See Also

Testosterone

Testosterone is a steroid Steroid

A steroid is a lipid [i] characterized by a carbon [i] skeleton with four fused rings. ... 

 hormone from the androgen group. Testosterone is primarily secreted in the testes Testicle

The testicles, or testes , are the male generative gland [i]s in animal [i]s. ... 

 of males and the ovaries Ovary

Ovaries are egg [i]-producing reproductive organs found in female [i] organisms. ... 

 of females although small amounts are secreted by the adrenal glands Adrenal gland

In mammal [i]s, the adrenal glands are the triangle-shaped endocrine gland [i]s that sit atop the kidney [i] ... 

. It is the principal male Malé

Mal , population 81,647 , is the capital [i] of the Republic of Maldives [i]. ... 

 sex hormone and an anabolic steroid Anabolic steroid

Anabolic androgenic steroids are a class of natural and synthetic steroid [i] hormone [i]s that promote ... 

. In both males and females, it plays key roles in health and well-being. Examples include enhanced libido, energy, immune function, and protection against osteoporosis. On average, the adult male body produces about twenty times the amount of testosterone an adult female's body does.

Discussions

  Discussion Features

   Ask a question about 'Testosterone'

   Start a new discussion about 'Testosterone'

   Answer questions about 'Testosterone'

   'Testosterone' discussion forum


Encyclopedia

Testosterone is a steroid Steroid

A steroid is a lipid [i] characterized by a carbon [i] skeleton with four fused rings. ... 

 hormone from the androgen group. Testosterone is primarily secreted in the testes Testicle

The testicles, or testes , are the male generative gland [i]s in animal [i]s. ... 

 of males and the ovaries Ovary

Ovaries are egg [i]-producing reproductive organs found in female [i] organisms. ... 

 of females although small amounts are secreted by the adrenal glands Adrenal gland

In mammal [i]s, the adrenal glands are the triangle-shaped endocrine gland [i]s that sit atop the kidney [i] ... 

. It is the principal male Malé

Mal , population 81,647 , is the capital [i] of the Republic of Maldives [i]. ... 

 sex hormone and an anabolic steroid Anabolic steroid

Anabolic androgenic steroids are a class of natural and synthetic steroid [i] hormone [i]s that promote ... 

. In both males and females, it plays key roles in health and well-being. Examples include enhanced libido, energy, immune function, and protection against osteoporosis. On average, the adult male body produces about twenty times the amount of testosterone an adult female's body does.

Sources of testosterone


Like other steroid hormones, testosterone is derived from cholesterol Cholesterol

Cholesterol is a sterol [i] and a lipid [i] found in the cell membrane [i]s of all body [i] ... 

. The largest amounts of testosterone are produced by the testes Testicle

The testicles, or testes , are the male generative gland [i]s in animal [i]s. ... 

 in men, but it is also synthesized in smaller quantities in women by the thecal cells Theca

A theca refers to any case, covering, or sheath.
... 

 of the ovaries Ovary

Ovaries are egg [i]-producing reproductive organs found in female [i] organisms. ... 

, by the placenta Placenta

The placenta is an ephemeral [i] organ [i] present only in female [i] placental [i] ... 

, as well as by the zona reticularis of the adrenal cortex Adrenal gland

In mammal [i]s, the adrenal glands are the triangle-shaped endocrine gland [i]s that sit atop the kidney [i] ... 

 in both sexes.

In the testes, testosterone is produced by the Leydig cells. The male generative glands also contain Sertoli cells which require testosterone for spermatogenesis. 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 .

Some drugs have the side-effect of elevating testosterone levels, such as finasteride Finasteride

Finasteride is an antiandrogen [i] which acts by inhibiting 5-alpha reductase [i], the enzyme [i] that c... 

 which prevents testosterone from burning into DHT thereby raising the level of testosterone in the body.

Mechanism of effects


The effects of testosterone in humans and other vertebrates occur by way of two main mechanisms: by activation of the androgen receptor , and by conversion to estradiol Estradiol

Estradiol is a sex hormone [i].... 

 and activation of certain estrogen Estrogen

Estrogens are a group of steroid [i] compounds, named for their importance in the oestrus [i] ... 

 receptors.

