Testosterone
Testosterone is the primary hormone involved in male sexual development and
fertility
Testosterone also plays an important role in muscle formation, body composition,
bone health, and cognitive function
Testosterone is produced and secreted by the Sertoli cells of the testes. The testes
produce 3 - 10 mg of testosterone a day.
A small amount of testosterone (5 - 8%) is converted to dihydrotestosterone (DHT)
by the enzyme 5-alpha reductase. DHT is a more potent androgen than testosterone.
It is involved in prostate development, external genitalia development, hair follicles
(hair loss), and skin sebum production.
An even smaller amount of testosterone (0.3 - 0.5%) is converted to the hormone
estradiol by the enzyme aromatase. Estradiol plays an important role in bone health,
cognitive function (memory), and plasma lipid levels. Aromatase is present in fatty
tissue, so obese men may overproduce estradiol
Mechanism of action:
The effect of testosterone act mainly by two ways-
By activation of androgen receptors
By conversion to estradiol and activation of certain androgen receptors
Free testosterone is transported to the target tissue cells. Where it can bind tp
androgen receptor and reduced to 5 a-dihydrotestosterone(DHT) by cytoplasmic
enzyme 5 a- reductase.
The DHT receptors complex undergoes a structural change that allow it to move to
cell nucleus and bind directly to specific nucleotide sequences of chromosomal
DNA. Then the RNA begins to increase in prostatic cells.
Leydig cells in the testes function to turn cholesterol into testosterone. LH regulates
the initial step in this process. Two important intermediates in this process are
dehydroepiandrosterone (DHEA) and androstenedione. Androstenedione is
converted to testosterone by the enzyme 17-beta-hydroxysteroid dehydrogenase.
The majority of testosterone is bound to plasma proteins such as sex-hormone-
binding-globulin and albumin. This majority supply of protein-bound testosterone
acts as a surplus of testosterone hormone for the body. The small amounts of free
testosterone in the blood act at the level of the tissues, primarily the seminal
vesicles, bone, muscle, and prostate gland.
Regulation:
Testosterone limits its own secretion via negative feedback. High levels of testosterone in
the blood feedback to the hypothalamus to suppress the secretion of GnRH and also
feedback to the anterior pituitary, making it less responsive to GnRH stimuli.
Male sexual functions:
Control of sexual functions in both male and female is beginning with secretion of
GnRH by hypothalamus.
Stimulation of anterior pituitary gland leads to secretion of two hormones
LH and FSH
LH is primary stimulus for secretion of testosterone by testes The quantity of
testosterone secreted increases in direct proportion to the amount of LH available.
Testosterone is responsible for the development of primary sexual development,
which includes testicular descent, spermatogenesis, enlargement of the penis and
testes, and increasing libido.
Testosterone is also involved in regulating secondary male characteristics, which
are those responsible for masculinity.
These secondary sex characteristics include male hair patterns, vocal changes, and
voice deepening, anabolic effects, which include growth spurts in puberty
(testosterone increases tissue growth at the epiphyseal plate early on and eventual
closure of plate later in puberty) and skeletal muscle growth (testosterone stimulates
protein synthesis).
Testosterone also stimulates erythropoiesis, which results in a higher haematocrit in
males versus females.
Testosterone levels tend to drop with increasing age; because of this, men tend
to experience a decrease in testicular size, a drop in libido, lower bone density,
muscle mass decline, increased fat production, and decreased erythropoiesis, which
leads to possible anaemia
