If you're a man in your thirties or forties noticing that your energy isn't what it used to be, you've probably come across the idea that testosterone levels are falling across the population. The headlines make it sound alarming. The reality is more complex, and understanding it properly matters if you're thinking about getting tested.
Here's what the evidence actually shows, what a blood test can reveal, and where the limits of testing lie.
Testosterone Does Decline With Age
This part is straightforward. Testosterone levels naturally fall by roughly 1 to 2% per year from around age 30, a pattern first established in large longitudinal studies including the Massachusetts Male Aging Study [Feldman et al., 2002]. By age 50, most men have measurably lower testosterone than they did at 25. That's normal physiology, not pathology.
The symptoms can overlap with dozens of other conditions. Fatigue, low mood, difficulty concentrating, reduced libido, loss of muscle mass, increased body fat, and erectile dysfunction are all associated with low testosterone. But every one of those symptoms can also be caused by thyroid dysfunction, iron deficiency, depression, poor sleep, undiagnosed diabetes, or simply being chronically stressed and under-rested.
That overlap is important. Assuming these symptoms are hormonal without proper investigation is one of the most common mistakes I see.
A Population-Level Trend That Goes Beyond Ageing
What's genuinely striking is the evidence showing that testosterone levels are declining across generations, not just within individual men as they age.
Travison et al. (2007) demonstrated a substantial age-independent decline in American men over a 17-year period, meaning that a 45-year-old man in 2004 had measurably lower testosterone than a 45-year-old man in 1987, even after accounting for differences in health and body composition [Travison et al., 2007]. Lokeshwar et al. (2021) confirmed this trend in younger men using NHANES data from over 4,000 US males, finding that mean testosterone in adolescent and young adult men declined between 1999 and 2016, even after adjusting for BMI [Lokeshwar et al., 2021].
The biggest single driver identified in these studies is rising obesity. Excess body fat, particularly visceral fat around the abdomen, increases the activity of an enzyme called aromatase, which converts testosterone into oestrogen. There's a well-documented two-way relationship: low testosterone promotes fat storage, and increased fat drives testosterone further down.
But obesity doesn't explain all of it. A 2025 systematic review by Santi et al., analysing over 1.06 million subjects across 1,256 studies, found that both testosterone and luteinising hormone (LH) are declining at a population level, even after adjusting for BMI and age [Santi et al., 2025]. The fact that LH is falling alongside testosterone is significant. If the testes were simply failing, you'd expect LH to rise as the brain tries to compensate. Instead, the decline in both hormones points to something happening higher up, at the level of the hypothalamus and pituitary gland.
The researchers describe this as an ongoing "resetting" of the hypothalamic-pituitary-gonadal axis. The likely contributors include sedentary behaviour, chronic psychological stress, sleep disruption, dietary changes, and exposure to endocrine-disrupting chemicals. The evidence is still evolving, and no single factor has been proven as the dominant cause. But dismissing these as irrelevant would be a misreading of the current science.
What Do the UK Guidelines Actually Say?
The British Society for Sexual Medicine (BSSM) provides the clearest framework for diagnosing testosterone deficiency in UK clinical practice [Hackett et al., 2023].
The key thresholds are:
Total testosterone below 8 nmol/L is considered definitively low and typically warrants treatment if symptoms are present
Total testosterone between 8 and 12 nmol/L is a borderline zone that requires further investigation
Total testosterone above 12 nmol/L makes testosterone deficiency unlikely as the primary cause of symptoms
But those numbers only tell part of the story. The BSSM is explicit that for borderline results, or in men with conditions that alter sex hormone-binding globulin (SHBG) levels, calculated free testosterone must be assessed. This is critical for men who are overweight or obese, because excess body fat suppresses SHBG production. When SHBG is low, total testosterone can appear artificially reduced even though the biologically active free testosterone may be perfectly adequate.
A calculated free testosterone below 0.225 nmol/L is considered diagnostic of deficiency in these borderline presentations.
The BSSM also highlights that men with pre-diabetes or metabolic syndrome should be considered for testosterone assessment when total testosterone falls below 14 nmol/L, given the established relationship between testosterone deficiency and progression to type 2 diabetes.
