TESTOSTERONE ∙ 3 minute read

What is testosterone? The ultimate guide 

By Joe Young | Medically reviewed by Dr Luke Pratsides

Testosterone is the main male sex hormone. Yet there’s a lot more to it than just body hair, deep voices, and manliness.

But first things first...

What is testosterone?

Testosterone isn’t your average hormone - it’s integral for developing male characteristics. 

And it starts from the onset of conception, shaping a male fetus and moulding the genitals from as early as 7 weeks. It’s mainly produced in the testicles, with minor additions from the adrenal gland. 

But it’s not just for men. Women also produce testosterone in their ovaries and adrenal glands, though in lesser quantities. It’s vital for their fertility and bone density.

The role of testosterone in men

Puberty, fertility, and muscle growth are all dependent on testosterone. Let’s take a closer look at its main claims to fame.


From the age of about 9, testosterone powers through the body, triggering changes associated with male adolescence (voice breaking, facial and body hair growth, and genital development). A low amount of testosterone at this stage can hinder the development of these features.

Sex and fertility

Testosterone gives you a steady supply of sperm in the testicles. It also regulates your sex drive so your body can perform at its best when the time comes. When you have low testosterone, it’s harder to have healthy erections.

Muscles, bones, and fat

When you hit the gym, testosterone goes into overdrive. It enhances muscle growth and protein synthesis. This is why some athletes abuse testosterone to enhance athletic performance. 

It also plays a significant role in red blood cell production. This helps to maintain healthy bone density, whilst regulating fat distribution and enhancing fat burn during physical activity. 

What are ideal testosterone levels for men?

Optimal levels of testosterone are different for each man but to ensure healthy bodily functions, you must have a certain level of the hormone. Significantly low levels can have a profound impact on your quality of life.

In your late teens, testosterone surges to its highest levels. All those seemingly random erections in the classroom or on the school bus suddenly make a lot of sense. And it stays high throughout your twenties and beyond. 

But here's the catch: testosterone levels decrease by 1–2% each year, starting at around 40.  By the time the average man hits 70, testosterone production has plunged by 30% from its peak. 

For an average man, total testosterone levels of less than 8 nmol/l highly support a diagnosis of low testosterone whereas levels greater than 12 nmol/l are likely to be normal. 

The grey zone between 8 and 12 nmol/l requires further evaluation, considering the impact any symptoms are having on an individual’s quality of life. A trial period of testosterone treatment may be beneficial.

There are several issues that can result from high or low testosterone levels:

High testosterone levels

Whilst you’ve heard of low testosterone, it’s also possible to have high testosterone. Although it’s much rarer - it’s usually found in those using anabolic steroids but can be a side effect of testosterone replacement therapy (TRT) or adrenal or testicular tumours.

Symptoms include: 

  • Low sperm count
  • Testicle shrinkage
  • Erectile dysfunction
  • High blood pressure
  • Increased blood clot risk
  • Weight gain
  • Fluid retention
  • Mood swings

Low testosterone levels

Low testosterone can be caused by hormonal disorders, underlying health conditions, or ageing. It can lead to health problems such as:

  • Loss of sexual desire
  • Erectile dysfunction
  • Low energy
  • Fatigue
  • Decreased muscle mass and bone density
  • Depression
  • Irritability
  • Loss of body and facial hair
  • Insomnia 

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The numan take

Testosterone isn't just for pumping your guns in the gym or boosting bedroom performance. It's a vital hormone which keeps your body in check.

Don't let low testosterone hold you back. Explore potential treatments and regain control of your health.