Chemical structure of testosterone.
Find information on thousands of medical conditions and prescription drugs.

Testosterone

Testosterone is a steroid hormone from the androgen group. Testosterone is secreted in the testes of men and the ovaries of women. It is the principal male sex hormone and the "original" anabolic steroid. 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. more...

Home
Diseases
Medicines
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
Oxytetracycline
Phentermine
Tacrine
Tacrolimus
Tagamet
Talbutal
Talohexal
Talwin
Tambocor
Tamiflu
Tamoxifen
Tamsulosin
Tao
Tarka
Taurine
Taxol
Taxotere
Tazarotene
Tazobactam
Tazorac
Tegretol
Teicoplanin
Telmisartan
Temazepam
Temocillin
Temodar
Temodar
Temozolomide
Tenex
Teniposide
Tenoretic
Tenormin
Tenuate
Terazosin
Terbinafine
Terbutaline
Terconazole
Terfenadine
Teriparatide
Terlipressin
Tessalon
Testosterone
Tetrabenazine
Tetracaine
Tetracycline
Tetramethrin
Thalidomide
Theo-24
Theobid
Theochron
Theoclear
Theolair
Theophyl
Theophyl
Theostat 80
Theovent
Thiamine
Thiomersal
Thiopental sodium
Thioridazine
Thorazine
Thyroglobulin
Tiagabine
Tianeptine
Tiazac
Ticarcillin
Ticlopidine
Tikosyn
Tiletamine
Timolol
Timoptic
Tinidazole
Tioconazole
Tirapazamine
Tizanidine
TobraDex
Tobramycin
Tofranil
Tolazamide
Tolazoline
Tolbutamide
Tolcapone
Tolnaftate
Tolterodine
Tomoxetine
Topamax
Topicort
Topiramate
Tora
Toradol
Toremifene
Tracleer
Tramadol
Trandate
Tranexamic acid
Tranxene
Tranylcypromine
Trastuzumab
Trazodone
Trenbolone
Trental
Trest
Tretinoin
Triacetin
Triad
Triamcinolone
Triamcinolone hexacetonide
Triamterene
Triazolam
Triclabendazole
Triclosan
Tricor
Trifluoperazine
Trilafon
Trileptal
Trimetazidine
Trimethoprim
Trimipramine
Trimox
Triprolidine
Triptorelin
Tritec
Trizivir
Troglitazone
Tromantadine
Trovafloxacin
Tubocurarine chloride
Tussionex
Tylenol
Tyrosine
U
V
W
X
Y
Z

Sources of testosterone

Like other steroid hormones, testosterone is derived from cholesterol. The largest amounts of testosterone are produced by the testes in men, but it is also synthesized in smaller quantities in women by the theca cells of the ovaries, by the placenta, as well as by the zona reticulosa of the adrenal cortex in both sexes.

In the testes, testosterone is produced by the Leydig cells. Due to the dual function of the male gonad, testosterone directly influences 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 (SHBG).

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 (directly or as DHT), and by conversion to estradiol and activation of certain estrogen receptors.

Free testosterone (T) is transported into the cytoplasm of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5α-dihydrotestosterone (DHT) by the cytoplasmic enzyme 5α-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 and bind directly to specific nucleotide sequences of the chromosomal DNA. The areas of binding are called hormone response elements (HREs), and influence transcriptional activity of certain genes, 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 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 (especially affecting LH secretion). In many mammals, prenatal or perinatal "masculinization" of the sexually dimorphic 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 height growth and bone maturation. Virilizing effects include maturation of the sex organs, particularly the penis and the formation of the scrotum in fetuses, and after birth (usually at puberty) a deepening of the voice, growth of the beard and torso hair. Many of these fall into the category of male secondary sex characteristics. Increased testosterone causes deepening of the voice in both sexes at puberty. To take advantage of its virilizing effects, testosterone is often administered to transmen (female-to-male transsexual and transgender people) as part of the hormone replacement therapy, with a "target level" of the normal male testosterone level. And like-wise, male-to-female transsexuals are prescribed drugs to decrease the level of testosterone in the body and allow for the effects of estrogen to develop. Testosterone is also often used by bodybuilders to enhance muscle build.

Read more at Wikipedia.org


[List your site here Free!]


High testosterone linked to prostate cancer risk
From Science News, 10/8/05

Men with naturally high concentrations of testosterone in their blood face an elevated risk of prostate cancer, according to data collected since 1961. If testosterone concentrations that have been artificially elevated carry the same danger, then men who use hormone supplements to combat age-related problems could be in for trouble, says a group of Baltimore researchers.

J. Kellogg Parsons of Johns Hopkins Medical Institutions in Baltimore and his colleagues used nearly 40 years of test results from 794 men to assess the relationship between prostate cancer and variations in blood concentrations of testosterone. All the volunteers were healthy and living in Baltimore when they joined the study decades ago, and each underwent multiple testosterone tests.

The scientists found that men with the highest concentrations of what's known as free testosterone were 2.6 times as likely to get prostate cancer as were men with the lowest concentrations of that sex hormone.

The link was less strong in men older than 45. Within that demographic, the subgroup with the highest free testosterone faced 1.9 times as great a cancer risk as the subgroup with the least of the hormone did. Physicians are most concerned about the effects of testosterone among older men because they're the group most likely to take synthetic testosterone for symptoms such as declining libido (SN: 5/10/03, p. 296).

"[O]lder men receiving testosterone therapy should be carefully monitored for the development of ... prostate cancer," the researchers say in the September Cancer Epidemiology Biomarkers & Prevention.--B.H.

COPYRIGHT 2005 Science Service, Inc.
COPYRIGHT 2005 Gale Group

Return to Testosterone
Home Contact Resources Exchange Links ebay