Definition
A potent androgenic steroid and major product secreted by the LEYDIG CELLS of the TESTIS. Its production is stimulated by LUTEINIZING HORMONE from the PITUITARY GLAND. In turn, testosterone exerts feedback control of the pituitary LH and FSH secretion. Depending on the tissues, testosterone can be further converted to DIHYDROTESTOSTERONE or ESTRADIOL.
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testicles of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid. [Wikipedia]
Description
Testosterone is the most important androgen in potency and quantity•_Оa steroid sex hormone found in both men and women. Testosterone is synthesized and released by the Leydig cells that lie between the tubules and comprise less than 5% of the total testicular volume. testosterone diffuses into the seminiferous tubules where it is essential for maintaining spermatogenesis. Some binds to an androgen-binding protein (ABP) that is produced by the Sertoli cells and is homologous to the sex-hormone binding globulin that transports testosterone in the general circulation. The ABP carries testosterone in the testicular fluid where it maintains the activity of the accessory sex glands and may also help to retain testosterone within the tubule and bind excess free hormone. Some testosterone is converted to estradiol by Sertoli cell-derived aromatase enzyme. Leydig cell steroidogenesis is controlled primarily by luteinizing hormone with negative feedback of testosterone on the hypothalamic-pituitary axis. The requirement of spermatogenesis for high local concentrations of testosterone means that loss of androgen production is likely to be accompanied by loss of spermatogenesis. Indeed, if testicular androgen production is inhibited by the administration of exogenous androgens then spermatogenesis ceases. This is the basis of using exogenous testosterone as a male contraceptive. testosterone is converted to dihydrotestosterone by 5a-reductase type 2 (EC 1.3.1.22, SRD5A2), the androgen with the highest affinity for the androgen receptor. SRD5A2 deficiency illustrates the importance of dihydrotestosterone for external virilization, as individuals with this condition have normal male internal structures but their external genitalia are of female appearance. There is now clear evidence that the human fetal testis and also the fetal adrenal gland is capable of testosterone biosynthesis during the first trimester. Regardless of the source of androgen production, the target tissue responds by male sexual differentiation of the external genitalia by the end of the first trimester. It is clear that testicular damage may result in loss of testosterone production or the loss of spermatogenesis or both. Loss of androgen production results in hypogonadism, the symptoms of which reflect the functions of testosterone. Male hypogonadism is defined as failure of the testes to produce normal amounts of testosterone, combined with signs and symptoms of androgen deficiency. Systemic testosterone levels fall by about 1% each year in men. Therefore, with increasing longevity and the aging of the population, the number of older men with testosterone deficiency will increase substantially over the next several decades. Serum testosterone levels decrease progressively in aging men, but the rate and magnitude of decrease vary considerably. Approximately 1% of healthy young men have total serum testosterone levels below normal; in contrast, approximately 20% of healthy men over age 60 years have serum testosterone levels below normal. (A3376, A3377). In men, testosterone is produced primarily by the Leydig (interstitial) cells of the testes when stimulated by luteinizing hormone (LH). It functions to stimulate spermatogenesis, promote physical and functional maturation of spermatozoa, maintain accessory organs of the male reproductive tract, support development of secondary sexual characteristics, stimulate growth and metabolism throughout the body and influence brain development by stimulating sexual behaviors and sexual drive. In women, testosterone is produced by the ovaries (25%), adrenals (25%) and via peripheral conversion from androstenedione (50%). Testerone in women functions to maintain libido and general wellbeing. Testosterone exerts a negative feedback mechanism on pituitary release of LH and follicle-stimulating hormone (FSH). Testosterone may be further converted to dihydrotestosterone or estradiol depending on the tissue
Top Gene Interactions
Related Pathways
Testosterone Health Effects
General Information
- Metabolism: Testosterone is metabolized to 17-keto steroids through two different pathways. The major active metabolites are estradiol and dihydrotestosterone (DHT). Route of Elimination: About 90% of a dose of testosterone given intramuscularly is excreted in the urine as glucuronic and sulfuric acid conjugates of testosterone and its metabolites; about 6% of a dose is excreted in the feces, mostly in the unconjugated form. Half Life: 10-100 minutes
- Uses/Sources: To be used as hormone replacement or substitution of diminished or absent endogenous testosterone. Use in males: For management of congenital or acquired hypogonadism, hypogonadism associated with HIV infection, and male climacteric (andopause). Use in females: For palliative treatment of androgen-responsive, advanced, inoperable, metastatis (skeletal) carcinoma of the breast in women who are 1-5 years postmenopausal; testosterone esters may be used in combination with estrogens in the management of moderate to severe vasomotor symptoms associated with menopause in women who do not respond to adequately to estrogen therapy alone.
- Health Effects: In women, excess testosterone may cause decreased breast size, a deep voice, increased genital size, irregular periods, oily skin, and unnatural hair growth. In men, excess testosterone may cause aggression, breast tenderness or enlargement, decreased testes size, and urinary urgency. Chronically high levels of testosterone are associated with at least 2 inborn errors of metabolism including: Adrenal Hyperplasia Type 3 and Aromatase deficiency.
- Symptoms: In women, testosterone may cause decreased breast size, a deep voice, increased genital size, irregular periods, oily skin, and unnatural hair growth. In men, excess testosterone may cause aggression, premature baldness, breast tenderness or enlargement, decreased testes size, and urinary urgency.
- Route of Exposure: Endogenous, Injection
Mechanism of Action
| Target Name | Mechanism of Action | References |
|---|---|---|
|
Estrogen receptor Estrogen receptor beta Androgen receptor Cytochrome P450 3A4 Cytochrome P450 19A1 Lanosterol 14-alpha demethylase Sigma non-opioid intracellular receptor 1 Sex hormone-binding globulin Mineralocorticoid receptor 3-beta-hydroxysteroid-Delta(8),Delta(7)-isomerase |
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