Testosterone is the primary male sex hormone. It is present in males from birth but levels remain low until a boy reaches puberty.
During puberty testosterone levels increase dramatically resulting in a deepening of the voice, the development of facial hair, an increase in bone and muscle mass and the onset of sperm production.
Below, we’ll take a close look at testosterone and discuss 21 things everyone should know about this fundamental hormone.
- Testosterone is the primary male sex hormone
- Testosterone also plays an important role in female development
- Testosterone levels can be affected by a variety of factors
What Is Testosterone?
Testosterone is the primary male sex hormone. It is a natural anabolic steroid produced by both males and females, although adult males typically produce 15 to 20 times as much testosterone as adult females (1).
Testosterone plays a central role in male sexual development while simultaneously promoting secondary characteristics such as facial and body hair along with increased bone and muscle mass (2). Testosterone can also influence mood and behavior (in the sense that it regulates libido) and it plays a key role in male fertility (3).
In females, small amounts of testosterone are produced by the ovaries (4) and play a discreet but important role in breast, heart, brain, nervous system, bladder, bone and ovarian health (5).
The Difference Between Free And Total Testosterone
Researchers and medical professionals divide testosterone into two main types: free and total. But what’s the difference?
Testosterone is considered “free” when it has no chemical receptors bound to it. This allows the testosterone (6) to act as a receptor for any cell in the body. This ability to freely bond with other cells enables it to work with those cells to facilitate their functions, such as regulating metabolism, bolstering the strength of bones and building muscles. Low levels of free testosterone are considered an important indicator of hypogonadism (7).
Only about 1-2% of testosterone in the body is classified as free testosterone with the rest falling into the category of “total” testosterone. But although the amount of free testosterone is small it serves a range of vitally important functions.
Total testosterone levels include all testosterone that is bound to SHBG (sex hormone-binding globulin), albumin, or cortisol-binding globulin (8).
The bond between testosterone and the various compounds mentioned above may be strong or weak. If the bond is weak that testosterone is considered bioavailable testosterone and may or may not be used by the body to enable certain functions.
On the whole, total testosterone affects a variety of processes including the production of red blood cells, the regulation of libido and muscle development.
When checking for hypogonadism (low testosterone) doctors will look at both free and total testosterone levels since levels of one may appear normal while levels of the other may be low.
21 Interesting Things About Testosterone
Testosterone may well be the most talked-about hormone on the planet, but there are still plenty of myths, misconceptions and poorly understood facts surrounding it. Since both men and women produce and rely on this fundamental hormone here are 21 things everyone should know about testosterone.
1: Puberty in boys is driven by testosterone
When boys enter puberty testosterone production skyrockets. Their bodies start producing 30 times as much testosterone as they did during their pre-pubescent years (9). All that newly circulating testosterone has a profound effect on the body and produces an array of physical changes including:
- A deepening of the voice
- The development of facial and body hair
- The growth of both penis and testicles
- Increased bone and muscle mass
- Increased height
- The onset of sperm production
The male body undergoes its most radical transformation in the years immediately following the onset of puberty and the associated increase in testosterone production.
2: Testosterone is synthesized from cholesterol
Humanity’s recent relationship with cholesterol has been something of a roller coaster ride. First we were told it was bad, then it was good. Finally, we discovered some was good and some was bad (10). Yet in spite of all the discussion surrounding cholesterol, its central role in testosterone production has rarely been discussed.
The fact is testosterone is synthesized from cholesterol by Leydig cells (11) located in the testicles. The Leydig cells glean the cholesterol they need to produce testosterone from the blood. If there is not enough in the blood the testicles produce a small amount to compensate and then the Leydig cells work their biochemical magic to produce testosterone. The average adult male produces about 6-7mg of testosterone per day (12) in this fashion.
3: Testosterone is vital for bone health
Both men and women rely on testosterone to help build and maintain a healthy skeletal system (13). Later in life, both men and women become susceptible to osteoporosis when bones become thin and brittle. Studies indicate that, just as low estrogen levels put women at risk of osteoporosis, so too low testosterone levels may expose men to an increased risk of the same condition (14).
4: Testosterone production begins in the brain
We tend to think of testosterone production as being a continuous process. But it’s not. Production of testosterone is only initiated after the brain decides you’re running low and could use a top-up.
