Testosterone, or 17-hydroxyandrost-4-en-3-one, is a C19 steroid hormone with a molecular weight of 288.4 daltons. Testosterone is the main circulating hormone of an androgenic type. Under the action of 5-alpha-reductase, it is converted into dihydrotestosterone (DHT). Although testosterone is considered to be the primary male androgen, in many sites of action it is actually dihydrotestosterone that is the active form of this steroid. Dihydrotestosterone (DHT) is a more potent form of testosterone, shown to be roughly three to four times more active in the human body in comparison.
Testosterone exists in two forms in the blood stream: approximately 99% of the testosterone is bound to plasma proteins and the remainder is unbound. Testosterone is metabolized primarily in the liver. Enzymes have been identified in the skin and the reticuloendothelial system which are capable of metabolizing testosterone. Testosterone is the major androgen in males and is controlled by luteinizing hormone (LH). LH is released from the anterior pituitary exerting the primary control on testosterone production, and acting directly on the Leydig cells in the testes, where testosterone is produced. Testosterone levels are much lower in females compared to males. The major sources of testosterone in females are the ovaries, the adrenal glands, and the peripheral conversion of precursors, specifically the conversion of androstenedione to testosterone. Testosterone strongly binds to plasma proteins such as sex hormone-binding globulin (SHBG) or testosterone-estradiol-binding globulin (TEBG). Testosterone also binds with low affinity to CBG (cortisol-binding globulins) and albumin. Testosterone is the principal androgen synthesized in the testis, the ovary, and the adrenal cortex. Androgens are compounds which stimulate secondary sex characteristics and produce male secondary sex characteristics.
Testosterone stimulates adult maturation of external genitalia and secondary sex organs, and the growth of beard, axillary and pubic hair. Testosterone has anabolic effects leading to increased linear growth, nitrogen retention, and muscular development. Clinical evaluation of serum testosterone, along with serum LH, assists in evaluation of hypogonadal males. Testosterone is the main circulating hormone of an androgenic type. Androgens have been reported to increase protein anabolism and decrease protein catabolism. Androgens are responsible for the growth spurt of adolescence and for the eventual termination of linear growth brought about by fusion of the epiphyseal growth centers. In the circulation, testosterone serves as a prohormone for the formation of two classes of steroids: 5-alpha-reduced androgens, which act as the intracellular mediators of most androgen action, and estrogens, which enhance some androgenic effects and block others. Thus the net effect of the action of endogenous androgens is the sum of the effects of the secreted hormone (testosterone), its 5-alpha-reduced metabolite (dihydrotestosterone, and estradiol). Adequate amounts of these hormones are required for proper physical development and physiological homeostasis. Testosterone is crucial for male health. Testosterone and its metabolite, dihydrotestosterone, are the primary endogenous androgenic hormones (androgens). Endogenous androgens are responsible for the normal growth and development of the male sex organs and for development and maintenance of secondary sex characteristics. These effects include the growth and maturation of the prostate, seminal vesicles, penis, and scrotum; the development of male hair distribution; laryngeal enlargement, vocal cord thickening, alterations in body musculature, and fat distribution. In men, conditions responsive to testosterone elevation may be caused by primary hypogonadism (congenital or acquired) including testicular failure due to cryptorchidism, bilateral torsion, orchitis, vanishing testis syndrome, or orchidectomy, Klinefelter's syndrome, chemotherapy, or toxic damage from alcohol or heavy metals. Testosterone is also important in women's health. Androgens are secreted by both the ovaries and adrenal glands in women. Provision of testosterone in androgen-deficient women can improve their libido, energy, muscle mass and strength, and bone mineral density.
It is recognized that testosterone in males and females decreases with age. When diminished or absent from the body, pathological conditions can arise in the body due to a testosterone deficiency which are treatable by testosterone replacement. In men, increasing age leads to a reduction of testicular androgen production and androgen concentration in the organism. Testosterone measurements are useful in the evaluation of hypogonadal states. After physical maturity, men often notice a slow decline in the level of testosterone produced by the body. Dubbed andropause, subnormal androgen levels can lead to a decline in muscle mass, libido, sexual functioning and overall sense of well being later in life. In many instances this indicates a need for some form of androgen replacement. Common causes of decreased testosterone in males include: hypogonadism, orchidectomy, estrogen therapy, Klinefelter's syndrome, hypopituitarism, testicular feminization and hepatic cirrhosis. In contrast to the situation in women, in whom estrogen production drops to castration values within a comparatively short period, this takes decades in men and involves a gradual drop. Additional conditions can be treated or ameliorated through the supplementation of endogenous testosterone. Testosterone replacement therapy has been used to improve the status of this hormone. Testosterone therapy is currently indicated for treatment of male hypogonadism, anemia, breast cancer, and hereditary angioedema. It is also being considered for treating a variety of other conditions such as male osteoporosis that appear to be mediated by androgen deficiency. Testosterone replacement therapy primarily involves administration of testosterone by intramuscular injection, transdermal patch, or gels. |
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