UNDERSTANDING YOUR RESULTS
Blood Tests Normal But Still Feel Ill: Why “In Range” Doesn't Mean Optimal
Your GP says your blood tests came back normal. You still feel exhausted, foggy, heavy, and not like yourself. You are not imagining it. The gap between “no diagnosable disease” and “actually feeling well” is one of the most common — and least acknowledged — problems in British healthcare. An estimated 1 in 5 GP consultations involve fatigue, and the majority of those patients leave with normal blood tests and no clear explanation.
This guide explains why NHS reference ranges were designed to detect disease rather than to optimise health — and identifies the 10 biomarkers most commonly caught in what functional medicine practitioners call the “grey zone”: technically in range, but at a level that is associated with real, measurable symptoms.
Understanding the difference between “normal” and “optimal” is the core of what Helvy was built to solve.
Why Your GP Says “Everything's Normal”
NHS reference ranges are derived from population statistics. A reference range is typically defined as the interval within which 95% of a healthy population falls — technically, the 2.5th to 97.5th percentile. This means that by mathematical design, 5% of people who are genuinely well will fall outside the reference range, and — critically — many people who feel unwell will fall inside it.
These ranges were designed for a specific purpose: detecting established disease in a clinical setting. They were not designed to identify the level at which you feel your best, perform cognitively, maintain energy, or support hormonal balance. They were designed to answer the question “does this patient have a condition requiring medical intervention?” — not “is this patient functioning optimally?”
The result is a systematic blind spot in routine healthcare. A ferritin of 14 μg/L is “in range” by NHS standards (the lower cutoff is 12 μg/L) but is associated with significant fatigue, hair loss, and poor concentration. A TSH of 3.9 mU/L sits comfortably within the NHS range of 0.27–4.2 mU/L but may represent subclinical hypothyroidism with real, measurable symptoms. A vitamin D level of 35 nmol/L is above the NHS deficiency threshold of 25 nmol/L but is associated with muscle weakness, bone pain, and low mood.
The gap between “diseased” and “optimal” is where millions of people in the UK live. They are not sick enough to trigger a diagnosis. They are not well enough to feel like themselves. And the healthcare system, optimised as it is to detect and treat disease, has very little to offer them.
This is not a failure of individual GPs. It is a systemic limitation of how reference ranges were constructed and how healthcare systems are designed. A 10-minute GP consultation does not allow for nuanced interpretation of borderline values, discussion of symptoms in the context of multiple borderline biomarkers, or proactive optimisation. Understanding this is the first step to taking back control.
The 10 Biomarkers Most Commonly “Normal But Not Optimal”
These are the biomarkers most frequently found in the grey zone — technically within NHS reference ranges, but at levels associated with real symptoms in peer-reviewed research.
1. Ferritin (stored iron)
NHS RANGE
12–300 μg/L
GREY ZONE
12–50 μg/L
FUNCTIONAL OPTIMAL
50–150 μg/L
Ferritin is the body's primary iron storage protein. The NHS lower threshold of 12 μg/L was set to detect frank iron deficiency anaemia, not to identify the level at which people feel well. Research consistently shows that symptoms including fatigue, brain fog, hair loss, and breathlessness on exertion appear well before ferritin falls below 12 — often at levels between 15 and 50 μg/L. NICE NG4 (iron deficiency) acknowledges that functional iron deficiency can occur with higher ferritin levels, particularly in women of childbearing age and endurance athletes.
Symptoms in the grey zone: persistent fatigue that doesn't improve with sleep, diffuse hair thinning, difficulty concentrating, restless legs at night, breathlessness climbing stairs.
