How to Read Your Blood Test Results: A UK Guide

What is a blood test actually measuring?

A blood test measures the concentration of specific substances in your blood — proteins, hormones, vitamins, minerals, enzymes, fats, sugars, and cell counts. Each substance is called a biomarker, and each biomarker tells you something about how a particular system in your body is functioning.

For example, ferritin measures your iron stores. A low ferritin level doesn't necessarily mean you're anaemic yet — but it can explain fatigue, poor recovery, and brain fog weeks before your haemoglobin drops low enough for a GP to flag it.

A standard NHS blood test typically checks around 10 markers. A comprehensive panel (like the ones Helvy offers) tests 50 or more, covering hormones, thyroid function, inflammation, liver, kidney, vitamins, and metabolic health. The more markers you test, the more complete the picture.

What do reference ranges mean?

Every blood test result comes with a reference range — a pair of numbers that define the “normal” range for that biomarker. For example, the NHS reference range for TSH (thyroid-stimulating hormone) is typically 0.27–4.2 mIU/L.

These ranges are calculated by testing a large population of apparently healthy people and taking the middle 95%. That means 2.5% of healthy people naturally fall above the range and 2.5% fall below it — even when nothing is wrong. The BMJ notes that reference intervals are “not diagnostic thresholds” but statistical distributions.

Reference ranges also vary between laboratories. The TSH upper limit might be 4.2 at one lab and 5.0 at another. This is because different labs use different analysers, reagents, and calibration methods. According to the NICE thyroid guidelines, results should always be interpreted against the specific laboratory's reference range, not a generic one from Google.

Key point: being “in range” means you're within the statistical norm. It does not mean your level is ideal for your age, sex, activity level, or goals.

Why “normal” doesn't mean optimal

This is the single most important concept in understanding blood test results. The standard reference range tells you whether you're likely to have a disease. It does not tell you whether you're performing at your best.

Take vitamin D. The NHS considers anything above 25 nmol/L (10 ng/mL) as “sufficient.” But the Endocrine Society recommends 75–150 nmol/L (30–60 ng/mL) for optimal immune function, bone density, and mood. You can be “in range” at 30 nmol/L and still feel terrible.

The same applies to ferritin. The NHS lower limit is around 15 µg/L, but sports medicine research consistently shows that athletes and active people feel significantly better above 50–80 µg/L. A level of 20 won't trigger a GP flag, but it can absolutely explain chronic fatigue.

Performance-optimised ranges — the kind used by sports physicians and functional doctors — narrow the window from “not sick” to “actually thriving.” This is exactly what Helvy reports use: your results plotted against both the clinical range and the performance range, so you can see the gap.

Full blood count (FBC)

The full blood count is the most commonly ordered blood test in the UK. It measures your red blood cells, white blood cells, haemoglobin, haematocrit, and platelets. The NHS uses it as a screening tool for anaemia, infection, and blood disorders.

What to watch for: low haemoglobin with low MCV typically indicates iron-deficiency anaemia — the most common nutritional deficiency in the UK according to the WHO. High MCV with low haemoglobin often points to vitamin B12 or folate deficiency.

Liver function tests (LFTs)

Liver function tests measure enzymes and proteins produced by the liver. The key markers are ALT (alanine aminotransferase), AST (aspartate aminotransferase), GGT (gamma-glutamyl transferase), ALP (alkaline phosphatase), bilirubin, and albumin.

ALT is the most liver-specific enzyme. The NICE guidelines on non-alcoholic fatty liver disease recommend testing ALT as part of routine health checks, especially if you have metabolic risk factors. A normal ALT is generally below 40 IU/L for men and 32 IU/L for women, though optimal is arguably lower.

Common causes of elevated liver enzymes: alcohol, non-alcoholic fatty liver disease (NAFLD), medications (including common painkillers like paracetamol), intense exercise (transient AST/ALT rise is normal after heavy training), and viral infections.

What to watch for: persistently elevated ALT above twice the upper limit warrants further investigation. A single mildly elevated reading after a heavy training week or a weekend of drinking is usually nothing to panic about — but should be rechecked.

Kidney function tests

Kidney tests typically include creatinine and eGFR (estimated glomerular filtration rate), urea, and electrolytes (sodium, potassium). eGFR is the single most important kidney marker — it estimates how well your kidneys are filtering waste from the blood.

An eGFR above 90 mL/min is considered normal. Between 60 and 89 may indicate early kidney function decline, especially if sustained over three months. Below 60 is classified as chronic kidney disease (CKD) by NICE CKD guidelines.

What to watch for: creatinine is affected by muscle mass — muscular people naturally have higher creatinine, which can make eGFR look falsely low. If you train with weights and your eGFR comes back at 75–85, this may be completely normal for your body composition. Context matters.

Thyroid function (TSH, T3, T4)

Your thyroid controls your metabolic rate, energy, body temperature, and weight regulation. The TSH test is the standard screening tool. NICE recommends TSH as the first-line investigation for suspected thyroid dysfunction.

The reference range for TSH is typically 0.27–4.2 mIU/L. But there's growing evidence that the optimal range for most adults is 0.5–2.5 mIU/L. A TSH of 3.8 is technically “normal” but may indicate your thyroid is starting to struggle — especially if you also have symptoms like fatigue, weight gain, or feeling cold.

Why TSH alone isn't enough: TSH tells you how hard your pituitary gland is working to stimulate the thyroid. It does not tell you how much active thyroid hormone (free T3 and free T4) is actually circulating. A comprehensive thyroid panel should include TSH, free T4, and ideally free T3 — which is what Helvy's Performance panel tests.

