester, androgen level is associated with gender formation.
A mother's testosterone level during pregnancy is correlated with her daughter's sex-typical behavior as an adult, and the correlation is even stronger than with the daughter's own adult testosterone level. Early infancy androgen effects are the least understood. In the first weeks of life for male infants, testosterone levels rise. The levels remain in a pubertal range for a few months, but usually reach the barely detectable levels of childhood by 4—6 months of age.
It has been theorized that brain masculinization is occurring since no significant changes have been identified in other parts of the body. Before puberty effects of rising androgen levels occur in both boys and girls. These include adult-type body odor , increased oiliness of skin and hair, acne , pubarche appearance of pubic hair , axillary hair armpit hair , growth spurt , accelerated bone maturation , and facial hair.
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.
Growth of spermatogenic tissue in testicles, male fertility , penis or clitoris enlargement, increased libido and frequency of erection or clitoral engorgement.
Growth of jaw , brow, chin, nose, and remodeling of facial bone contours, in conjunction with human growth hormone. This occurs indirectly via estradiol metabolites and hence more gradually in men than women.
Increased muscle strength and mass, shoulders become broader and rib cage expands, deepening of voice, growth of the Adam's apple. Enlargement of sebaceous glands. This might cause acne, subcutaneous fat in face decreases. Pubic hair extends to thighs and up toward umbilicus , development of facial hair sideburns , beard , moustache , loss of scalp hair androgenetic alopecia , increase in chest hair , periareolar hair, perianal hair, leg hair , armpit hair.
Testosterone is necessary for normal sperm development. It activates genes in Sertoli cells , which promote differentiation of spermatogonia. It regulates acute HPA hypothalamic—pituitary—adrenal axis response under dominance challenge. Testosterone also regulates the population of thromboxane A 2 receptors on megakaryocytes and platelets and hence platelet aggregation in humans. Adult testosterone effects are more clearly demonstrable in males than in females, but are likely important to both sexes.
Some of these effects may decline as testosterone levels might decrease in the later decades of adult life. Testosterone does not appear to increase the risk of developing prostate cancer.
In people who have undergone testosterone deprivation therapy, testosterone increases beyond the castrate level have been shown to increase the rate of spread of an existing prostate cancer.
Conflicting results have been obtained concerning the importance of testosterone in maintaining cardiovascular health. High androgen levels are associated with menstrual cycle irregularities in both clinical populations and healthy women. When testosterone and endorphins in ejaculated semen meet the cervical wall after sexual intercourse, females receive a spike in testosterone, endorphin, and oxytocin levels, and males after orgasm during copulation experience an increase in endorphins and a marked increase in oxytocin levels.
This adds to the hospitable physiological environment in the female internal reproductive tract for conceiving, and later for nurturing the conceptus in the pre-embryonic stages, and stimulates feelings of love, desire, and paternal care in the male this is the only time male oxytocin levels rival a female's [ citation needed ]. Testosterone levels follow a nyctohemeral rhythm that peaks early each day, regardless of sexual activity. There are positive correlations between positive orgasm experience in women and testosterone levels where relaxation was a key perception of the experience.
There is no correlation between testosterone and men's perceptions of their orgasm experience, and also no correlation between higher testosterone levels and greater sexual assertiveness in either sex.
Sexual arousal and masturbation in women produce small increases in testosterone concentrations. Studies conducted in rats have indicated that their degree of sexual arousal is sensitive to reductions in testosterone. When testosterone-deprived rats were given medium levels of testosterone, their sexual behaviors copulation, partner preference, etc.
Therefore, these mammals may provide a model for studying clinical populations among humans suffering from sexual arousal deficits such as hypoactive sexual desire disorder. In every mammalian species examined demonstrated a marked increase in a male's testosterone level upon encountering a novel female. The reflexive testosterone increases in male mice is related to the male's initial level of sexual arousal.
In non-human primates, it may be that testosterone in puberty stimulates sexual arousal, which allows the primate to increasingly seek out sexual experiences with females and thus creates a sexual preference for females. Testosterone levels are shown to increase as a response to previously neutral stimuli when conditioned to become sexual in male rats.
