Summary: Low testosterone, also called testosterone deficiency or hypogonadism, occurs when the body produces insufficient amounts of the hormone to support normal physical, sexual, and psychological health. A 2024 UK survey found that 49% of men showed a high likelihood of testosterone deficiency based on symptom scoring, yet fewer than 5% had a formal diagnosis. A morning blood test, interpreted alongside symptoms, is the starting point for any accurate assessment.
What Is Low Testosterone?
Testosterone is the primary male sex hormone, regulating energy, libido, mood, muscle mass, bone density, and red blood cell production. From around age 30, levels decline at approximately 1% per year. Not every man will develop symptoms, but for a significant proportion the drop becomes clinically relevant. The medical term is hypogonadism, or testosterone deficiency syndrome (TDS).
In the UK, the British Society for Sexual Medicine (BSSM) defines low testosterone as a total testosterone level below 8 nmol/L, with a grey zone between 8 and 12 nmol/L where free testosterone and symptoms together guide clinical decisions. A 2024 cross-sectional survey published in BMJ Open, conducted with Imperial College London, found that 49% of 973 respondents showed a high likelihood of testosterone deficiency based on symptom scoring, yet only 5% had ever received a formal diagnosis.
Symptoms of Low Testosterone in Men
Symptoms are non-specific and develop gradually, which is why they are so often attributed to stress, poor sleep, or ageing. Two are worth flagging specifically. Mood changes, including irritability and emotional flatness, can resemble depression closely enough that some men receive antidepressants before any hormonal assessment, and certain antidepressants can in turn suppress testosterone production. Cognitive symptoms, including brain fog and reduced mental sharpness, are among the least recognised indicators. The 2024 Imperial College survey found that 31% of respondents reported low libido and 27% reported reduced erectile function.
| Symptom Category | Common Symptoms | Often Mistaken For |
| Physical | Fatigue, reduced muscle mass, increased body fat, reduced bone density, hot flushes | Overwork, ageing, poor diet |
| Sexual | Low libido, erectile difficulties, reduced semen volume | Relationship problems, stress |
| Psychological | Low mood, irritability, poor concentration, brain fog | Depression, anxiety, burnout |
| Metabolic | Weight gain (especially around the abdomen), increased cholesterol | Diet, sedentary lifestyle |
What Causes Low Testosterone?
Common contributing factors include:
- Age, the most prevalent factor in men over 40
- Obesity, which is strongly associated with testosterone deficiency, with research consistently showing significantly higher rates of low testosterone in overweight men compared with those at a healthy weight
- Type 2 diabetes: NHS data indicates around 16% of male patients with type 2 diabetes have lower-than-normal testosterone levels
- Chronic poor sleep and elevated stress, both of which suppress testosterone production via cortisol
- Certain medications, including opioid-based pain relief and some antidepressants
- Testicular injury, infection (orchitis), or previous cancer treatment
- Rare genetic conditions, including Klinefelter syndrome
How Is Low Testosterone Tested?
Testosterone peaks in the morning, so the BSSM recommends blood samples before 11am. A single low result is not conclusive. Most guidelines require two separate morning tests before a diagnosis is confirmed. At the 108 Harley Street Men’s Health Clinic, a comprehensive assessment typically includes the following panel.
| Test | What It Tells Your Doctor |
| Total testosterone (morning blood test) | Overall hormone level; the starting point for any diagnosis |
| Free testosterone | The active portion of testosterone available to the body; important when total T is borderline |
| LH (luteinising hormone) | Whether the problem originates in the testes or the brain |
| FSH (follicle-stimulating hormone) | Assesses testicular function and fertility |
| SHBG (sex hormone binding globulin) | Affects how much testosterone is freely available |
| Full blood count and metabolic panel | Rules out anaemia, diabetes, thyroid issues, and liver or kidney problems |
A total testosterone reading in the grey zone of 8 to 12 nmol/L does not automatically indicate deficiency. Free testosterone, symptoms, and wider health context all inform the picture, which is why specialist review adds value that an at-home test alone cannot.
| Reference Ranges (UK BSSM Guidelines) |
| Below 8 nmol/L: Consistent with testosterone deficiency. Treatment is generally indicated when accompanied by symptoms. |
| 8 to 12 nmol/L: Grey zone. Free testosterone and clinical symptoms guide the decision. |
| Above 12 nmol/L: Generally within normal range, though free testosterone and symptoms should still be considered. |
| Target on treatment: 15 to 30 nmol/L for symptom optimisation. |
When Should You See a Specialist?
Consider a consultation if three or more of the following represent a change from your baseline:
- Persistent fatigue not explained by sleep or workload
- Noticeably reduced sex drive
- Low mood, irritability, or emotional flatness that is new for you
- Difficulty building or maintaining muscle despite regular exercise
- Increased abdominal fat without a clear dietary cause
- Reduced morning erections
- Brain fog or a noticeable decline in mental sharpness
A review is also warranted if you have risk factors including obesity, type 2 diabetes, a history of testicular problems, or are taking medications known to affect hormone levels.
NHS vs Private: Why the Pathway Matters
Testosterone replacement therapy for men is classified in the UK as an amber-list drug, meaning it must be initiated by a specialist in a secondary or tertiary care setting, unlike female HRT which GPs can prescribe directly. In practice, this creates delays at GP level. Private assessment at 108 Harley Street offers same-day or next-day access to a specialist, a full blood panel, and a clear clinical plan.
Treatment
If testing confirms deficiency, TRT is the primary treatment, available as topical gels, injections, or patches. The decision to treat should always be made jointly, with a clear discussion of benefits and risks, and followed by regular monitoring of blood levels, haematocrit, and cardiovascular markers. Weight management, sleep quality, and regular resistance exercise also have a meaningful effect on testosterone levels and should be addressed alongside any medical treatment.
Conclusion
If you recognise several of the symptoms above, a morning blood test reviewed by a specialist is the most reliable next step. At 108 Harley Street, no referral is required and appointments are typically available within 24 hours.
