¹Arlt, Wiebke, et al. “Health Status of Adults with Congenital Adrenal Hyperplasia: A Cohort Study of 203 patients,” The Journal of Clinical Endocrinology and Metabolism, vol. 95, no. 11, 2010:5110-5121 https://pmc.ncbi.nlm.nih.gov/articles/PMC3066446/.
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.
Key takeaways
- Testosterone is a naturally-occurring steroid hormone.
- Having balanced testosterone keeps you at your best, physically and mentally.
- Testosterone is responsible for sperm production, libidio and erectile function.
- It also impacts your muscle growth, bone density and mood.
- Low testosterone levels can be treated with testosterone replacement therapy (TRT) medication.
- You can check your testosterone levels with an at-home blood test.
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.
Feeling drained and unfocused?
It might be low testosterone. Get answers fast with an at-home blood test.
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.
Age range
25-34
35-44
45-54
55-64
65-74
75-84
85-100
Av. total testosterone (nmol/l)
21.38
23.14
21.02
19.49
18.15
16.32
13.05
Av. free testosterone (nmol/l)
0.428
0.356
0.314
0.288
0.239
0.207
0.186
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 (this varies based on your region in the UK).
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. Not everyone who has low levels of testosterone will have symptoms but majority of men don't know what their baseline is.
Erectile dysfunction, loss of early morning erections and low libido are the three most common. Other include:
- Reduced energy
- Depression, anxiety, irritability
- Reduced strength
- Reduced enjoyment in life
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:
- Acne
- Prostate enlargement and trouble urinating
- Risk of clots
It's important to note the goal of testosterone replacement therapy is to normalise low testosterone
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 and can’t be put to use in your body. Free tesosterone measures the testosterone that isn't stuck to proteins, that's free to be used by the body.
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. Find out your level.