What to watch for: TSH above 4.0 with symptoms warrants further investigation. Subclinical hypothyroidism (TSH 4–10 with normal T4) affects approximately 5–8% of UK adults, according to the British Thyroid Foundation.

Cholesterol and lipid profile

A lipid panel typically measures total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides. The NHS targets a total cholesterol below 5 mmol/L and LDL below 3 mmol/L for most adults.

However, total cholesterol alone is a blunt instrument. ApoB (apolipoprotein B) is increasingly recognised as a superior marker of cardiovascular risk because it counts the actual number of atherogenic particles, not just cholesterol mass. The European Society of Cardiology now recommends ApoB alongside standard lipids for risk assessment.

Similarly, Lp(a) — lipoprotein(a) is a genetically determined cardiovascular risk factor that most standard NHS tests don't include. Elevated Lp(a) significantly increases your risk of heart disease regardless of LDL levels. It only needs to be measured once in your lifetime because it's genetically fixed.

What to watch for: the ratio of triglycerides to HDL is a better predictor of metabolic health than any single lipid number. A ratio below 1.0 (in mmol/L) is excellent; above 2.0 warrants attention. If your LDL is “high” but your HDL is also high and triglycerides are low, the clinical picture is often much less concerning than the headline number suggests.

Vitamins and minerals

The most commonly tested nutritional biomarkers in the UK are vitamin D, vitamin B12, folate, ferritin (iron stores), magnesium, and zinc.

Notice the gap between NHS “normal” and optimal. This is where most people live — technically in range, practically suboptimal. A comprehensive blood test with performance ranges makes that gap visible.

Hormones (testosterone, cortisol, DHEA-S)

Testosterone is the marker most men ask about — and the one GPs are most reluctant to test. The NHS reference range for total testosterone is roughly 8.6–29 nmol/L, which is enormously wide. A 35-year-old man with a level of 10 nmol/L is technically “normal” but likely experiencing fatigue, low motivation, reduced muscle mass, and poor recovery.

Cortisol is your primary stress hormone. Morning cortisol should be at its highest (the cortisol awakening response), typically 166–507 nmol/L. Consistently high cortisol suggests chronic stress or overtraining; consistently low cortisol may indicate adrenal fatigue (a pattern, not a formal diagnosis).

DHEA-S is a precursor to both testosterone and oestrogen. It declines steadily from your late twenties onward. Low DHEA-S relative to your age group can indicate accelerated biological ageing and is increasingly used in longevity medicine as a vitality marker.

What to watch for: hormones are best interpreted together, not in isolation. Low testosterone with high SHBG tells a different story from low testosterone with low SHBG. Context and clinical symptoms matter as much as the number.

Inflammation markers (CRP, hs-CRP)

High-sensitivity CRP (hs-CRP) measures low-grade systemic inflammation — the kind that doesn't cause obvious symptoms but correlates with long-term cardiovascular risk, metabolic dysfunction, and accelerated ageing.

The British Heart Foundation recognises hs-CRP as a useful adjunct to standard cardiovascular risk assessment. Levels below 1.0 mg/L indicate low cardiovascular risk; 1.0–3.0 moderate risk; above 3.0 high risk.

What to watch for: a single elevated hs-CRP doesn't mean you have heart disease. It can spike after infection, intense exercise, poor sleep, or even a bad week of eating. The trend over time is what matters. If hs-CRP is consistently above 1.0, it's worth investigating lifestyle factors like sleep, stress, diet, and body composition.

Blood sugar and HbA1c

HbA1c (glycated haemoglobin) measures your average blood sugar over the past 2–3 months. It's the gold standard for diabetes screening and the single best marker of long-term metabolic health.

The NICE diabetes guidelines define the thresholds as: below 42 mmol/mol = normal, 42–47 = pre-diabetic, 48 or above = diabetic. But research in the Lancet Diabetes & Endocrinology shows that cardiovascular risk starts increasing above 37 mmol/mol — well within the “normal” range.

What to watch for: an HbA1c of 40 mmol/mol won't trigger a GP flag, but it's a yellow light. Combined with elevated triglycerides and a waist circumference above 94 cm (men) or 80 cm (women), it suggests early metabolic dysfunction that lifestyle changes can still reverse.

What to do when results are flagged

A flagged result — one that falls outside the reference range — is not an automatic cause for alarm. Here's a structured approach:

1. 1.

   Don't panic at a single result

   One abnormal reading could be a lab error, a transient fluctuation, or the result of something you did the day before (heavy exercise, alcohol, dehydration). Always retest before drawing conclusions.

2. 2.

   Look at the pattern, not the number

   A vitamin D of 30 nmol/L in January is expected (low sunlight). The same level in August, after a sunny summer, is more concerning. Trends over time matter more than any single snapshot.

3. 3.

   Consider the context

   Your age, sex, activity level, medications, menstrual cycle, and recent illness all affect results. A creatinine level that would be abnormal for a sedentary 60-year-old may be perfectly normal for a 30-year-old who lifts weights five times a week.

4. 4.

   Talk to someone qualified

   If a result is significantly outside range, speak to your GP or the doctor reviewing your results. Every Helvy blood test is reviewed by a GMC-registered doctor who can flag anything that needs clinical follow-up.

5. 5.

   Retest in 8–12 weeks

   If you make changes — supplementation, diet, training adjustments — retest after 2–3 months to confirm the intervention worked. This is where most people fail: they test once, never follow up, and never know if anything changed.

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