What a Proper Assessment Includes
A testosterone blood test is a starting point, not a diagnosis. The BSSM recommends a full clinical assessment alongside the blood work, including:
Evaluation of symptoms using a structured clinical history
Additional hormone testing: LH, FSH, prolactin, SHBG, oestradiol, and DHEA-S
Screening for contributing factors such as obesity, type 2 diabetes, obstructive sleep apnoea, depression, and medication effects (opioids, corticosteroids, and some antidepressants can all suppress testosterone)
A fasting morning blood sample taken before 10am, repeated on a second occasion if the first result is borderline
The morning timing matters because testosterone follows a diurnal rhythm, peaking in the early hours and dropping through the day. An afternoon sample can produce a result that's 20 to 30% lower than the same man's morning level.
Understanding Your Results
If you get a testosterone result back and it falls outside the reference range, that doesn't automatically mean you have a clinical deficiency.
Laboratory reference ranges represent the middle 95% of a healthy population. A result at 9 nmol/L in an otherwise well man with no symptoms may be completely physiological for him. Equally, a man at 11 nmol/L with profound fatigue, loss of libido, and depressive symptoms deserves thorough investigation even though his number sits within the "borderline" range.
Context matters. Your result needs to be interpreted alongside your symptoms, your weight, your other blood markers (particularly SHBG and free testosterone), and your overall clinical picture. A number in isolation tells you very little.
When to See Your GP
If you're experiencing any of the following, see your GP before ordering a private blood test:
Persistent low mood, depression, or thoughts of self-harm require clinical assessment regardless of your hormonal status
Erectile dysfunction can be an early warning sign of cardiovascular disease and should always be investigated by a doctor, not just attributed to hormones
Rapid or unexplained weight gain needs metabolic and endocrine investigation
Gynaecomastia (breast tissue development) needs assessment to exclude other causes
If you're under 30 and experiencing symptoms, this may indicate a more serious underlying condition that needs specialist referral
These symptoms deserve face-to-face medical assessment. A postal blood test is not the appropriate first step when serious pathology needs to be excluded.
How Brooksby Medical Can Help
If you've already seen your GP, if your symptoms are mild and you want a broader picture, or if you're preparing for a follow-up conversation with data in hand, a structured hormone panel can be genuinely useful.
The Male Hormone Profile measures total testosterone alongside the markers that give it context: LH, FSH, SHBG, oestradiol, prolactin, and DHEA-S. This is the panel that allows for proper clinical interpretation rather than a single number in isolation.
For a wider health assessment that also covers thyroid function, metabolic markers, liver and kidney function, and inflammatory markers, the Men's Health Profile provides a thorough baseline.
Every Brooksby report is written personally by a GP who explains what your results mean together, flags anything that needs further investigation, and tells you clearly what to do next.
If your results suggest genuine testosterone deficiency, we'll recommend you discuss this with your GP or a specialist endocrinologist. Private testing gives you data. It doesn't replace the clinical relationship you need for treatment decisions.
References
Feldman HA, Longcope C, Derby CA, et al. Age trends in the level of serum testosterone and other hormones in middle-aged men. J Clin Endocrinol Metab. 2002;87(2):589-598. doi:10.1210/jcem.87.2.8201
Travison TG, Araujo AB, O'Donnell AB, Kupelian V, McKinlay JB. A population-level decline in serum testosterone levels in American men. J Clin Endocrinol Metab. 2007;92(1):196-202. doi:10.1210/jc.2006-1375
Lokeshwar SD, Patel P, Fantus RJ, et al. Decline in serum testosterone levels among adolescent and young adult men in the USA. Eur Urol Focus. 2021;7(4):886-889. doi:10.1016/j.euf.2020.02.006
Santi D, Spaggiari G, Furini C, et al. Temporal trends in serum testosterone and luteinizing hormone levels indicate an ongoing resetting of hypothalamic-pituitary-gonadal function in healthy men: a systematic review. J Endocrinol Invest. 2025;48(11):2721-2734. doi:10.1007/s40618-025-02671-9
Hackett G, Kirby M, Rees RW, et al. The British Society for Sexual Medicine guidelines on adult testosterone deficiency, with statements for UK practice. World J Mens Health. 2023;41(3):508-537. doi:10.5534/wjmh.221027
Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.
Medically reviewed: March 2026 | Next review due: March 2027
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.
Medical disclaimer. This article is for informational purposes and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, history, and clinical picture. If you have concerns about your health, please consult your GP.