Free testosterone is transported into the cytoplasm Cytoplasm

Cytoplasm is a jelly-like material that fills cell [i]s. ... 

 of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5a-dihydrotestosterone Dihydrotestosterone

Dihydrotestosterone is a biologically active metabolite of the hormone [i] testosterone [i], formed prim ... 

  by the cytoplasmic enzyme 5a-reductase. DHT binds to the same androgen receptor even more strongly than T, so that its androgenic potency is about 2.5 times that of T. The T-receptor or DHT-receptor complex undergoes a structural change that allows it to move into the cell nucleus Cell nucleus

In cell biology [i], the nucleus is an organelle [i] found in most eukaryotic [i] cells [i]... 

 and bind directly to specific nucleotide Nucleotide

A nucleotide is a chemical compound [i] that consists of a heterocyclic base [i] ... 

 sequences of the chromosomal Chromosome

A chromosome is a large macromolecule [i] into which DNA [i] is normally packaged in a cell [i].... 

 DNA. The areas of binding are called hormone response elements , and influence transcriptional activity of certain gene Gene

A gene is the unit of heredity [i] in living organisms [i].... 

s, producing the androgen effects.

Androgen receptors occur in many different vertebrate body system tissues, and both males and females respond similarly to similar levels. Greatly differing amounts of testosterone prenatally, at puberty, and throughout life account for a large share of biological differences between males and females.

The bones and the brain are two important tissues in humans where the primary effect of testosterone is by way of aromatization Aromaticity

Aromaticity is a chemical property in which a conjugated [i] ring of unsaturated bonds ... 

 to estradiol. In the bones, estradiol accelerates maturation of cartilage into bone, leading to closure of the epiphyses and conclusion of growth. In the central nervous system, testosterone is aromatized to estradiol. Estradiol rather than testosterone serves as the most important feedback signal to the hypothalamus . In many mammals, prenatal or perinatal "masculinization" of the sexually dimorphic Sexual dimorphism

Sexual dimorphism is the systematic difference in form between individuals of different sex [i] in the s ... 

 areas of the brain by estradiol derived from testosterone programs later male sexual behavior.

Effects of testosterone on humans


In general, androgens promote protein synthesis and growth of those tissues with androgen receptors. Testosterone effects can be classified as virilizing and anabolic effects, although the distinction is somewhat artificial, as many of the effects can be considered both. Anabolic effects include growth of muscle mass and strength, increased bone density and strength, and stimulation of linear growth and bone maturation.
Virilizing effects include maturation of the sex organs, particularly the penis Penis

The penis is an external male [i] sexual organ [i]. ... 

 and the formation of the scrotum Scrotum

[i] containing the [[testicle]... 

 in fetuses, and after birth a deepening of the voice, growth of the beard Beard

A beard is the hair [i] that grows on a man's chin, cheeks, neck, and the area above the upper lip. ... 

 and axillary hair. Many of these fall into the category of male secondary sex characteristic Secondary sex characteristic

Secondary sex characteristics are traits that distinguish the two sex [i]es of a species, but that are n ... 

s.

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.

Most of the prenatal androgen effects occur between 7 and 12 weeks of gestation.
  • Genital virilization
  • Development of prostate Prostate

    The prostate is an exocrine gland [i] of the male mammal [i]ian reproductive system [i].... 

     and seminal vesicles


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. The function of this rise in humans is unknown. It has been speculated that "brain masculinization" is occurring since no significant changes have been identified in other parts of the body.

Early postnatal effects are the first visible effects of rising androgen levels in childhood, and occur in both boys and girls in puberty.
  • Adult-type body odour
  • Increased oiliness of skin and hair, acne
  • Pubarche
  • Axillary hair
  • Growth spurt, accelerated bone maturation
  • Fine upper lip and sideburn hair


Advanced postnatal effects begin to occur when androgen has been higher than normal adult female levels for months or years. In males these are normal late pubertal effects, and only occur in women after prolonged periods of excessive levels of free testosterone in the blood.
  • Phallic enlargement
  • Increased libido and erection Erection

    An erection of the penis [i] occurs when two tubular structures that run the length of the penis, the corpora cavernosa [i] ... 

     frequency
  • Pubic hair extends to thighs and up toward umbilicus
  • Facial hair Facial hair

    Facial hair is a secondary sex characteristic [i] in human male [i]s. ... 