If low levels of testosterone are detected the hypothalamus gland deep within the brain sends a substance called gonadotropin-releasing hormone (15) to the pituitary gland in another part of the brain. The pituitary then secretes both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) which travel to the testicles and prompt them to produce testosterone.
5: Testosterone levels change throughout the day
It can be difficult to define exactly what constitutes “normal” testosterone levels because they fluctuate throughout the day. As a general rule, they tend to be at their highest in the morning and reach their lowest point about 12 hours later. Not accounting for this diurnal fluctuation (16) could skew diagnostic efforts, so doctors and researchers looking for dependable baseline data typically take testosterone measurements at the same time: early in the morning.
6: It’s rare for a man to have high testosterone
You can find all kinds of articles on the web warning of the dangers of high testosterone in men. But the fact is high testosterone is pretty rare. It almost never occurs naturally. When it is detected, either by symptoms or by testing, it is usually the result of either Testosterone Replacement Therapy (17) or men attempting to pack on muscle in the gym by taking anabolic steroids (18).
Women tend to suffer from (relatively) high testosterone levels more often than men. Some of the most common causes of high testosterone in women are congenital adrenal hyperplasia (CAH), polycystic ovary syndrome (PCOS) and thyroid problems. High testosterone in women can cause thinning hair, loss of libido, irregular periods, body hair growth, mood swings, deepening of the voice and more.
7: Testosterone was identified in 1935
In the late 19th century scientists became aware that the testicles produced what one researcher called “a rejuvenating elixir” which, when injected, created temporary feelings of invigoration.
However, the hormone wasn’t properly isolated and identified until May 1935 when a paper was published by researchers working for the Organon pharmaceutical company in the Netherlands. That paper, entitled “On Crystalline Male Hormone from Testicles”, both identified the hormone and assigned it the name we use today, “testosterone”. A copy of the original scientific paper in German can be found here.
8: Testosterone levels peak around age 19
Testosterone production swings into high gear during puberty and remains high throughout adolescence, helping to drive the enormous physical changes that occur during that stage of life.
Research indicates that testosterone levels peak toward the end of that period of maximum growth, at around 19 years of age (19). They then begin a steady decline that continues until around age 40. After that, any decline is usually slower and less obvious, but it may be accelerated as a result of injury or illness including injury to the testicles, kidney disease, alcoholism, HIV and steroid use.
9: Testosterone does not produce aggressive behavior
For some time testosterone has had its reputation dragged through the cultural mud as the cause of aggressive behavior in men. Turns out that’s probably not true. New research strongly suggests that there is no direct correlation between aggressive behavior and elevated testosterone levels (20).
That said, the same research suggests that while elevated testosterone doesn’t cause aggressive behavior the belief that it does can prompt some men to act aggressively, even if their testosterone levels are normal.
10: Testosterone replacement therapy is popular
Nearly 2.9% of men in the US over 40 are undergoing some form of Testosterone Replacement Therapy (21). That adds up to about 2.5 million men. The most common reasons men choose Testosterone Replacement Therapy are to counteract feelings of low energy, as a way to treat erectile dysfunction, and as a way to restore their libido, all of which can result from low testosterone levels.
11: Testosterone replacement therapy is not without its downside
The fact that millions of men in the US undergo Testosterone Replacement Therapy does not mean it’s a risk-free endeavor. Whether administered via patch, gel, pill or injection there is the possibility of side effects including but not necessarily limited to:
- Acne or oily skin
- Enlargement of the breasts
- Heart problems (22)
- Reduced sperm count
- Obstructive sleep apnea
- Benign prostatic hyperplasia
- Shrinking testicles
Obviously, if you decided to undergo TRT to counteract low sex drive or erectile dysfunction the last thing you want is for the therapy to backfire and cause problems with your testicles or prostate. Therefore it is highly recommended that you consult your doctor before you start taking supplemental testosterone to make sure you are a suitable candidate.
12: Obesity and low testosterone are linked
There is a clear and well-established link between obesity and low testosterone in men (23). What has never been particularly clear is whether low testosterone causes obesity, or obesity causes low testosterone.
Recent research, however, suggests both are true (24)(25). Low testosterone levels open the door for higher concentrations of adipose tissue, also known as “fat”. At the same time increasing amounts of fat are also believed to reduce testosterone levels. So it seems that no matter how you look at it, obesity and low testosterone go hand in hand.