2. TSH (thyroid-stimulating hormone)
NHS RANGE
0.27–4.2 mU/L
GREY ZONE
2.5–4.2 mU/L
FUNCTIONAL OPTIMAL
0.5–2.5 mU/L
TSH is the pituitary's signal to the thyroid to produce more hormone. A rising TSH typically indicates the thyroid is struggling. The NICE CKS on hypothyroidism acknowledges that subclinical hypothyroidism — TSH elevated but FT4 still normal — is common and can be symptomatic. A 2018 BMJ review found that up to 10% of the UK population may have subclinical thyroid dysfunction. The upper end of the NHS range (TSH of 3.5–4.2 mU/L) is associated with a higher risk of progression to overt hypothyroidism and can be symptomatic in sensitive individuals.
Symptoms in the grey zone: unexplained tiredness, weight gain despite no dietary change, cold intolerance, brain fog, dry skin, constipation, low mood.
3. Vitamin D (25-hydroxyvitamin D)
NHS DEFICIENCY THRESHOLD
<25 nmol/L
GREY ZONE
25–75 nmol/L
FUNCTIONAL OPTIMAL
75–150 nmol/L
The NHS defines vitamin D deficiency as below 25 nmol/L. However, the Scientific Advisory Committee on Nutrition (SACN) 2016 report sets a “at risk” threshold of 25 nmol/L and a population target above 50 nmol/L. Levels between 25 and 75 nmol/L — which affect a significant proportion of the UK population, particularly between October and March — are associated with muscle weakness, bone pain, fatigue, impaired immune function, and low mood. This is the British vitamin D paradox: millions of people above the official “deficiency” threshold but still significantly below optimal.
Symptoms in the grey zone: general fatigue, muscle weakness and aching, low mood particularly in winter months, frequent infections, bone tenderness.
4. Vitamin B12
NHS RANGE
200–900 ng/L
GREY ZONE
200–500 ng/L
FUNCTIONAL OPTIMAL
500–900 ng/L
The standard NHS B12 test measures total serum B12 — both active (holotranscobalamin) and inactive fractions. A result of 250 ng/L is technically “normal” but neurological symptoms associated with B12 deficiency can begin at levels below 500 ng/L. The NHS acknowledges that B12 deficiency can cause neurological problems before anaemia develops, and NICE CKS notes that grey-zone results between 100–200 pmol/L (roughly 140–270 ng/L) should prompt further investigation. The standard test cannot distinguish between bound and unbound B12, so active B12 (holotranscobalamin) testing may be warranted in symptomatic patients.
Symptoms in the grey zone: numbness or tingling in hands and feet, fatigue, memory and concentration difficulties, low mood, mouth ulcers.
5. Folate (serum folate)
NHS THRESHOLD
>3 μg/L
GREY ZONE
3–8 μg/L
FUNCTIONAL OPTIMAL
>8 μg/L
Folate works in tandem with B12 for cell division and neurological function. The NHS lower cutoff of 3 μg/L identifies frank deficiency. NICE CKS guidance on iron-deficiency anaemia and B12/folate deficiency notes that grey-zone folate levels are associated with fatigue, irritability, and mood disturbance before anaemia is detectable. Women planning pregnancy and those eating restricted diets are particularly at risk.
Symptoms in the grey zone: fatigue, irritability, difficulty concentrating, mouth ulcers, sore tongue.
6. Free T4 (free thyroxine)
NHS RANGE
12–22 pmol/L
GREY ZONE
12–15 pmol/L
FUNCTIONAL OPTIMAL
15–18 pmol/L
Most GP thyroid panels include only TSH. Free T4 — the actual thyroid hormone in circulation — is often not requested unless TSH is abnormal. But a patient can have a borderline TSH of 2.8 mU/L and an FT4 of 13 pmol/L — both technically “normal” — while experiencing classic hypothyroid symptoms. British Thyroid Association guidelines acknowledge the importance of measuring free thyroid hormones, not just TSH, when symptoms persist. The BMJ review of subclinical hypothyroidism highlights that symptom burden does not always correlate with TSH levels alone.
Symptoms in the grey zone: persistent fatigue, dry skin, constipation, feeling cold, low mood, slower reflexes.
