You have had a blood test. The results are back. There is a number next to “testosterone” on the report, and now you are trying to work out what it means. Is it normal? Is it low? Is it low enough to explain how you have been feeling? And why does the reference range on the lab report seem so wide that it is almost meaningless?
If you are feeling confused, you are in good company. Testosterone levels are one of those areas of medicine where the numbers can feel simultaneously precise and frustratingly vague. The reference ranges are broad, they vary between laboratories, and they do not always correspond neatly to how you actually feel. Understanding what your testosterone level means requires more context than a single number can provide.
This article will explain how testosterone is measured in the UK, what the reference ranges are, how levels change with age, and why the relationship between your numbers and your symptoms is more nuanced than you might expect.
How Testosterone Is Measured in the UK
In the UK, testosterone is measured in nanomoles per litre (nmol/L). This is different from the United States, where the measurement is typically in nanograms per decilitre (ng/dL). If you are reading American resources, which are common online, it is important to convert the values to avoid confusion. As a rough guide, 1 nmol/L is approximately 28.8 ng/dL.
When your GP or specialist orders a testosterone blood test, they are typically measuring total testosterone. This represents all the testosterone in your blood, including testosterone that is bound to proteins and not immediately available for use by the body. We will discuss the distinction between total and free testosterone in more detail below, as it is clinically important.
The blood sample should ideally be taken in the morning, between 7am and 11am, because testosterone levels follow a circadian rhythm and are highest early in the day. An afternoon sample may show levels that are 20% to 30% lower than a morning sample simply due to this natural fluctuation, which could lead to a misleadingly low result if the timing is not accounted for.
Reference Ranges for Total Testosterone in the UK
The reference range for total testosterone in most UK laboratories is approximately 8.64 to 29 nmol/L, though the exact figures can vary slightly between laboratories depending on the assay used and the population the range was derived from. Some laboratories use slightly different ranges, such as 7.6 to 31.4 nmol/L or 9.0 to 29.0 nmol/L.
The NHS and most UK endocrinology guidelines generally interpret these ranges as follows:
- Below 8 nmol/L: Clearly low. Most clinicians would consider this unequivocally deficient and would typically recommend further investigation and potentially treatment if symptoms are present.
- 8 to 12 nmol/L: Borderline or grey zone. This is where clinical judgement becomes particularly important. Some men in this range will have significant symptoms; others will feel fine. Further testing, including free testosterone and SHBG, is often helpful in this range.
- Above 12 nmol/L: Generally considered within the normal range, though men at the lower end may still experience symptoms depending on their individual physiology and their free testosterone levels.
It is worth emphasising that these are population-level reference ranges. They tell you where you fall relative to a large sample of men, but they do not tell you what is optimal for you as an individual. A man who has spent most of his adult life with a testosterone level of 25 nmol/L may experience significant symptoms if his level drops to 13 nmol/L, even though 13 nmol/L is technically within the reference range.
How Testosterone Levels Change with Age
Testosterone levels are not static. They change throughout a man’s life in a predictable but individually variable pattern.
Adolescence and early adulthood (puberty to late 20s): Testosterone levels rise dramatically during puberty, reaching their peak in the late teens to early 20s. During this period, levels of 20 to 30 nmol/L or higher are common and expected.
30s: Testosterone levels typically begin a slow, gradual decline starting around age 30. The rate is approximately 1% to 2% per year on average, though individual variation is substantial. Most men in their 30s will have levels comfortably within the normal range, and the decline is usually not symptomatic at this stage.
40s: The cumulative effect of the gradual decline begins to become more apparent. A man who had a level of 25 nmol/L at age 25 might be around 18 to 20 nmol/L by his mid-40s, still well within the reference range. However, men who started with lower levels, or whose decline has been steeper due to contributing factors like weight gain or chronic stress, may begin to experience symptoms during this decade.
50s and 60s: By this age, a more significant proportion of men have levels that fall into the borderline or low range. The Baltimore Longitudinal Study of Aging found that approximately 20% of men over 60 had total testosterone levels below the lower limit of the normal range. Symptoms of deficiency become more common, though they continue to be frequently attributed to ageing alone.
70s and beyond: The proportion of men with low testosterone continues to increase. Some studies suggest that up to 30% to 50% of men over 70 have total testosterone levels below 12 nmol/L. However, interpreting these levels in older men is complicated by changes in SHBG and overall health status.
It is important to recognise that these are population averages. Some men maintain high testosterone levels throughout their lives, while others experience a more pronounced decline. The trajectory is influenced by genetics, body composition, physical activity, sleep quality, stress levels, and the presence or absence of chronic disease.
Total Testosterone vs Free Testosterone: Why the Distinction Matters
When most people talk about their testosterone level, they are referring to total testosterone. But total testosterone is not the whole picture, and in many cases, it is not even the most informative number.
In the bloodstream, testosterone exists in three forms:
- Bound to sex hormone-binding globulin (SHBG): Approximately 60% to 70% of testosterone is tightly bound to SHBG and is not available for use by the body’s tissues. It is essentially inactive in this form.
