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? Use this page to learn all about this major androgen.
What is testosterone?
Testosterone is a hormone, i.e. a chemical messenger, that is responsible for the development of male sexual characteristics, although females also produce testosterone, albeit usually in smaller amounts. 1
A type of androgen, testosterone is produced mostly by the testes/testicles in cells called the Leydig cells.2-4
Without adequate testosterone, men are infertile. This is because the process of spermatogenesis (development of mature sperm) requires testosterone to develop germ cells beyond the stage of meiosis (reproductive cell division).4
In men, testosterone is thought to regulate a number of functions in addition to sperm production:1
Most testosterone is produced in the testes.
- 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 grow older 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% every year in most men, decreasing to a level in many men aged over 60 years that would lead to a diagnosis of hypogonadism in younger men.5,6
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.5,7
Men's sexual performance can be affected by erection and ejaculation problems, collectively known as male sexual dysfunction.
The following symptoms of late-onset hypogonadism - many of which are familiar as a part of aging - are attributed to low testosterone levels:5,7,8
- 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.5
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 60.7
Some research has looked into whether testosterone supplementation can benefit older men. One study was randomized and placebo-controlled, with neither investigators nor subjects knowing who was receiving the testosterone for 6 months (known as a double-blind study).7
This study investigated the effects of testosterone supplementation (80 mg a day) in older men who had low to normal levels. Specifically, they looked at testosterone's potential effects on:
- Functional mobility
- Bone mineral density
- Body composition
- Quality of life
- Treatment safety.
They also assessed the safety of testosterone supplementation, finding no adverse effects of twice-daily testosterone. They also 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 in the men taking testosterone compared to placebo, including:
Compared with placebo, testosterone resulted in:
- Lean body mass increase
- Fat mass decrease.
Can men get thrush?
Thrush is more common in women, but men can also be affected by the genital fungal infection.
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% was seen over the 5 years to 2012.9
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.9
One of the most recent studies of testosterone found that its effects vary widely between different men in terms of the relative doses and serum levels at which body composition, strength and sexual function start to decline.8
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 benefits, who might benefit most, and whether there are negative effects of testosterone therapy.
Testosterone and obesity
Average penis size
Men worry more than their female partners about penis size...
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 and waist circumference. Researchers have also found links between low testosterone, cardiovascular risk and insulin resistance.10
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 and declining testosterone may be part of a complex downward cycle.10
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.10-12 Such modifications, in addition to standard interventions for metabolic disorders, can normalize moderately reduced testosterone levels in men.10
Testosterone and smoking
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 do not appear to have beneficial effects on reproductive capacity as smokers also have compromised sperm production.14,15
A recent 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% lower sperm concentrations, and 29% lower total sperm count, while those who regularly combined marijuana with other recreational drugs had a 52% lower sperm concentration and 55% lower total sperm count.13
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 also found that men with low testosterone had significantly lower levels of magnesium and iron, as well as low levels of zinc, while two toxic metals, cadmium and lead, appeared to dramatically reduce the level of bioavailable testosterone.16
Increasing life expectancy means that the number of older men has increased in recent years in the US. 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, a number of 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.17
The latest evidence now does suggest that there is a benefit for men whose testosterone levels normalize with testosterone treatment.18 This latest study looked at national data on 83,010 men over 50 years old 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, body mass index, various chronic diseases, LDL-cholesterol levels, and the use of aspirin, beta-blockers, or statins, the researchers looked at the incidence of heart attack, stroke and death in the men. What they found was that the men who were treated and had normalized testosterone were 56% less likely to die compared to the untreated men in the 4.6 to 6.2 years of follow-up. The treated men were also 24% less likely to suffer a heart attack, and 36% less likely to have a stroke.
The men who were treated and whose levels normalized were also 37% less likely to die than the men who were treated but whose levels did not normalize. The former group also had an 18% lower likelihood of heart attack, and were 30% less likely to have a stroke. Even the men who were treated and whose levels did not normalize were 16% less likely to die than the untreated men.
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.
Recent developments on testosterone
Low testosterone in men linked to CVD risk. This September 2013 paper in the Journal of Clinical Endocrinology and Metabolism concluded there was a "growing body of evidence" suggesting "a modest connection" between testosterone and cardiovascular disease.
Parkinson's in men may be linked to testosterone decline. Researchers publishing in The Journal of Biological Chemistry in July 2013 argued the case for preserving testosterone levels in men.
Testosterone makes men more honest. This study showed that men with higher testosterone levels told lies less often. The result - published in PLoS ONE in October 2012 - "clearly contradicts the one-dimensional approach that testosterone results in anti-social behavior."
Written by Markus MacGill
Reviewed by Dr Helen Webberley MBChB MRCGP MFSRH