7. HbA1c (glycated haemoglobin)
NHS NORMAL
<42 mmol/mol
GREY ZONE
36–42 mmol/mol
FUNCTIONAL OPTIMAL
<33 mmol/mol
HbA1c measures average blood glucose over approximately three months. The NHS pre-diabetes threshold is 42–47 mmol/mol, and diabetes is diagnosed at 48 mmol/mol or above. But metabolic dysfunction — including post-meal blood glucose spikes, insulin resistance, and energy instability — can occur well below 42 mmol/mol. Diabetes UK acknowledges that lifestyle changes in the “at risk” range can prevent progression to pre-diabetes. Someone with HbA1c of 38–41 mmol/mol may already be experiencing the energy crashes and afternoon brain fog characteristic of early insulin resistance, despite being well within the NHS “normal” range. NICE NG28 provides guidance on preventing type 2 diabetes in high-risk individuals.
Symptoms in the grey zone: energy crashes 1–2 hours after meals, afternoon brain fog, carbohydrate cravings, difficulty losing weight, poor sleep.
8. Magnesium (serum magnesium)
NHS RANGE
0.7–1.0 mmol/L
GREY ZONE
0.7–0.85 mmol/L
FUNCTIONAL OPTIMAL
0.85–1.0 mmol/L
Serum magnesium is a poor proxy for total body magnesium — less than 1% of body magnesium is in the blood, meaning serum levels can appear normal while intracellular magnesium is depleted. The NHS notes that magnesium is essential for muscle and nerve function, blood sugar regulation, and sleep. Serum levels at the lower end of the reference range (0.7–0.85 mmol/L) are associated with muscle cramps, poor sleep quality, anxiety, and headaches, particularly in those with high-stress lifestyles or diets low in green vegetables.
Symptoms in the grey zone: muscle cramps particularly at night, difficulty falling or staying asleep, anxiety, headaches, fatigue.
9. Testosterone (men)
NHS RANGE
8–29 nmol/L
GREY ZONE
8–15 nmol/L
FUNCTIONAL OPTIMAL
15–25 nmol/L
The British Society for Sexual Medicine (BSSM) guidelines acknowledge that symptoms of testosterone deficiency can occur at levels the NHS considers “normal.” A man with total testosterone of 9 nmol/L sits within the NHS reference range but may have significant fatigue, reduced libido, poor muscle recovery, low mood, and reduced motivation. The BSSM guidelines note that clinical symptoms should always be considered alongside biochemical levels, and that some men are symptomatic at levels the standard range would consider acceptable.
Symptoms in the grey zone: persistent fatigue, reduced libido, poor recovery after exercise, reduced muscle mass, low mood, difficulty concentrating.
10. hs-CRP (high-sensitivity C-reactive protein)
NHS NORMAL
<5 mg/L
GREY ZONE (elevated risk)
1–3 mg/L
FUNCTIONAL OPTIMAL
<1 mg/L
Standard CRP tests (not high-sensitivity) typically only flag results above 5–10 mg/L, which indicates active inflammation. hs-CRP, measured with more sensitive assays, can detect low-grade chronic inflammation in the 1–3 mg/L range. The American Heart Association and Centers for Disease Control (AHA/CDC) cardiovascular risk stratification categorises hs-CRP of 1–3 mg/L as “intermediate risk” for cardiovascular events. Chronic low-grade inflammation at this level is also associated with fatigue, joint stiffness, and general unwellness that resists explanation on routine testing.
Symptoms in the grey zone: general malaise, joint stiffness (particularly morning stiffness), fatigue without clear cause, slow recovery from illness or exercise.