  • Chest hair Chest hair

    The term chest hair is generally used to describe hair [i] that grows on the chest [i] of human males, i ... 

    , periareolar hair, perianal hair
  • Subcutaneous fat in face decreases
  • Increased muscle strength and mass
  • Deepening of voice
  • Growth of the adam's apple Adam's apple

    The human larynx [i] rests in a frame of cartilage [i] bound by ligament [i]s and muscle [i]s. ... 

  • Growth of spermatogenic tissue in testes, male fertility
  • Growth of jaw, brow, chin, nose, and remodeling of facial bone contours
  • Shoulders widen and rib cage expands
  • Completion of bone maturation and termination of growth. This occurs indirectly via estradiol metabolites and hence more gradually in men than women.


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 decline in the later decades of adult life.
  • Maintenance of muscle mass and strength
  • Maintenance of bone density and strength
  • Libido and erection frequency
  • Mental and physical energy

Testosterone in athletes

Testosterone may be administered to an athlete in order to improve performance, and is considered to be a form of doping in most sports. There are several application methods for testosterone, including intramuscular injections, transdermal gels and patches, and implantable pellets.

Anabolic steroids have also been taken to enhance muscle development, strength, or endurance. After a series of scandals and publicity in the 1980s , prohibitions of anabolic steroid use were renewed or strengthened by many sports organizations. Testosterone and other anabolic steroids were designated a "controlled substance" by the United States Congress in 1990, with the Anabolic Steroid Control Act.

Therapeutic use of testosterone


Testosterone was first isolated from a bull in 1935. There have been many pharmaceutical forms over the years. Forms of testosterone for human administration currently available in North America include injectable , oral , buccal , transdermal skin patches, and transdermal creams or gels and . In the pipeline are a "roll on" delivery method and a nasal spray.

The original and primary use of testosterone is for the treatment of males who have too little or no natural endogenous testosterone production; males with hypogonadism. Appropriate use for this purpose is legitimate hormone replacement therapy, which maintains serum testosterone levels in the normal range.

However, over the years, as with every hormone, testosterone or other anabolic steroids has also been given for many other conditions and purposes besides replacement, with variable success but higher rates of side effects or problems. Examples include infertility, lack of libido or erectile dysfunction, osteoporosis, penile enlargement, height growth, bone marrow Bone marrow

Bone marrow is the tissue comprising the center of large bone [i]s. ... 

 stimulation and reversal of anemia, and even appetite stimulation. By the late 1940s testosterone was being touted as an anti-aging wonder drug .

To take advantage of its virilizing effects, testosterone is often administered to transmen  as part of the hormone replacement therapy, with a "target level" of the normal male testosterone level. And like-wise, transwomen are sometimes prescribed drugs [anti-androgens] to decrease the level of testosterone in the body and allow for the effects of estrogen to develop.

There is a myth that exogenous testosterone can more or less definitively be used for male birth control. However, the vast majority of physicians will agree that to prescribe exogenous testosterone for this purpose is inappropriate. But perhaps more importantly, many men of first hand found this myth to be untrue or at least, unreliable. This is especially true when exogenous testosterone is used in conjunction with hCG.

Exogenous testosterone supplementation comes with a number of health risks. Fluoxymesterone and methyltestosterone are synthetic derivatives of testosterone. In 2006 it was reported that women taking Estratest, a combination pill including estrogen and methyltestosterone, were at considerably heightened risk of breast cancer.

The "testosterone deficiency" of aging and the andropause controversy


Testosterone levels decline gradually with age in men. The clinical significance of this decrease is debated . There is disagreement about if and when to treat aging men with testosterone replacement therapy. The American Society of Andrology's position is that testosterone therapy "is indicated when both clinical symptoms and signs suggestive of androgen deficiency and decreased testosterone levels are present". The American Association of Clinical Endocrinologists says "Hypogonadism is defined as a free testosterone level that is below the lower limit of normal for young adult control subjects. Previously, age-related decreases in free testosterone were once accepted as normal. Currently, they are not considered normal....Patients with low-normal to subnormal range testosterone levels warrant a clinical trial of testosterone."

