What is Testosterone? What Does It Do?

What is Testosterone? What Does It Do?



Testosterone is the primary male sex hormone. It is also an androgenic/anabolic steroid. It is androgenic in that promotes the development of testes and prostate during puberty as well as secondary male sex characteristics (deepening of the voice and growth of body hair). It is anabolic in that it stimulates skeletal muscle and bone growth.

Testosterone also helps promote fat lipolysis (mobilization and breakdown), control blood sugar, regulate cholesterol and maintain a healthy immune system. It even affects key functions of the brain.
The vast majority (more than 95%) of testosterone is manufactured by the Leydig’s cells in the testes at various amounts throughout a male’s life. The other 5% is produced by the adrenal glands. Testosterone production peaks during puberty and then declines with age following puberty.
This natural decline in testosterone production leads to the prevalence of low testosterone in middle-aged and older-aged men. It is estimated that between 20-40% of older men have low testosterone and/or suffer from symptoms associated with low testosterone.

Total Testosterone vs. Free Testosterone

Testosterone can either be bound or free in the bloodstream. Most testosterone (>98%) is bound. Less than 2% of total circulating testosterone is free. When bound, testosterone is not biologically active. That is, it cannot exert its effects.

As mentioned above, the testes produce testosterone. After the testes produce it, testosterone enters and circulates through the bloodstream, so that it is available for tissue uptake. As it circulates through the bloodstream, it is typically bound to a carrier protein. The carrier protein helps it move through the bloodstream and safeguards it from being eliminated by the body too quickly. Once testosterone enters the tissue, the carrier protein dissociates. The two primary carrier proteins that transport testosterone are sex hormone binding globulin (SHBG) and albumin. Most circulating testosterone is bound to either SHBG or albumin. Typically, only about 2% exists in the free, unbound state.

When bound, testosterone is not biologically active. Therefore, only a small percentage of testosterone is active at any given time.

As an advanced note, testosterone bound to either SHBG or albumin is inactive. However, testosterone bound to albumin dissociates easily, thereby becoming readily available for tissue uptake. Consequently, testosterone bound to albumin is considered to be bioavailable but not necessarily biologically active. Testosterone bound to SHBG is neither bioavailable nor active.
Blood tests to measure testosterone levels will report a total testosterone level as well as a free testosterone level. Your free testosterone level is as important, if not more important, as the total testosterone level because free testosterone is the only biologically active form of testosterone. It is not uncommon for total testosterone levels to be normal and free testosterone levels to be low or vice versa.

Normal Testosterone Levels

The normal range for total testosterone levels in men is approximately 300 ng/dL to 1050 ng/dL. There is no absolute consensus among different medical organizations for the exact cutoff for low testosterone. In general, the cutoff ranges from high 200s to low-to-mid 300s ng/dL. This range is over a broad age range and there is no “normal” testosterone level based on age that men can look to as a reference.

Symptoms of Low Testosterone

Low testosterone may lead to many unwanted effects. Signs and symptoms of low testosterone in adult men may include:
  • Erectile dysfunction
  • Reduced sex drive
  • Reduced muscle mass
  • Decreased energy
  • Increase in body fat
  • The decrease in bone strength
  • Loss of body hair
  • Depressed mood
  • Increase in breast size
If you have any of these symptoms, do not ignore them. Talk to your doctor about these symptoms. Your doctor can perform simple blood tests to determine whether your testosterone levels are low or not. Only a blood test can definitively determine whether your testosterone levels are low or not.
It is important to note that experiencing one or more of these symptoms of low testosterone does not necessarily mean you have low testosterone. These symptoms may be related to one of other, unrelated medical issues.

If you suspect you have low testosterone, start by talking about these symptoms with your doctor. Then, ask your doctor for a simple blood test to measure your testosterone levels. If your doctor won’t perform a blood test, either get a different doctor or get some blood work done yourself. Plenty of companies now offer hormone panel testing services. While you can’t get a TRT prescription from them, you can arm yourself with the results by figuring out whether or not your levels are low.

Below is a good starter list of values to get tested:
  • Total Testosterone
  • Bioavailable testosterone (aka Free and Loosely Bound)
  • Free Testosterone
  • SHBG
  • DHT
  • Estradiol (specify “sensitive” assay for males)
  • LH
  • FSH
  • Prolactin
  • Cortisol
  • Thyroid Panel (complete)
  • PSA ( age dependent)
  • CBC
  • Comprehensive Metabolic Panel
  • Lipid Panel
  • Vitamin D

Additional Tests for Low Testosterone

In addition to measuring your total testosterone and free testosterone, measuring your LH (luteinizing hormone), FSH (follicle stimulating hormone), prolactin, and thyroid function may provide useful information for understanding the potential underlying cause of low testosterone. To understand why these values are useful, it is helpful to understand the difference between primary and secondary hypogonadism (low testosterone) and how testosterone is produced and regulated in the body.

There are two basic types of low testosterone (hypogonadism): primary and secondary hypogonadism:
  • Primary hypogonadism originates from a problem in the testes. In this case, the testes literally fail and cannot produce adequate amounts of testosterone even if feedback loop in the body that tells the testes to produce testosterone is working properly.
  • Secondary hypogonadism originates from a problem with the feedback loop, not the tests themselves. In this case, the feedback loop, the mechanism that tells the testes to produce testosterone, does not properly tell the testes to produce adequate amounts of testosterone. The testes would work fine, if not for a problem in the feedback loop.
LH and FSH are important to test for because they are the two hormones that are responsible for telling the testes to produce testosterone. When testosterone levels are low, the hypothalamus (a small gland of the brain) releases GnRH (gonadotropin releasing hormone). Then, GnRH stimulates the pituitary gland (another small gland in the brain) to release LH and FSH, which tell the testes to produce testosterone.

Ideally, if testosterone levels are low, LH an FSH levels should be high and vice versa. Measuring LH and FSH levels can help determine the cause of your low testosterone. If you have low testosterone and your LH and FSH levels are low, the feedback loop is likely not working properly. On the other hand, if you have low testosterone and your LH and FSH levels are high, the testes themselves likely may not be working properly. Normal LH levels for adult men are between 1.8 and 8.6 mIU/mL or IU/L. Normal FSH levels for adult men are between 1.5 and 12.4 mIU/mL or IU/L.

Prolactin is important to test for because high levels of prolactin suppress the secretion of GnRH and in turn LH and FSH. Thus high levels of prolactin suppress normal testosterone production. In addition to lower testosterone, elevated prolactin levels have been shown to increase your risk for breast enlargement and erectile dysfunction.

Lastly, a full thyroid panel is important because hypothyroidism (underproduction of the thyroid gland) suppresses the secretion of LH. Thus hypothyroidism, like high levels of prolactin, also suppresses normal testosterone production.

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