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Getting a testosterone blood test is straightforward. Understanding what the results mean is where most men - and some GPs - go wrong.
Total testosterone is just the starting point. To understand your hormonal status properly, you need to read the markers together. Here's how.
My first NHS result: total testosterone 11.2 nmol/L - reported as "normal." I felt terrible. When I paid for a full private panel, my SHBG was 52 nmol/L and my free testosterone was 0.19 nmol/L. The total number was misleading. The full picture told a different story.
Total Testosterone
What it measures: All testosterone circulating in the blood - both protein-bound (inactive) and free (active). Approximately 98% of testosterone is protein-bound at any given time.
UK reference range: Typically 8-35 nmol/L. NHS deficiency threshold: under 8-10 nmol/L.
Optimal range: 18-30 nmol/L for most men. Below 15 with symptoms warrants investigation.
The problem with total T alone: It doesn't tell you how much is biologically available. A man with total T of 18 nmol/L and SHBG of 65 nmol/L has much less active testosterone than a man with total T of 15 nmol/L and SHBG of 25 nmol/L.
Free Testosterone
What it measures: The fraction of testosterone not bound to proteins - specifically the portion not bound to SHBG or albumin. This is the biologically active testosterone that can actually enter cells and exert its effects.
UK reference range: Approximately 0.2-0.6 nmol/L (varies by lab and calculation method). Some labs calculate this from total T and SHBG; others measure directly (direct measurement is considered less reliable than calculation).
Optimal range: Above 0.4 nmol/L. Below 0.3 nmol/L with symptoms is clinically significant.
Key insight: This is often the number that explains symptoms when total T looks "normal." Men with total T in range but high SHBG frequently have low free T and full symptomatic deficiency.
SHBG (Sex Hormone-Binding Globulin)
What it measures: A protein produced by the liver that binds tightly to testosterone (and oestradiol), making it unavailable to cells. Higher SHBG = less free testosterone, regardless of total T.
UK reference range: 16-55 nmol/L. Rises significantly with age - men in their 50s often have SHBG 20+ nmol/L higher than they did at 30.
What raises SHBG:
- Ageing
- Low insulin / very low carbohydrate intake
- Hyperthyroidism
- Liver disease or elevated liver enzymes
- Chronic alcohol intake
- Caloric restriction
What lowers SHBG:
- Higher insulin (higher carbohydrate intake, insulin resistance)
- Obesity
- Hypothyroidism
- Anabolic steroids
Why it matters: If your SHBG is elevated (above 40-45 nmol/L), your free testosterone is likely lower than your total T suggests. This is the most commonly missed pattern in men over 40 presenting with low testosterone symptoms.
LH (Luteinising Hormone)
What it measures: Pituitary hormone that signals the testes to produce testosterone. The "upstream signal" in the testosterone production pathway.
UK reference range: 1.7-8.6 IU/L.
How to interpret with testosterone:
| LH | Testosterone | Interpretation | |----|-------------|----------------| | High | Low | Primary hypogonadism - testes not responding to signal | | Low/normal | Low | Secondary hypogonadism - signal isn't being sent | | Normal | Normal (but symptomatic) | Look at free T and SHBG | | Low | Normal | May indicate suppression (exogenous testosterone, pituitary issue) |
Primary vs secondary hypogonadism: Primary (high LH) usually requires TRT - the testes are damaged or not responding. Secondary (low LH) can sometimes be addressed with clomiphene citrate or lifestyle changes - the issue is upstream.
FSH (Follicle-Stimulating Hormone)
What it measures: Another pituitary hormone, primarily regulating sperm production. Less directly relevant to testosterone symptoms but important context.
UK reference range: 1.5-12.4 IU/L.
Why it matters:
- Very high FSH suggests testicular failure or significant testicular damage
- Important if fertility is a consideration - TRT suppresses FSH and sperm production significantly
- Low FSH alongside low LH = secondary hypogonadism (pituitary or hypothalamic origin)
Oestradiol (E2)
What it measures: The primary oestrogen in men. Produced partly in the testes, partly via conversion of testosterone to oestradiol by the aromatase enzyme in fat tissue.
UK reference range: 41-159 pmol/L in men.
Optimal range: 70-130 pmol/L. Both high and low cause problems.
Symptoms of high E2 (above 150+ pmol/L):
- Water retention and bloating
- Mood swings and emotional sensitivity
- Reduced libido and erectile dysfunction
- Gynecomastia (breast tissue)
Symptoms of low E2 (below 60 pmol/L):
- Joint pain
- Poor exercise recovery
- Low bone density
- Low mood and libido (similar to high E2)
What raises E2: More body fat (more aromatase), alcohol, unmonitored TRT.
What lowers E2: Low body fat, low testosterone, aromatase inhibitors (AI) on TRT.
Prolactin
What it measures: Pituitary hormone associated with lactation but present in men at low levels. Elevated prolactin directly suppresses LH and FSH, reducing testosterone production.
UK reference range: Under 450 mIU/L in men (some labs use different units - confirm with your lab).
When elevated prolactin matters:
- Persistently above 600-700 mIU/L warrants further investigation
- Can be transiently elevated by stress, recent exercise, or sexual activity - the test should be fasting and resting
- Significant elevation (above 1,500 mIU/L) should prompt pituitary imaging
- Common causes: stress, medications (PPIs, antidepressants, antipsychotics), pituitary adenoma
Reading the Panel Together
Here are the three most common patterns:
Pattern 1: Classic primary hypogonadism
- Total T: low (under 10 nmol/L)
- LH: high (above 6-8 IU/L)
- Free T: low
- Meaning: testes aren't responding to the signal. Usually requires TRT.
Pattern 2: SHBG-mediated low free T (most common in men over 40)
- Total T: normal (12-20 nmol/L)
- SHBG: elevated (45+ nmol/L)
- Free T: low (under 0.3 nmol/L)
- LH: normal
- Meaning: total T looks fine, but high SHBG is binding most of it. Symptomatic men often overlooked by NHS.
Pattern 3: Secondary hypogonadism
- Total T: low
- LH: low or low-normal
- FSH: low or low-normal
- Meaning: the signal isn't being sent. Cause is pituitary or hypothalamic. Investigate prolactin, thyroid, and consider MRI pituitary if prolactin elevated.
Getting the Full Panel
Most NHS tests cover total testosterone only. To get the full picture:
Never interpret total testosterone in isolation. Free testosterone and SHBG are the markers that explain symptoms when total T looks "normal." LH tells you why it's low. Always test fasting, between 7-10am.
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