There isn't total agreement on the threshold of testosterone value below which a man would be considered hypogonadal. Testosterone can be measured as "free" or more commonly, "total" . In the United States, male total testosterone levels below 200 to 300 ng/dl from a morning sample are generally considered low. However these numbers are typically not age-adjusted, but based on an average of a test group which includes elderly males with low testosterone levels. Therefore a value of 300 ng/dl might be normal for a 90 year old male, but not normal for a 30 year old. Identification of inadequate testosterone in an aging male by symptoms alone can be difficult. The signs and symptoms are non-specific, and might be confused with normal aging characteristics, such as loss of muscle mass and bone density, decreased physical endurance, decreased memory ability and loss of libido.

Replacement therapy can take the form of injectable depots, transdermal patches and gels, subcutaneous pellets and oral therapy. Adverse effects of testosterone supplementation include minor side effects such as acne and oily skin, and more significant complications such as increased hematocrit Hematocrit

The hematocrit and packed cell volume are measures of the proportion of blood [i] volume that is o ... 

, exacerbation of sleep apnea Sleep apnea

Sleep apnea is a sleep disorder [i] characterized by pauses in breathing [i] during sleep. ... 

 and acceleration of pre-existing prostate cancer Prostate cancer

Prostate cancer is a disease [i] in which cancer [i] develops in the prostate [i], a gland in the male [i]... 

 growth. Exogenous testosterone also causes suppression of spermatogenesis Spermatogenesis

Spermatogenesis is the process by which stem cell [i]s develop into mature spermatozoon cells. ... 

 and can lead to infertility. It is recommended that physicians screen for prostate cancer with a digital rectal exam and PSA level prior to initiating therapy, and monitor hematocrit and PSA levels closely during therapy.

Large scale trials to assess the efficiency and long-term safety of testosterone are still lacking. Many caution against embracing testosterone replacement therapy because of lessons from the female hormone replacement therapy trials, where initially promising results were later refuted by larger studies.

Synthesis

Testosterone is synthesized from pregnenolone Pregnenolone

Pregnenolone is a steroid [i] hormone involved in the steroidogenesis of progesterone [i], mineralocorticoids [i] ... 

, which is the precursor of all steroid Steroid

A steroid is a lipid [i] characterized by a carbon [i] skeleton with four fused rings. ... 

 hormones and is made from cholesterol Cholesterol

Cholesterol is a sterol [i] and a lipid [i] found in the cell membrane [i]s of all body [i] ... 

 by a series of enzymatic reactions. two pathways are possible, In the delta-5 pathway, pregnenolone is converted to DHEA Dehydroepiandrosterone

Dehydroepiandrosterone , is a natural steroid [i] hormone [i] produced from cholesterol [i] by the adrenal gland [i] ... 

 to androstenedione Androstenedione

Androstenedione is a 19-carbon [i] steroid [i] hormone [i] produced in the adrenal gland [i]s and the gonad [i] ... 

.



In the delta-4 pathway there is hydroxylation of C-17 of progesterone Progesterone

Progesterone is a C-21 steroid [i] hormone involved in the female [i] menstrual cycle [i], pregnancy [i] ... 

, to yield 17a-hydroxyprogesterone. The side chain is then cleaved to form androstenedione Androstenedione

Androstenedione is a 19-carbon [i] steroid [i] hormone [i] produced in the adrenal gland [i]s and the gonad [i] ... 

. Androstenedione is the immediate precursor to testosterone.



The keto Ketone

A ketone is either the functional group [i] characterized by a carbonyl [i] group linked to two other carbon [i]... 

 group on C-17 is reduced to an alcohol Alcohol

In chemistry [i], an alcohol is any organic compound [i] in which a hydroxyl [i] group [i] ... 

 to yield testosterone. Testosterone is a potential precursor of estradiol Estradiol

Estradiol is a sex hormone [i].... 

.

Zinc Zinc

Zinc is a chemical element [i] in the periodic table [i] that has the symbol Zn and atomic number [i] ... 

 supplementation is known to result in increased levels of testosterone synthesis, especially in those who are zinc deficient. Zinc is critical to the proper function of steroid receptors and plays a vital role as a cofactor to many enzyme Enzyme

Enzymes are protein [i]s that accelerate, or catalyze [i], chemical reaction [i]s. ... 

s which is the likely mechanim for its effect on testosterone synthesis.

Notes


External links