- Bound to albumin: Approximately 25% to 35% is loosely bound to albumin. This testosterone can dissociate from albumin relatively easily and become available to tissues, so it is considered partially bioavailable.
- Free testosterone: Only about 1% to 3% of testosterone circulates in a free, unbound form. This is the fraction that is immediately available to enter cells and exert biological effects.
The concept of bioavailable testosterone encompasses both free testosterone and albumin-bound testosterone, representing the testosterone that is actually available for the body to use.
Why does this matter? Because SHBG levels vary significantly between individuals and change with age and medical conditions. SHBG tends to increase with age, meaning that an older man may have a total testosterone level that appears adequate while his free testosterone, the fraction that actually does the work, is genuinely low. Conversely, conditions that lower SHBG, such as obesity and type 2 diabetes, can make total testosterone appear lower while free testosterone may be relatively preserved.
Normal ranges for free testosterone in the UK are typically around 0.2 to 0.62 nmol/L (or 5.7 to 17.9 pg/mL), though again these vary between laboratories.
In clinical practice, measuring free testosterone alongside total testosterone and SHBG provides a much more complete picture of a man’s hormonal status. This is particularly important in the borderline range (total testosterone 8 to 15 nmol/L), where total testosterone alone may be insufficient to make a clinical decision.
The Role of SHBG
Sex hormone-binding globulin deserves its own discussion because of the significant impact it has on how testosterone levels are interpreted.
SHBG is a protein produced primarily by the liver. It binds testosterone with high affinity, effectively sequestering it and preventing it from reaching target tissues. Factors that increase SHBG include:
- Ageing (SHBG tends to increase with age)
- Liver disease
- Hyperthyroidism
- Certain medications, including anticonvulsants
- Low body weight
Factors that decrease SHBG include:
- Obesity
- Type 2 diabetes and insulin resistance
- Hypothyroidism
- Nephrotic syndrome
- Androgen use
A man with high SHBG may have a total testosterone level that looks normal while his free testosterone is actually low, because more of his testosterone is being bound up and rendered unavailable. This is a common scenario in older men and is one of the reasons why a total testosterone level alone can be misleading.
When clinicians at specialist services like Evernu assess testosterone levels, SHBG is routinely included because it is essential for accurate interpretation of the results.
Morning vs Afternoon Variation
Testosterone secretion follows a circadian pattern, with the highest levels occurring in the early morning and the lowest in the late afternoon and evening. In younger men, this diurnal variation can be substantial, with morning levels up to 30% higher than afternoon levels. In older men, the circadian rhythm tends to flatten somewhat, but a meaningful difference between morning and afternoon levels still exists.
This is why clinical guidelines consistently recommend that testosterone blood tests be performed in the morning, ideally between 7am and 11am. An afternoon blood test may produce a result that is significantly lower than a morning test in the same individual, potentially leading to a diagnosis of deficiency that does not accurately reflect the man’s true hormonal status.
If your initial testosterone test was taken in the afternoon and showed a low result, it is worth repeating the test in the morning before drawing conclusions. Conversely, if a morning test shows a low result, the finding is more reliable and more likely to reflect genuine deficiency.
Why a Single Test Is Not Enough
Testosterone levels can fluctuate from day to day based on sleep quality, stress, illness, exercise, alcohol consumption, and other transient factors. A single low reading does not necessarily indicate a chronic deficiency. For this reason, the NICE and most endocrinology guidelines recommend that a low testosterone result be confirmed with a second test, taken on a separate morning, before a diagnosis of hypogonadism is made.
This is not bureaucratic caution. It reflects the genuine variability of the measurement. A man who slept poorly, was under unusual stress, or was recovering from illness might produce a transiently low result that does not represent his usual hormonal state. Confirming the finding on a second occasion provides much greater confidence in the diagnosis.
In addition to repeating the testosterone measurement, a comprehensive hormonal assessment should typically include:
- LH and FSH: To distinguish between primary and secondary hypogonadism
- SHBG: To calculate free testosterone and assess bioavailability
- Prolactin: To screen for pituitary disorders
- Thyroid function: To rule out thyroid disease as a contributing factor
- Full blood count: As a baseline before any potential treatment
- PSA (prostate-specific antigen): In men over 40, as a baseline prostate marker
Why Numbers Alone Do Not Tell the Whole Story
This is perhaps the most important point in this entire article: a testosterone level is a number, and numbers require interpretation. Two men with identical testosterone levels can have completely different symptom profiles. One may feel fine; the other may be profoundly symptomatic.
There are several reasons for this:
Individual sensitivity to testosterone varies. Androgen receptor density and sensitivity differ between individuals, meaning that some men’s tissues respond more robustly to a given testosterone level than others. A man with highly sensitive receptors may function well at a level that would produce significant symptoms in a man with less responsive receptors.