NHS vs Optimal Ranges — Comparison Table
The table below summarises the gap between NHS reference ranges, the grey zone where symptoms are common despite “normal” results, and functional optimal ranges referenced in the clinical literature.
| Biomarker | NHS “Normal” | Grey Zone | Functional Optimal | What You Might Feel |
|---|---|---|---|---|
| Ferritin | 12–300 µg/L | 12–50 µg/L | 50–150 µg/L | Fatigue, hair loss, brain fog |
| TSH | 0.27–4.2 mU/L | 2.5–4.2 mU/L | 0.5–2.5 mU/L | Tiredness, weight gain, cold intolerance |
| Vitamin D | >25 nmol/L | 25–75 nmol/L | 75–150 nmol/L | Muscle weakness, low mood, fatigue |
| Vitamin B12 | 200–900 ng/L | 200–500 ng/L | 500–900 ng/L | Numbness, fatigue, brain fog |
| Folate | >3 µg/L | 3–8 µg/L | >8 µg/L | Fatigue, irritability, mouth ulcers |
| Free T4 | 12–22 pmol/L | 12–15 pmol/L | 15–18 pmol/L | Fatigue, dry skin, constipation |
| HbA1c | <42 mmol/mol | 36–42 mmol/mol | <33 mmol/mol | Energy crashes, afternoon fog |
| Magnesium | 0.7–1.0 mmol/L | 0.7–0.85 mmol/L | 0.85–1.0 mmol/L | Cramps, poor sleep, anxiety |
| Testosterone (men) | 8–29 nmol/L | 8–15 nmol/L | 15–25 nmol/L | Fatigue, low libido, poor recovery |
| hs-CRP | <5 mg/L | 1–3 mg/L | <1 mg/L | Malaise, joint stiffness, fatigue |
Grey zones and optimal ranges are based on published clinical research and functional medicine literature. They are not diagnostic thresholds. Discuss your individual results with a clinician.
Why GPs Use Such Wide Reference Ranges
Understanding why NHS reference ranges are wide — and why this is not a failure of individual doctors — is important context for anyone navigating this issue.
Population-based statistics
Reference ranges are typically set at the 2.5th and 97.5th percentile of a “healthy” reference population. This means that by design, 5% of healthy people will always fall outside any given range — they are not sick, the range just doesn't accommodate all normal variation. Conversely, someone can sit at the edge of the reference range and be symptomatic, because the range reflects population averages rather than individual optimum. Some people feel best with ferritin at 100 μg/L; for others, 70 μg/L is sufficient. Reference ranges cannot capture this individual variation.
GPs are trained to exclude disease, not optimise health
This is not a criticism — it is a description of what the NHS is designed and resourced to do. General practice operates on approximately 10-minute consultations, and the clinical priority is identifying patients who require immediate investigation or intervention. The concept of “functional optimisation” — identifying and correcting borderline biomarkers to improve subjective wellbeing — falls largely outside the scope of routine NHS care. GPs are also constrained by clinical guidelines: prescribing vitamin D or iron for levels that are technically “normal” is not supported by NICE guidelines, even if those levels are associated with symptoms.
The 10-minute consultation problem
Interpreting multiple borderline biomarkers in the context of a patient's symptoms, lifestyle, and individual history requires time that NHS consultations do not routinely provide. A GP who sees that ferritin is 18 μg/L, vitamin D is 35 nmol/L, and TSH is 3.5 mU/L — all technically normal — would need significant time to discuss the cumulative significance of these findings with a patient. The system is not designed for this, and most patients leave with a single concern addressed rather than a holistic metabolic picture.
The result is that the gap between “no disease” and “actually optimal” falls, largely unaddressed, between the NHS and a healthcare system that doesn't yet exist at scale in the UK. This is the gap that comprehensive private testing and functional health interpretation is designed to fill.
The 5 Most Common “Normal Bloods, Still Unwell” Patterns
These are the five clinical patterns most frequently seen in patients who present with feeling unwell despite “normal” blood test results.