13: Low testosterone can cause a cascade of health issues
Men whose bodies do not produce enough testosterone often experience some or all of the following conditions:
- Reduced bone mass and flexibility
- Decreased muscle mass
- Erectile dysfunction
- Reduced sex drive (libido)
- Low sperm count
- Reduced energy levels
- Enlarged breasts
Women who are not producing enough testosterone often experience:
- Loss of sex drive
- Sleep disruptions
- Hair loss
- Reduced energy
- Irregular periods
14: But there are ways to naturally boost testosterone
There are a variety of ways a person can naturally increase their testosterone levels. These include:
- Strength training
- Eating protein-rich foods
- Getting more sleep
- Taking vitamin D
- Reducing stress
- Losing weight
As mentioned earlier millions of people opt to engage in Testosterone Replacement Therapy. Most medical professionals, however, would prefer to see people try to boost their testosterone levels naturally.
15: Higher testosterone levels can reduce the risk of Type 2 diabetes
Some 35 million Americans have Type 2 diabetes and that number has been growing at an alarming rate in recent years. That’s the bad news. The good news is that a recent review of 13 cohort studies involving nearly 17,000 men (26) revealed that those who had higher levels of both free and total testosterone enjoyed a significantly reduced risk of developing Type 2 diabetes. The results also strongly suggests a link between low testosterone levels and Type 2 diabetes in men.
16: Caffeine intake may increase testosterone levels
Both human and animal studies suggest that caffeine has the ability to raise testosterone levels (27). What’s more, daily caffeine intake may boost testosterone levels even further.
Caffeine is not without its downside, however. So, before you go doubling your caffeine intake to try and boost your testosterone levels it’s a good idea to discuss the matter with your doctor.
17: Alcohol consumption decreases testosterone levels
Men who have a few drinks and then feel like they can take on the world are even more delusional than previously thought. That’s because not only will their reactions be impaired, opening them up to physical harm, but the alcohol they consumed will also likely decrease their testosterone levels thereby reducing available energy and undermining stamina. Alcohol reduces testosterone levels by negatively impacting the function of the Leydig cells in the testicles that produce testosterone (28).
18: Marijuana use may reduce testosterone levels
A study published in the prestigious New England Journal of Medicine concluded that young men who smoke marijuana on a regular basis (at least 4 times per week) experienced reduced testosterone levels compared to similar-aged men who did not smoke pot (29). In addition, some of the men in the study had low sperm counts which researchers concluded was likely linked to their marijuana use.
A more recent review of the medical literature (30) agreed that there is cause for concern related to cannabis use and male fertility. The same review concluded that marijuana use may increase the likelihood of ED and pose a risk to other aspects of urologic health.
19: Steroid use reduces testosterone levels
The primary reason men use anabolic steroids is to increase muscle mass in order to make themselves more physically appealing. Ironically, embracing anabolic steroids as a way to enhance your appeal may simultaneously reduce your ability to perform should you attract a partner.
This is because anabolic steroids suppress the release of gonadotropin from the brain which is a necessary part of the signal chain sent to the testicles prompting them to produce testosterone. When this signal chain breaks down testosterone production slows with the result being steroid-induced hypogonadism, or low testosterone. What’s more, the negative effects on testosterone production may be long-term (31).
20: High testosterone in women can produce a variety of health problems
Higher than normal levels of testosterone are slightly more common in women than in men and they can cause a rash of negative health outcomes. The most notable among these health problems is Polycystic Ovarian Syndrome or PCOS (32).
A woman with PCOS may experience infrequent periods or stop having periods altogether. This disruption in ovulation also affects a woman’s ability to conceive. PCOS has also been known to cause hair growth on the back, chest and face similar to that experienced by men. And PCOS may cause skin problems including acne, patchy skin and dandruff.
In addition to PCOS, high testosterone in women can cause insulin resistance (33), high cholesterol and high blood pressure and increase the risk of Type 2 diabetes.
21: Low testosterone may negatively affect the immune system
Researchers looking at covid-19 cases discovered that while men and women were equally likely to get the coronavirus (technically SARS-CoV-2) men were more likely to develop a serious case of covid-19 than women. Further investigation discovered that men who developed the most serious cases almost always had low testosterone levels.
Related: our testosterone boosters
Following up on this discovery an Italian team conducted a small study on 31 male covid-19 patients in intensive care units and discovered that men with low testosterone levels were more likely to die from covid-19 than men with higher testosterone levels (34). This strongly suggests low testosterone may reduce a person’s immune response leaving them vulnerable to serious illness.