The rate of decline matters as much as the absolute level. A man whose testosterone has been stable at 12 nmol/L for years may feel quite different from a man whose testosterone has dropped from 25 to 12 nmol/L over the past two years. The body adapts to gradual changes but may struggle with more rapid shifts.
Comorbidities, medications, and overall health influence symptom expression. Depression, poor sleep, chronic pain, and other conditions can amplify the symptoms of low testosterone, making a moderate deficiency feel more severe than the number alone might suggest.
Free testosterone may not correlate with total testosterone. As discussed, a man with high SHBG may have a normal total testosterone but low free testosterone, explaining symptoms that the total number would not predict.
This is why responsible clinical practice involves treating the patient, not just the lab result. A comprehensive assessment considers the blood tests, the symptoms, the medical history, and the individual’s own experience of how they feel. At Evernu, we take this approach because we believe it leads to better outcomes and better care.
What to Do If Your Levels Are Low
If your testosterone levels are confirmed as low on two morning blood tests, and you are experiencing symptoms consistent with testosterone deficiency, the next step is a thorough clinical assessment to determine the underlying cause and discuss treatment options.
Treatment decisions should consider:
- The severity and impact of your symptoms
- The underlying cause of the deficiency
- Your overall health and any contraindications
- Your fertility goals (testosterone replacement can suppress sperm production)
- Your personal preferences and values regarding treatment
For some men, addressing contributing factors such as weight, sleep, stress, or medication may be sufficient. For others, testosterone replacement therapy (TRT) may be appropriate and can provide significant improvement in symptoms and quality of life when properly managed and monitored.
Whatever your situation, the important thing is to have the conversation with someone who understands the complexity of male hormonal health and who will take your experience seriously. If your GP has not been receptive, or if you want a more thorough assessment than primary care can typically provide, specialist services are available.
Evernu provides comprehensive testosterone assessments for men across England, Scotland, Wales, and Northern Ireland. Our clinicians understand that a number on a blood test is only the beginning of the story, and we are committed to helping you understand what it means for you as an individual. Start your assessment today.
When to Retest Your Testosterone Levels
If your initial test showed a borderline or low result, retesting is important. Here are some general guidelines:
- Confirming a low result: Repeat the test within 2 to 4 weeks, in the morning, on a day when you have slept reasonably well and are not acutely unwell
- After lifestyle changes: If you have made significant changes to weight, sleep, exercise, or stress management, retesting after 3 to 6 months can show whether these changes have had an impact on your levels
- During treatment: Men on testosterone replacement therapy are typically monitored every 3 to 6 months initially, then annually once levels are stable
- If symptoms change: Any significant change in symptoms, whether improvement or deterioration, warrants retesting to ensure that levels are appropriate
Frequently Asked Questions About Testosterone Levels
What is a normal testosterone level for a 40-year-old man in the UK?
There is no single “normal” level that applies to all 40-year-old men. The reference range for total testosterone in UK laboratories is typically 8.64 to 29 nmol/L, and most healthy 40-year-old men will fall somewhere within this range. However, levels vary significantly between individuals. A level of 15 nmol/L might be entirely normal for one man and represent a significant decline for another who previously had levels of 25 nmol/L or higher. Symptoms and clinical context matter as much as the absolute number.
Can I have low testosterone symptoms even if my levels are in the normal range?
Yes, this is possible and more common than many clinicians appreciate. There are several explanations. Your total testosterone may be normal while your free testosterone is low due to elevated SHBG. Your level may have declined significantly from your personal baseline, even though it still falls within the population reference range. Or you may have heightened sensitivity to a lower level due to the rate of decline or individual receptor characteristics. This is why a comprehensive assessment that considers symptoms alongside blood results is essential.
Why does the time of day matter for a testosterone blood test?
Testosterone follows a circadian rhythm, with levels peaking in the early morning (typically between 7am and 11am) and declining throughout the day. In some men, afternoon levels can be 20% to 30% lower than morning levels. Clinical guidelines recommend morning testing to capture the most accurate and reproducible measurement. An afternoon test may produce a misleadingly low result that does not reflect your true hormonal status.
What is the difference between total and free testosterone?
Total testosterone measures all testosterone in the blood, including testosterone bound to SHBG (which is biologically inactive) and testosterone bound to albumin (partially available). Free testosterone measures only the unbound fraction, approximately 1% to 3% of the total, which is immediately available for the body to use. In many clinical situations, particularly in older men or those with conditions that affect SHBG levels, free testosterone provides a more accurate picture of functional hormonal status than total testosterone alone.
How many times should testosterone be tested before starting treatment?
Most clinical guidelines, including those from the British Society for Sexual Medicine, recommend that low testosterone be confirmed on at least two separate morning blood tests before a diagnosis of hypogonadism is made and treatment is considered. This is because testosterone levels can fluctuate day to day due to sleep, stress, illness, and other factors. A single low result may not represent a chronic deficiency. Confirming the finding on a second occasion, ideally 2 to 4 weeks later, provides much greater diagnostic confidence and helps ensure that treatment decisions are based on accurate information.
