What is testosterone? What does testosterone do?

The word testosterone is used in everyday language by people referring to specifically male characteristics - and not always in a positive light. It is indeed the 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 infamous androgen.

What is testosterone?

Testosterone is a hormone, a chemical messenger - it is the sex hormone that is responsible for making males male, although women also have the hormone in small amounts.

Also known as an androgen, testosterone is produced mostly by the testes/testicles. The Leydig cells, present in certain parts of the testis, produce testosterone.

Men are infertile without testosterone - during the process of spermatogenesis (development of mature sperm), germ cells are unable to progress beyond meiosis (reproductive cell division) in the absence of testosterone.

In men, testosterone is thought to regulate a number of functions in addition to sperm production:¹

Most testosterone is produced in the testicles.

• 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.

The concentration of circulating testosterone falls by about 1.6% every year with age over 40, and reaches levels in many men aged over 60 years that would represent 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.⁵

Men's health

Men's sex performance can be affected by erection and ejaculation problems, collectively known as male sexual dysfunction.

The following symptoms - many of which are familiar as a part of aging - accompany low testosterone levels to make up the syndrome of late-onset hypogonadism:

• Lower desire for sex
• Diminished erectile quality, particularly for nighttime erections
• Changes in mood
• Reduced intellectual and cognitive powers
• Fatigue, depression and anger
• Decrease in muscle mass and strength
• Less 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.

Testosterone supplements

The amount by which testosterone levels decline in aging men varies, but more men are becoming affected as greater numbers live beyond the age of 60.

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 six months.

It investigated the effects of testosterone supplementation, in older men who had low-normal levels, on:

• Functional mobility
• Cognition
• Bone mineral density
• Body composition
• Lipids
• Quality of life
• Treatment safety.

No adverse effects of twice-daily testosterone were reported in the large study, and there were effects on body composition and metabolic risk factors. No beneficial effects of the 80 mg supplementation were found, however, on functional mobility, bone mineral density, or cognitive function.

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 be affected by the gential fungal infection, too.

Testosterone prescribing in older men has increased over the five years to 2012 - by more than 170%. This is in spite of fairly limited evidence in its favor such as the above.

Specialists in the field say that until the evidence improves, testosterone treatment in elderly men should be restricted to those with clearly low testosterone levels plus clinical symptoms.

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.

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, all the research into testosterone replacement calls for more work to better understand whether it gives benefits and who is most likely to enjoy them.

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 - lowering levels of the hormone are accompanied by an increase in body mass index and waist circumference. Researchers have also found links between low testosterone, cardiovascular risk and insulin resistance.

Therefore, testosterone has been suggested as an option in the treatment of obese men. However, as well as the other way around, obesity itself can affect testosterone levels - a viscous cycle means that each negatively affects the other.

Researchers suggest that lifestyle measures are more appropriate to the treatment of obesity than testosterone supplementation. Such diet and activity measures, and standard control of metabolic disorders, can normalize men's moderately reduced testosterone levels.¹