What is testosterone? The ultimate guide

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.

Puberty

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 testosterone levels aren’t the same for every man; what’s “normal” on paper might not feel normal for you. While the body needs a certain amount of testosterone to function properly, even a small drop from your personal baseline can have a noticeable effect on energy, mood, and overall quality of life. Testosterone peaks in your late teens, explaining those unpredictable erections on the bus or in class, and stays high through your twenties.

From around 40, it naturally begins to fall by about 1–2% a year, so by 70, most men produce around 30% less than they did at their peak.Guidelines can help frame what’s typical (stating that testosterone levels over 12nmol/L as being normal, and anything under 8nmol/L requiring treatment), but they don’t tell the whole story.

Someone with a testosterone level that sits comfortably in the “normal” range might still feel the effects of low testosterone if their usual baseline was higher. This is what we call the grey zone, where numbers alone don’t capture how you actually feel.That’s why testosterone management is never just about blood results. It requires a conversation with a clinician who can interpret your results alongside your symptoms, history, and lifestyle to understand what’s really going on, and whether treatment is right for you.

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 

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.