The word "testosterone" is sometimes used in everyday language to excuse or explain behavior or characteristics regarded specifically as male.
Testosterone is, indeed, the key male sex hormone - but what exactly does it do, and should men worry about their levels getting lower as they grow older?
Contents of this article:
Fast facts on testosterone
Here are some key points about testosterone. More detail and supporting information is in the main article.
- Testosterone regulates a number of processes in the male body
- Levels of testosterone tend to drop as men age
- There is little, if any, evidence to indicate that prohormone supplements have any effect on testosterone level
What is testosterone?
Testosterone is required for the development of mature sperm.
Testosterone is a hormone - a chemical messenger - that is responsible for the development of male sexual characteristics. Females also produce testosterone, but usually in smaller amounts.
Testosterone is a type of androgen produced primarily by the testes/testicles in cells called the Leydig cells.
In men, testosterone is thought to regulate a number of functions in addition to sperm production:
- Sex drive
- Bone mass
- Fat distribution
- Muscle size and strength
- Red blood cell production
How lower testosterone levels affect men as they age
The effects of gradually lowering testosterone levels as men age have received increasing attention in recent years, leading to a term known as late-onset hypogonadism.
After the age of 40, the concentration of circulating testosterone falls by about 1.6 percent every year in most men. By the age of 60, the low levels of testosterone would lead to a diagnosis of hypogonadism in younger men.
Low serum testosterone levels have even been associated with increased mortality in male veterans, and late-onset hypogonadism has become an increasingly recognized medical condition, although many of the symptoms are associated with normal aging.
The following symptoms of late-onset hypogonadism - many of which are familiar as a part of aging - are attributed to low testosterone levels:
A decrease in libido is just one of many side effects attributed to low testosterone levels.
- Decreased desire for sex (libido)
- Diminished erectile quality, particularly for nighttime erections
- Changes in mood
- Reduced intellectual and cognitive function
- Fatigue, depression, and anger
- Decrease in muscle mass and strength
- Decreased body hair
- Skin alterations
- Decreased bone mass/mineral density
- Increase in abdominal fat mass
In addition to sexual dysfunction, late-onset hypogonadism has also been associated with metabolic disease and cardiovascular disease.
The degree to which testosterone levels decline varies between men, but an increasing number of men are experiencing the effects of reduced testosterone levels as more men now live beyond the age of 60.
One study investigated the effects of testosterone supplementation (80 milligrams a day) in older men who had low to normal testosterone levels. Specifically, they looked at testosterone's potential effects on:
- Functional mobility
- Bone mineral density
- Body composition
- Quality of life
They also assessed the safety of testosterone supplementation, finding no adverse effects of twice-daily doses. The researchers observed no beneficial effects of supplementation on functional mobility, bone mineral density, or cognitive function. They did, however, observe some effects on body composition and metabolic risk factors.
Compared with placebo, testosterone resulted in:
- Lean body mass increase
- Fat mass decrease
In spite of fairly limited evidence to support its health benefits, the prescription of testosterone for older men has increased dramatically in recent years; an increase of 170 percent was seen over the 5 years to 2012.
Specialists in testosterone research and reproductive health say that until there is stronger evidence of its benefits and safety, testosterone treatment in elderly men should be restricted to those with clinical symptoms of demonstrably low testosterone.
One recent study of testosterone found that its effects varied widely between different men.
The 2013 study found that lean mass, muscle size, and strength were regulated by male hormones while fat accumulation was primarily a consequence of estrogen deficiency. Meanwhile, sexual function was regulated by both androgens and estrogens.
In summary, additional research into testosterone replacement is needed for physicians to be better able to understand its potential risks and benefits and which individuals may benefit most.
Testosterone and obesity
Testosterone has been implicated in obesity and metabolic risk factors - declining levels of the hormone are typically accompanied by an increase in body mass index (BMI) and waist circumference. Researchers have also found links between low testosterone, cardiovascular risk, and insulin resistance.
Testosterone has, therefore, been suggested as a treatment option for men who are obese. However, obesity itself can affect testosterone levels, meaning that separating out cause and effect is practically impossible; declining testosterone may be just a part of the complex downward cycle.
Researchers suggest that lifestyle and dietary modifications are more appropriate than testosterone supplementation for people who are obese and who want to reduce body weight. Such modifications, in addition to standard interventions for metabolic disorders, can normalize moderately reduced testosterone levels in men.
Testosterone and smoking
Despite higher levels of testosterone, smokers also have compromised sperm production.
Cigarette smoking has been linked to elevated levels of estrone and estradiol, as well as elevated levels of testosterone in men - higher levels of testosterone in smokers does not appear to have beneficial effects on reproductive capacity as smokers also have compromised sperm production.
A study looking at young Danish military recruits (aged 18-28) found that those who regularly smoked marijuana (more than once a week) had higher levels of testosterone (similar to the elevated testosterone seen in cigarette smokers).
However, regular marijuana smokers also had 28 percent lower sperm concentrations, and 29 percent lower total sperm count, while those who regularly combined marijuana with other recreational drugs had a 52 percent lower sperm concentration and 55 percent lower total sperm count.
Prohormone supplements, testosterone, and heart disease
A number of prohormone supplements are marketed at men to treat low testosterone levels. Such supplements can include dehydroepiandrosterone (DHEA), and the herbal product Tribulus terrestris.
There is little, if any, research to indicate that these prohormone supplements have any effect on testosterone levels, and such remedies may pose a risk to health.
One study found that men with low testosterone had significantly lower levels of magnesium, iron, and zinc, while two toxic metals - cadmium and lead - appeared to dramatically reduce the level of bioavailable testosterone.
Increasing life expectancy means that the number of older men has increased in recent years in the United States. This greater population of older men, and an increased focus on declining testosterone levels has led to a sharp increase in testosterone prescriptions, despite there being no clear understanding of the safety of testosterone replacement.
Given that cardiovascular disease increases in older men, while testosterone declines, some physicians have suggested that testosterone supplements may help mitigate the risk of cardiovascular disease (CVD). However, studies have found conflicting evidence about the benefits and risks of testosterone treatment in older men, with some studies finding an increased risk of CVD in some men taking testosterone.
Studies have found conflicting evidence on how testosterone treatment affects the risk of CVD.
A recent study does suggest that there is a benefit for men whose testosterone levels normalize with testosterone treatment.
This latest study looked at national data on 83,010 men over 50 who had low testosterone and who received care from the Veteran's Administration between 1999 and 2014.
Of these men, 43,931 had treatment that led to a normalization of their testosterone levels, a further 25,701 were treated but their testosterone levels did not normalize, and 13,378 were untreated and their testosterone levels remained low.
After matching these men for age, BMI, various chronic diseases, LDL-cholesterol levels, and the use of aspirin, beta-blockers, and statins, the researchers looked at the incidence of heart attack, stroke, and death in the men.
They found that those who were treated and had normalized testosterone were 56 percent less likely to die compared with the untreated men in the 4.6 to 6.2 years of follow-up. The treated men were also 24 percent less likely to suffer a heart attack, and 36 percent less likely to have a stroke.
The men who were treated and whose levels normalized were also 37 percent less likely to die than the men who were treated but whose levels did not normalize. The former group also had an 18 percent lower likelihood of heart attack, and were 30 percent less likely to have a stroke. Even the men who were treated and whose levels did not normalize were 16 percent less likely to die than the untreated men.
More randomized controlled studies on the effects of testosterone are necessary before significant conclusions can be drawn. This kind of research is also important given that testosterone is increasingly used by transmen in their transition, to enable the development of secondary male sexual characteristics.