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Getting blood test results back is one thing. Understanding what they actually mean - beyond "normal" or "abnormal" - is another.
Most labs return a column of numbers with a reference range next to each one. What they don't tell you is that "normal" is a population average, not an optimal target. They don't tell you which markers interact, which symptoms each one explains, or what to do when you're in the "normal" range but feeling anything but.
This is the guide to reading your own results.
I've been through this process many times - first confused by my own results, then reading enough to understand what each number means in context. This guide covers everything I wish someone had explained to me at the start. If you haven't tested yet, the best place to start in the UK is a comprehensive private panel.
Section 1: Hormones
Total Testosterone
What it is: The total amount of testosterone circulating in your blood, including both bound (inactive) and free (active) fractions.
UK reference range: Typically 8-35 nmol/L (varies by lab). NHS deficiency threshold: under 8-10 nmol/L.
What to aim for: 18-30 nmol/L is optimal for most men. Below 15 nmol/L with symptoms warrants investigation. The BSSM (British Society for Sexual Medicine) recommends considering treatment below 12 nmol/L if symptomatic.
Important: Total testosterone alone is insufficient. Always interpret alongside free testosterone and SHBG.
Free Testosterone
What it is: The fraction of testosterone not bound to proteins - the biologically active portion your cells can actually use. Typically 1-3% of total testosterone.
UK reference range: Approximately 0.2-0.6 nmol/L (varies significantly by lab and calculation method).
What to aim for: Above 0.4 nmol/L. Below 0.3 nmol/L with symptoms is clinically significant regardless of total testosterone.
Why it matters: You can have normal total testosterone and low free testosterone if SHBG is elevated. This is the most commonly missed pattern in men over 40.
SHBG (Sex Hormone-Binding Globulin)
What it is: A protein produced by the liver that binds tightly to testosterone (and oestradiol), making it unavailable to tissues. High SHBG reduces free testosterone even when total testosterone appears normal.
UK reference range: 16-55 nmol/L. Rises with age - men over 40 often see SHBG in the 30-50+ range.
What to aim for: 20-35 nmol/L. Below this, more testosterone is free (can be beneficial or indicate insulin resistance). Above 40, free testosterone is meaningfully suppressed.
What raises SHBG: Ageing, low insulin / low carbohydrate intake, hyperthyroidism, liver disease, excess alcohol, caloric restriction.
What lowers SHBG: Higher insulin (higher carbohydrate intake), obesity, hypothyroidism, anabolic steroids.
LH (Luteinising Hormone)
What it is: Pituitary hormone that signals the testes to produce testosterone. The "upstream" signal in the HPG axis.
UK reference range: 1.7-8.6 IU/L.
How to interpret:
- High LH + low testosterone: Primary hypogonadism. Testes aren't responding to the signal. Cause is testicular - typically requires TRT.
- Low LH + low testosterone: Secondary hypogonadism. The signal isn't being sent - cause is pituitary or hypothalamic. Can sometimes respond to clomiphene.
- Normal LH + normal testosterone but symptomatic: Look at free testosterone and SHBG.
FSH (Follicle-Stimulating Hormone)
What it is: Pituitary hormone involved in sperm production. Less directly relevant to testosterone symptoms but important for understanding testicular function.
UK reference range: 1.5-12.4 IU/L.
Why it matters: Very high FSH suggests testicular damage or failure. Important if fertility is a consideration before starting TRT (TRT suppresses FSH and sperm production).
Oestradiol (E2)
What it is: The primary oestrogen in men. Produced partly in the testes and partly via conversion of testosterone to oestradiol by the aromatase enzyme in fat tissue.
UK reference range: 41-159 pmol/L in men.
What to aim for: 70-130 pmol/L. Both too high and too low cause problems.
Symptoms of high E2: Water retention, mood swings, reduced libido, gynecomastia (breast tissue development), erectile dysfunction.
Symptoms of low E2: Joint pain, poor recovery, low bone density, mood issues, low libido (yes, the same as high E2 - context matters).
What raises E2: Excess body fat (more aromatase), alcohol, low testosterone therapy without monitoring.
Prolactin
What it is: Pituitary hormone primarily associated with lactation but present in men at low levels. Elevated prolactin suppresses LH and FSH, reducing testosterone.
UK reference range: Under 450 mIU/L in men.
Why it matters: Persistently elevated prolactin (above 600-700 mIU/L) warrants further investigation including pituitary imaging. Common causes: stress, certain medications (antipsychotics, PPIs), pituitary adenoma.
Symptoms of high prolactin: Low testosterone symptoms, reduced libido, erectile dysfunction, headaches, visual disturbances (if pituitary mass).
DHEA-S (Dehydroepiandrosterone Sulphate)
What it is: Adrenal androgen precursor. Converts to testosterone and oestrogens in peripheral tissues. Declines significantly with age - peaks in the 20s.
UK reference range: Age-dependent. At 40: 2.2-15.2 µmol/L.
Why it matters: Low DHEA-S in symptomatic men suggests adrenal insufficiency or significant HPA axis stress. Some clinicians supplement DHEA in deficient older men.