In men, higher levels of testosterone are associated with periods of sexual activity. The increase in testosterone levels was associated with the degree that the women thought the men were trying to impress them.
Men's levels of testosterone, a hormone known to affect men's mating behaviour, changes depending on whether they are exposed to an ovulating or nonovulating woman's body odour. Men who are exposed to scents of ovulating women maintained a stable testosterone level that was higher than the testosterone level of men exposed to nonovulation cues.
Testosterone levels and sexual arousal in men are heavily aware of hormone cycles in females. Men with lower thresholds for sexual arousal have a greater likelihood to attend to sexual information and that testosterone may work by enhancing their attention to the relevant stimuli. Androgens may modulate the physiology of vaginal tissue and contribute to female genital sexual arousal. In addition, a continuous increase in vaginal sexual arousal may result in higher genital sensations and sexual appetitive behaviors.
When females have a higher baseline level of testosterone, they have higher increases in sexual arousal levels but smaller increases in testosterone, indicating a ceiling effect on testosterone levels in females. Sexual thoughts also change the level of testosterone but not level of cortisol in the female body, and hormonal contraceptives may affect the variation in testosterone response to sexual thoughts.
Testosterone may prove to be an effective treatment in female sexual arousal disorders ,  and is available as a dermal patch. There is no FDA approved androgen preparation for the treatment of androgen insufficiency; however, it has been used off-label to treat low libido and sexual dysfunction in older women. Testosterone may be a treatment for postmenopausal women as long as they are effectively estrogenized.
Falling in love decreases men's testosterone levels while increasing women's testosterone levels. There has been speculation that these changes in testosterone result in the temporary reduction of differences in behavior between the sexes. Marriage or commitment could cause a decrease in testosterone levels. It is suggested that these single men with prior experience are in a more competitive state than their non-experienced counterparts.
Collectively, these results suggest that the presence of competitive activities rather than bond-maintenance activities are more relevant to changes in testosterone levels. Men who produce more testosterone are more likely to engage in extramarital sex. Fatherhood also decreases testosterone levels in men, suggesting that the resulting emotional and behavioral changes promote paternal care.
If the levels reduce, then there is more empathy by the father than in fathers whose levels go up. Testosterone levels play a major role in risk-taking during financial decisions. Most studies support a link between adult criminality and testosterone, although the relationship is modest if examined separately for each sex. Nearly all studies of juvenile delinquency and testosterone are not significant.
Most studies have also found testosterone to be associated with behaviors or personality traits linked with criminality such as antisocial behavior and alcoholism. About half the studies have found a relationship and about half no relationship.
Testosterone is only one of many factors that influence aggression and the effects of previous experience and environmental stimuli have been found to correlate more strongly. A few studies indicate that the testosterone derivative estradiol one form of estrogen might play an important role in male aggression. The sexual hormone can encourage fair behavior.
For the study subjects took part in a behavioral experiment where the distribution of a real amount of money was decided. The rules allowed both fair and unfair offers. The negotiating partner could subsequently accept or decline the offer. The fairer the offer, the less probable a refusal by the negotiating partner. If no agreement was reached, neither party earned anything. Test subjects with an artificially enhanced testosterone level generally made better, fairer offers than those who received placebos, thus reducing the risk of a rejection of their offer to a minimum.
Two later studies have empirically confirmed these results. Testosterone is significantly correlated with aggression and competitive behaviour and is directly facilitated by the latter. There are two theories on the role of testosterone in aggression and competition. The second theory is similar and is known as "evolutionary neuroandrogenic ENA theory of male aggression". By doing so, individuals with masculinized brains as a result of pre-natal and adult life testosterone and androgens enhance their resource acquiring abilities in order to survive, attract and copulate with mates as much as possible.
Higher pre-natal testosterone indicated by a low digit ratio as well as adult testosterone levels increased risk of fouls or aggression among male players in a soccer game. The rise in testosterone levels during competition predicted aggression in males but not in females. Estradiol is known to correlate with aggression in male mice. The brain is also affected by this sexual differentiation;  the enzyme aromatase converts testosterone into estradiol that is responsible for masculinization of the brain in male mice.