What is testosterone? What does testosterone do?

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. 

A type of androgen, testosterone is produced mostly by the testes/testicles in cells called the Leydig cells.

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)

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.

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:

• 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.

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 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 and declining testosterone may be part of a 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

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.

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.