Section 2: Thyroid
Thyroid dysfunction is the single most common condition mistaken for low testosterone. Hypothyroidism and low testosterone share almost identical symptoms: fatigue, brain fog, weight gain, low mood, poor recovery, cold intolerance, low libido.
TSH (Thyroid-Stimulating Hormone)
What it is: Pituitary hormone that drives thyroid hormone production. High TSH = thyroid underperforming (hypothyroidism). Low TSH = thyroid overactive (hyperthyroidism).
UK reference range: 0.4-4.0 mIU/L.
What to aim for: 1.0-2.5 mIU/L. Being "in range" at 3.5 is not the same as being optimal.
Free T3 and Free T4
What they are: The active (T3) and storage (T4) forms of thyroid hormone. T4 converts to T3 in peripheral tissues - this conversion can be impaired even with normal TSH and T4.
Why they matter: TSH alone can miss conversion problems. A man with normal TSH, normal T4, but low Free T3 has a functional thyroid deficiency - and will feel it.
What to aim for: Free T3 in the upper half of the reference range. Free T4 mid-range or above.
Section 3: Metabolic
HbA1c
What it is: Average blood glucose over the previous 2-3 months. The best single marker for insulin resistance and diabetes risk.
UK reference range:
- Under 42 mmol/mol: normal
- 42-47: pre-diabetic
- 48+: diabetic
Why it matters for testosterone: Insulin resistance suppresses testosterone through multiple mechanisms. Men with HbA1c above 42 reliably have lower testosterone than men with HbA1c below 35. Improving insulin sensitivity is one of the most effective testosterone interventions available.
What to aim for: Under 38 mmol/mol for optimal metabolic health.
Fasting Glucose
What it is: Blood glucose after 8+ hours fasting.
UK reference range: 4.0-6.0 mmol/L.
What to aim for: Under 5.5 mmol/L. Above 5.5 fasting suggests developing insulin resistance even if HbA1c is still normal.
Cholesterol Panel
| Marker | Normal Range | Notes | |--------|-------------|-------| | Total cholesterol | Under 5.0 mmol/L | Context-dependent | | LDL | Under 3.0 mmol/L | Lower is better | | HDL | Above 1.0 mmol/L | Higher is better | | Triglycerides | Under 1.7 mmol/L | Insulin resistance marker | | Total:HDL ratio | Under 4.0 | Best single cardiovascular risk marker |
Testosterone and cholesterol: Testosterone is synthesised from cholesterol. Very low cholesterol (under 4.0 mmol/L) is associated with lower testosterone. Very low fat diets suppress testosterone through this mechanism.
Section 4: Vitamins and Minerals
Vitamin D (25-OH)
What it is: Marker of vitamin D status. Vitamin D is technically a steroid hormone precursor - Leydig cells (testosterone-producing testicular cells) have vitamin D receptors.
UK reference range:
- Under 30 nmol/L: deficient
- 30-50: insufficient
- 50+: sufficient (NHS threshold)
What to aim for: 75-120 nmol/L. The NHS threshold of 50 is the floor, not the target.
Why it matters: 40% of UK adults are deficient in winter. Deficiency is consistently associated with lower testosterone in observational studies. Supplementation in deficient men improves testosterone in several RCTs.
Ferritin
What it is: Iron storage protein. The best marker of iron status - serum iron fluctuates too much to be useful alone.
UK reference range: 30-400 µg/L in men.
What to aim for: 70-200 µg/L. Below 50 causes symptoms (fatigue, brain fog, poor athletic performance) that closely mimic low testosterone - even when haemoglobin is normal.
Note: Elevated ferritin (above 300 in men) can indicate haemochromatosis (iron overload), inflammation, or liver disease - worth investigating.
Zinc
What it is: Essential mineral involved in LH receptor function and testosterone synthesis.
Note: Not included in most standard panels. If testing specifically: serum zinc 10-18 µmol/L is typical. Symptoms of deficiency overlap with low testosterone.
Section 5: Organ Function
Liver Function (ALT, AST, GGT)
Why it matters for hormones: The liver processes and clears hormones. Elevated liver enzymes suggest impaired clearance, which affects oestradiol metabolism, SHBG production, and overall hormonal balance. GGT is particularly sensitive to alcohol intake.
What to aim for: ALT under 30 IU/L (men), AST under 35 IU/L, GGT under 50 IU/L.
Kidney Function (eGFR, Creatinine)
Why it matters: Chronic kidney disease affects testosterone production and is associated with lower total and free testosterone. Creatinine is also used to assess whether creatine supplementation is affecting kidney load.
What to aim for: eGFR above 90 mL/min/1.73m².
How to Get the Full Panel
Most NHS tests don't cover the full picture above. To get comprehensive results:
"Normal" means you're within the population distribution - not that you're optimal. Know your actual numbers. Track them over time. A result of 12 nmol/L testosterone and 11.5 nmol/L three months later tells a story that a single snapshot doesn't.
Seb tracks his full panel quarterly. All views are his own - see affiliate disclosure above.
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