The normal gonadal function guarantees the execution of the reproductive and sexual mission of man. Alterations at this level lead to dysfunction of various organs such as the brain, muscle and sexual area. The functioning of the hypothalamic-pituitary-gonadal axis depends on the secretion of several hormones. Gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus stimulates the production of pituitary gonadotropins (follicle stimulating hormone, FSH) and luteinizing hormone (LH). FSH regulates spermatogenesis in the seminiferous tubules of the testis and LH controls the secretion of testosterone by Leydig cells. In turn, the control of GnRH, FSH and LH depends on the production of inhibin by Sertoli cells and testosterone by Leydig cells. Inhibin inhibits the secretion of FSH and testosterone controls the production of LH.
Androgens act in the differentiation of external genitalia, sexual desire, the growth of axillary and pubic hair, development of muscle mass, and skin texture, the tone of voice, bone formation and pubertal development.
Hypogonadism or decreased gonadal function represents an alteration of the hypothalamic-pituitary gonadal axis.
What Causes Low Testosterone Levels In Males?
Low testosterone levels in males or male hypogonadism occurs when the testicles stop producing sperm, testosterone or both. There are three basic mechanisms:
- An intrinsic testicular damage (primary hypogonadism) can cause low testosterone levels in males.
- An abnormality of the hypothalamic-pituitary axis (secondary hypogonadism) can cause Low testosterone levels in males.
- A diminished or absent response from the target organs (skin, hair and prostate) to androgens (androgenic resistance) can cause low testosterone levels in the males.
In the first two cases, hormonal production is decreased but the hormonal response is normal; in the third case the production is normal but the answer is diminished. Hormonal deficiency can occur before birth, before puberty or after puberty. The absence of testosterone in the fetus leads to the presence of ambiguous genitalia at birth. Prepubertal testicular failure is characterized by decreased testicle size, small penis, decreased body hair, female voice, small prostate, decreased libido (sexual desire) and muscle mass.
In some cases, there is a eunuchoid habitus (distance from the pubis to the floor is greater than from the cranium to the pubis and the distance between the tip of the middle fingers with the extended arms is greater than the height. If it occurs after puberty, the skeletal proportions and the size of the penis are normal, the testicles are not as small as in the prepubertal failure but they are soft and have a volume of fewer than 15 ml. The body hair disappears almost completely and the decrease of the libido (sexual desire) is notorious. Over the years, osteoporosis occurs if the patient is not treated.
It is useful to separate primary hypogonadism (testicular failure) from secondary hypogonadism (problems of the hypothalamic-pituitary axis). The term hypergonadotropic applies to the first and hypogonadotropic to the second.
The diagnosis of hypogonadism begins with a meticulous clinical history and the search for relevant findings during the physical examination. The laboratory only verifies the clinical suspicion. A total or free serum testosterone below the normal limits with increased FSH and LH confirms the presence of a primary hypogonadism. There is controversy about the utility of total versus free testosterone, since in some cases a decrease in total testosterone can be found with normal free testosterone. This is due to a decrease in the carrier protein of gonadal hormones (SHBG). However, the free testosterone is harder to measure and the test is more expensive. That is why doctors initially order a total testosterone and reserve free testosterone for doubtful cases. In cases of secondary hypogonadism, serum testosterone is barely diminished or is at a low normal limit. The FSH and LH may also be low or in the normal low limit. There are several stimulation tests that could be used in special cases but are not of routine clinical use.
Male hypogonadism is common and is neither diagnosed nor treated frequently. The concentration of plasma testosterone also decreases from 0.5% to 2.0% per year after 50 years. Overweight and diabetes contribute to the gonadal alterations of men.
The management of men with hypogonadism has two objectives: achieve fertility and improve sexual characteristics.
- Mayo Clinic – Male Hypogonadism: https://www.mayoclinic.org/diseases-conditions/male-hypogonadism/symptoms-causes/syc-20354881
- WebMD – Male Hypogonadism: Causes and Treatment: https://www.webmd.com/men/what-is-hypogonadism
- American Urological Association – Evaluation and Treatment of Testosterone Deficiency: https://www.auanet.org/guidelines/testosterone-deficiency-guideline
- National Institute on Aging – Testosterone and Aging: Clinical Research Directions: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2691482/
- Urology Care Foundation – Low Testosterone: https://www.urologyhealth.org/urologic-conditions/low-testosterone
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