Testosterone is the main male sex hormone. It’s primarily made in the testes and plays a key role in keeping you physically and mentally well.
So, what’s the function of testosterone and why does it matter? Here’s what to know.
What does testosterone do?
Testosterone is responsible for the development and function of your male features, from childhood through to puberty and beyond. That includes your reproductive organs growing, your voice deepening, sperm production, libido and erectile function.
But testosterone acts on loads of different systems, too. It regulates your muscle growth, bone density, metabolism and mood, and is linked to cognitive function.
Is testosterone a steroid?
Testosterone is a steroid hormone. While the word “steroid” is often linked to performance-enhancing anabolic drugs, testosterone itself is a naturally occurring hormone.
Our guide on testosterone and steroids unpacks how testosterone replacement therapy (TRT) differs from anabolic steroids, and why the two shouldn’t be confused.
How is testosterone measured?
Doctors measure your testosterone levels with a good old-fashioned blood test.
The test is done in the morning before you eat anything. Because testosterone levels can be influenced by different factors, from your diet to stress, you’ll be tested at least twice to confirm you’re actually deficient.
Many men also wonder whether habits like masturbation affect their testosterone levels, but research shows it doesn’t.
What are normal levels of testosterone for a man?
Here’s a chart showing average levels of testosterone by age.
The British Society of Sexual Medicine recommends that testosterone replacement therapy (TRT) should be offered to patients with levels below 12nmol/l total testosterone or free testosterone of 0.225nmol/l and below. But you can only usually get TRT on the NHS if your total testosterone levels are less than 8nmol/L.
Low testosterone
Up to 35% of men aged over 45 are estimated to have low T. It’s more common with age: testosterone naturally starts to decline after you turn 30.
Your provider might suggest a test if you’re having symptoms of low T. Erectile dysfunction, loss of early morning erections and low libido are the three most common. Others include:
- Fatigue
- Depression, anxiety, irritability
- Reduced strength
High testosterone
Having too much testosterone can be a problem, too. It’s not common for men to naturally have abnormally high T, though it can sometimes be caused by underlying medical issues. Or use of anabolic steroids. Symptoms include:
- Increased risk of heart problems
- Prostate enlargement and trouble peeing
- Acne
Learn more about high testosterone in men—its signs, causes and how to lower it safely.
Types of testosterone
There are two main types of testosterone to be aware of.
Free testosterone
Most testosterone in your blood is attached (or “bound”) to proteins. This T is not active: it can’t be put to use in your body. It’s kind of like a cab passing by with its light off, meaning that it’s not available for use. But it’s in reserve: later on, it could turn its light on (i.e. be released from the protein) and be used.
So, what is free testosterone? It’s T that’s not attached to anything, so it can be used right away. Here, the cab has its light on and is ready to go.
Even if your total testosterone levels are normal, you can still have testosterone deficiency if your free testosterone is low.
Total testosterone
This is the overall amount of testosterone in your blood, free and bound. It’s a good first test for low T because it gives a general picture of your hormone levels.
Which forms of testosterone are used for TRT?
Testosterone replacement therapy (TRT) is a synthetic version of T that’s used to treat testosterone deficiency.
But what types of testosterone are used for TRT? Available medicines include:
- Testosterone undecanoate (tablets or injection, such as Nebido)
- Testosterone enanthate (injection)
- Testosterone cypionate (injection)
Each medicine has its pros and cons, and your doctor will advise on what’s best for you. If you’re prescribed injections, understanding how and where to inject testosterone can make the process more comfortable and effective.
Make sure you get TRT from a registered provider (find this information on their website). Otherwise, you can’t guarantee that TRT is safe, authentic or in-date.
Why testosterone matters
Having balanced testosterone keeps you at your best, physically and mentally. And TRT can help get you there.
Take our short quiz and order an at-home, finger-prick blood test to get started. Our clinicians will then confirm whether you have low testosterone - and if TRT is right for you.
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