Physiology of Testosterone

Posted by Dr. Michael White, Updated on April 23rd, 2024
Reading Time: 4 minutes

What is Testosterone?

Testosterone is one of the many chemical messengers known as Hormones, and it is essential for the usual health and function of both men and women. However, the hormone is primarily associated with male wellness and development.

Testosterone belongs to a specific class of hormones known as steroids. Most people think of steroids as hormones or chemicals that boost anabolic, muscle-building performance. Still, a steroid represents a molecule with a specific structure that is most commonly used to deliver messages and exert physiological changes in the body.

Three common steroids are cholesterol, testosterone, and estrogen. In fact, the human body builds both Testosterone and Estrogen from a core of cholesterol, which is one reason why cholesterol is such an essential part of daily nutritional intake.

Testosterone and Prenatal Development

Testosterone is responsible for the development of primary male sex characteristics while the child is still in the womb. In the normal developmental cycle, Testosterone begins to affect the male fetus around seven weeks after conception. It encourages the formation of Leydig Cells, which are responsible for the production of Testosterone.

After the fetus develops its own ability to produce Testosterone, the internal male sexual organs begin to grow, and at around eight weeks, external sexual organs begin to develop fully.

Testosterone and Puberty

After primary sex characteristics are established, the influence of Testosterone diminishes almost completely until puberty, where Testosterone triggers the development of secondary sex characteristics.

Testosterone also triggers muscle development and growth. Human Growth Hormone is also responsible for growth, and the combination of Testosterone and Human Growth Hormone during puberty is why men grow taller than women.

When puberty begins, the hypothalamus starts to release the ultimate precursor hormone of Testosterone: Gonadotropin-Releasing Hormone, or GnRH. GnRH travels from the hypothalamus to the pituitary gland where the production of Follicle-Stimulating Hormone and Luteinizing Hormone begin, which are responsible for the production of sperm and testosterone, respectively.

Testosterone Levels in the bloodstream are mediated by a negative feedback loop. As serum Testosterone Levels increase, a small amount of Testosterone is converted into Estrogen, primarily by body fat, which then circulates to the hypothalamus and suppresses the production of Testosterone precursors.

Anabolic Actions of Testosterone

Testosterone is a hormone that encourages growth and development. During puberty, the male physiologically changes significantly. He begins to put on muscle mass, and this triggers a reduction in body fat. Testosterone amplifies the growth spurt induced by Human Growth Hormone, and also eventually leads to the closure of the epiphysial plates, which concludes the growth spurt.

The vocal cords get thicker, and the larynx grows, which leads to the deepening of the voice associated with male puberty. Finally, Testosterone encourages the full development and functionality of the male sex organs, and the testicles start to produce sperm.

The Influence of Testosterone During Adulthood

Beyond puberty, Testosterone Levels normalize, and no longer have a significant impact on the structure of the male body, except when Testosterone Levels become deficient.

Testosterone ultimately becomes an agent of homeostasis, helping the male body retain its health and potency. Testosterone contributes heavily to the ability to form an erection and produce sperm, and it also sustains muscle mass and body fat distribution. Testosterone also appears to have protective effects upon many systems in the body.

For example, there is convincing evidence that men that don't produce enough Testosterone are more likely to suffer from depression, anxiety, and even cognitive decline and Alzheimer's disease.

How Is Testosterone Found in the Body?

Natural Testosterone exists in the bloodstream in three different forms. Some Testosterone circulates through the body without being attached to any other molecule—This Testosterone is referred to as Unbound Testosterone.

Testosterone is also found flowing through the bloodstream attached to Sex hormone-binding Globulin, which stores Testosterone and cannot be used by the body while bound. A third form of Testosterone in the bloodstream is attached to Albumin. Unlike with SHBG, Albumin has a weak binding with Testosterone, and this Testosterone can still be made available to the body.

Most circulating Testosterone is attached to SHBG, meaning that most Testosterone in the bloodstream cannot be actively used by the body. If a person produces too much SHBG, this reduces the effectiveness of Testosterone released by the Leydig Cells and contributes to Testosterone Deficiency.

There are many factors that can lead to increased SHBG, including overactive thyroid, abnormally high estrogen levels, and natural aging.

Testosterone that travels through the bloodstream can be used by the body to make other vital hormones. For example, 5-alpha reductase is an enzyme that converts Testosterone into Dihydrotestosterone, also known as DHT.

Elevated DHT levels are associated with an enlarged prostate because DHT stimulates the reproduction of prostate cells. Testosterone also encourages the production of Estrogen via adipose fat tissue, and elevated Estrogen levels are the primary mechanism by which Testosterone production is controlled via the hypothalamus.

How Do Testosterone Levels Change as a Function of Age?

Testosterone Levels are vitally crucial to male (and to a lesser extent, female) homeostasis. Testosterone Production remains steady through the late teens and the twenties and starts to enter a steady decline starting at age thirty. This decline actually begins around thirty for both sexes.

Testosterone Secretion slows gradually, at a rate of about one percent annually. After many years, however, this decline can begin to produce symptoms associated with hormone imbalance, such as weight gain, fatigue, loss of muscle mass, depression, anxiety, cognitive decline, loss of bone mineral density, erectile dysfunction, and loss of libido.

Age-related Testosterone Decline is often referred to as Andropause because it manifests around the same age as Menopause in many men.

On the other hand, Testosterone is a slow drop in Testosterone Production, and Testosterone Production does not cease wholly and suddenly, as is the case with Estrogen and Menopause. Testosterone Deficiency can inhibit fertility when severe, but most men retain some fertility even in the case of significant deficiency.

For men with Testosterone Deficiency, Testosterone Therapy with Testosterone Injections, Testosterone Patches, or Testosterone Creams can help restore Hormonal Homeostasis and counteract many of the symptoms of abnormally low Testosterone Levels.

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