If your energy has tanked, your erections aren’t what they used to be, or you’re gaining belly fat no matter how hard you train, low testosterone (which affects millions of men) could be the missing link.
Testosterone fuels strength, libido and confidence, so when levels dip too low, the impact shows up everywhere: in the bedroom, in the gym, and even in your mood.
Here’s what low testosterone is, the top signs to watch for, and how to tackle it.
What is testosterone?
Testosterone is the primary male sex hormone. Men produce most of it in the testicles, with smaller amounts from the adrenal glands above your kidneys. A control circuit in your brain (the hypothalamus and pituitary gland) keeps levels steady.
In many parts of the body, testosterone also converts into dihydrotestosterone (DHT) or oestradiol (the most active form of the main female sex-hormone, oestrogen)l, which can explain its wide-ranging effects on everything from sexual function to mood, bone strength and hair growth.
Note: Women make much smaller amounts of testosterone in the ovaries and adrenals, but this page focuses on men.
What testosterone does for men?
When levels are healthy, testosterone powers your day-to-day performance, impacting many different body parts:
- Brain: boosts sex drive, motivation, clear thinking, memory and stable mood.
- Sexual and reproductive: supports sex organ growth, sperm production, erections, and overall sexual function.
- Heart and circulation: helps your heart pump efficiently and supports healthy blood flow.
- Kidneys and blood: boosts red blood cell production, improving stamina.
- Skin: supports collagen, hair growth and natural skin oils.
- Muscle and bone: builds strength, preserves muscle and maintains bone density.
What are the symptoms of low testosterone?
So how do you know if your levels are low? Testosterone touches nearly every part of male health, so the symptoms can be sexual, physical and psychological. Some are obvious, others are easy to miss. If you’ve noticed any of these, it’s worth exploring whether low T could be the cause.
*These ranges come from different large studies in men seen in GP clinics and specialist centres. Exact numbers vary depending on how “low T” is defined and whether doctors only count men who have both low test results and symptoms. Sexual symptoms, like low sex drive, fewer morning erections, or erectile problems, are generally the clearest warning signs for low testosterone.
Sources: BSSM guidance, HIM primary-care cohort, community ADAM screening study, and a JAMA Network Open testing cohort.
Low sex drive and erection changes
You think about sex less, morning and night-time erections are fewer or weaker, and it can be harder to get or keep an erection.
Fatigue and low energy
Men with low testosterone often feel constantly drained. This isn’t just “being tired”; it’s a persistent lack of energy that doesn’t improve with sleep or a rest day.
Loss of muscle and strength
Testosterone supports muscle building and recovery. When it dips, training results stall, lifts feel heavier, and many men may notice they’re losing size or strength even if they still train.
Increased body fat and weight gain
Fat, especially around the waist, creeps up more easily. Extra fat then pushes testosterone lower, which is why many men feel stuck in a loop until the hormone issue is addressed.
Reduced hair growth
Beard or body hair can thin with low T. Genetics still matter, so lack of chest hair on its own is not a reliable marker.
Other common symptoms of low testosterone in men include:
- Fertility issues due to reduced sperm count
- Shrinking testicles
- Gynaecomastia (“man boobs”)
- Hot flushes or sweating
- Low mood, irritability, or depression
- Difficulty concentrating (“brain fog”)
- Poor sleep quality
- Reduced bone density, increasing fracture risk
These problems have many possible causes. Low testosterone is one potential driver. The only way to know is proper testing.
At MANUAL, getting tested for low T is simple. Order a quick, at-home finger-prick blood test. Our clinicians will then confirm if your testosterone is low and whether TRT is right for you - without GP delays or waiting rooms.
What counts as “low testosterone” in men?
Spotting the signs is one thing, but confirming the diagnosis takes proper testing. A “low testosterone” diagnosis requires two early-morning blood tests on different days because testosterone peaks in the morning.
Clinicians first measure total testosterone. If that result is borderline, free testosterone and related hormones are checked to understand the cause.
The numbers your clinician uses
- If you have symptoms and your total testosterone is below about 12 nmol/L, or your free testosterone is below 0.225 nmol/L (225 pmol/L) on two morning tests, you usually meet criteria for treatment under UK specialist guidance.
- If your total testosterone sits in the “grey zone” (8–14 nmol/L), or if you have other risk factors such as type 2 diabetes, your clinician will check free testosterone and review your symptoms. If everything points to low T, they may suggest a monitored 6 month trial of treatment.
- Evidence also suggests that, in rare cases, patients with free testosterone levels up to 0.35nmol/L may still be considered for treatments, although this decision is made on an individual basis.
- Once on treatment, UK specialist guidance aims for a total testosterone (TT) of 15–30 nmol/L to optimise symptom relief
How common is low testosterone and who does it affect?
Low testosterone impacts around one in four men, and it can occur at any age. Although your testosterone naturally drops ~1% per year after 30, gradual age-related decline isn’t the same as a true deficiency that affects energy, sex drive, muscle, mood and performance.
Some conditions or medications put you at greater risk of low T, such as:
- Getting older
- Obesity or type 2 diabetes
- Chronic illness (for example chronic kidney disease)
- Testicular injury, surgery, or infection
- Long-term opioids, some antipsychotics, or anabolic steroid use
- Post-viral problems, including after COVID-19
What are the risks of untreated low testosterone?
Leaving low testosterone unchecked doesn’t just impact your sex life. Over time, untreated testosterone deficiency is linked to:
- Reduced bone density and osteoporosis
- Higher rates of insulin resistance, metabolic syndrome and type 2 diabetes
- Adverse cardiovascular risk markers and poorer cardiometabolic outcomes
- Mood disorders and a lower quality of life
- Possible risk of dementia
Low testosterone treatment options
The good news: if your tests confirm testosterone deficiency, you don’t have to just “live with it”. Testosterone replacement therapy (TRT) is a proven treatment that helps restore healthy levels, though it’s important to be aware of potential side effects.
With the right plan, most men regain their vitality, sex drive and energy.
Get started with TRT today
If you’re wondering whether low testosterone is holding you back, now’s the time to take action. With MANUAL’s expert medical team, you can get tested, diagnosed, and treated - without GP delays or waiting rooms.
Take our short quiz to see if TRT could be the right fit for you.
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