The hypothalamus in the male body secretes GnRH pulsatilely to stimulate the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. LH gets to the testes through the bloodstream and binds to an LH receptor. This starts a chain of biochemical reactions that change LDL cholesterol into testosterone. The bloodstream carries the secreted testosterone from the testes to target tissues, where it produces its biological effects.

Testosterone Production, Action, and Clinical Manifestations of Testosterone Deficiency in the Male Body

Testosterone Production in the Male Body

The pituitary gland releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in response to the hypothalamus' pulsatile secretion of GnRH. LH gets to the testes through the bloodstream and binds to an LH receptor. This starts a chain of biochemical reactions that turn LDL cholesterol into testosterone. The bloodstream carries the secreted testosterone from the testes to target tissues, where it produces its biological effects.

Hypogonadism: Testosterone Deficiency

Hypogonadism is the inability of the testes to produce physiological levels of testosterone. Primary hypogonadism refers to hypogonadism resulting from abnormalities in the testes, while secondary hypogonadism refers to a defect in the hypothalamic-pituitary axis.   Dual or mixed forms can also occur. If an insufficient amount of testosterone reaches the target tissues, manifestations of testosterone deficiency may appear in the liver, muscle, and adipose tissue.

Mechanism of Testosterone Action

Effects on Liver and Muscle

Testosterone binds directly to the androgen receptor in the liver. In the liver, testosterone enhances protein synthesis. While in muscle, testosterone enhances muscle mass. Given its role in muscle, men with reduced levels of testosterone may complain of muscle weakness, lethargy, or decreased energy.

Conversion to Estradiol: Effects on Brain and Bone

In other tissues, testosterone must first undergo conversion before becoming biologically active. Estradiol binds to the estrogen receptor following the aromatization of testosterone in the brain and bone. Testosterone enhances bone development by promoting bone accretion. Men with reduced testosterone levels may develop osteoporosis. Testosterone acts in the brain to stabilize mood, enhance libido, and may even have a positive effect on cognition. As a result, men with testosterone deficiency can experience mood changes, a lack of motivation, and reduced libido.

Conversion to Dihydrotestosterone: Effects on Skin, Hair, and Sexual Organs

5-alpha-reductase changes testosterone into dihydrotestosterone (DHT) so that it can bind to androgen receptors on hair, skin, and the prostate. Testosterone supports the growth of facial, body, axillary, and pubic hair in the adult. However, in some genetically susceptible men, testosterone may also inhibit hair growth in some areas of the scalp, leading to baldness.

In the sexual organs, testosterone contributes to penile growth, spermatogenesis, and prostate growth and function. Profound hypogonadism interferes with the vascular function of the penis (corpora) and may lead to erectile dysfunction and a decrease in orgasm quality.

Other Effects of Testosterone

Other effects of testosterone include promotion of erythropoiesis and regulation of immune function.

Clinical Manifestations of Testosterone Deficiency

Men with testosterone deficiency may experience:

  • Muscle weakness, lethargy, or decreased energy
  • Mood changes, lack of motivation, and reduced libido
  • Osteoporosis
  • Erectile dysfunction and decreased orgasm quality
  • Anemia due to decreased erythropoiesis
  • Altered immune function

Summary

In summary, testosterone production plays a key role in many body tissues, and testosterone deficiency can result in a myriad of clinical manifestations.

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