1. The iron-depleted runner
Ferritin: 20 µg/L | Hb: 125 g/L | MCV: 82 fL — all “normal”
A woman who runs regularly, often presents with fatigue on exertion, breathlessness climbing stairs, and poor post-run recovery. Her haemoglobin and MCV are normal — she is not anaemic — but her ferritin is 20 μg/L. She is in functional iron depletion: her body has enough iron to maintain red blood cells, but not enough to support optimal energy metabolism. Exercise-induced iron loss through foot-strike haemolysis, sweat, and gastrointestinal micro-bleeds is well documented. Most GP panels report her iron as normal because they look at haemoglobin, not ferritin.
2. The subclinical thyroid
TSH: 3.8 mU/L | FT4: 13.5 pmol/L — both in range
A man in his 40s with unexplained weight gain, persistent fatigue, and difficulty concentrating. His TSH of 3.8 mU/L is within the NHS upper limit of 4.2 mU/L, and his FT4 of 13.5 pmol/L is above the lower limit of 12 pmol/L. Both are technically normal. But his pituitary is working harder than average to stimulate the thyroid, and his free T4 — the actual hormone available to his cells — is at the low end of the range. The BMJ review of subclinical hypothyroidism notes that quality of life impairment can occur in the subclinical range.
3. The British vitamin D paradox
Vitamin D: 40 nmol/L — above NHS deficiency threshold
Forty nmol/L is above the NHS deficiency threshold of 25 nmol/L, so it is reported as “normal.” But the SACN 2016 vitamin D report identifies a population target above 50 nmol/L, and functional research consistently places optimal levels above 75 nmol/L. This person is not deficient by NHS definition, but is well below what many clinicians consider optimal — especially common between October and March in the UK, where 40–50% of the adult population has levels below 50 nmol/L. Muscle aching, fatigue, and low mood in winter are frequently attributed to seasonal affective disorder when low vitamin D is the underlying driver.
4. The pre-diabetic energy roller coaster
HbA1c: 39 mmol/mol — well below 42 NHS threshold
An HbA1c of 39 mmol/mol is clearly within the NHS normal range of below 42 mmol/mol. But this person experiences significant energy crashes an hour or two after eating, strong afternoon fatigue, difficulty maintaining concentration, and carbohydrate cravings. These are classic signs of post-meal glucose dysregulation and early insulin resistance — conditions that develop gradually over years and are well established before HbA1c moves into the pre-diabetes range. A fasting glucose and fasting insulin measurement (or, ideally, a continuous glucose monitor for a week) would reveal the pattern that HbA1c alone cannot capture. NICE NG28 guidance on type 2 diabetes prevention applies to those with identified risk factors even before HbA1c reaches the pre-diabetes threshold.
5. The low-B12 neurological gap
B12: 280 ng/L — above 200 ng/L NHS cutoff
A B12 of 280 ng/L is reported as normal. But the standard NHS test measures total serum B12, not the active fraction (holotranscobalamin) that is actually available to cells and tissues. Neurological symptoms associated with functional B12 deficiency — tingling in the hands and feet, memory difficulties, persistent fatigue — can begin at total B12 levels below 500 ng/L in sensitive individuals. The NHS acknowledges that neurological symptoms can precede haematological changes, meaning the standard test may miss early deficiency. Active B12 (holotranscobalamin) testing provides a more accurate picture.
What to Do If Your GP Says “Everything's Normal” — A Step-by-Step Guide
Being told your results are normal when you still feel unwell can be frustrating and isolating. Here is a practical approach to getting the information you need.
Ask for your exact numbers
You are legally entitled to your medical records under the Data Protection Act 2018 and UK GDPR. Ask your GP practice for the specific values of each biomarker tested, not just the interpretation. Online access via the NHS App gives you results directly in many GP practices. Once you have the numbers, you can compare them against the optimal ranges in the table above.
Identify which tests were actually run
A standard GP ‘blood test’ often includes a full blood count (FBC), basic metabolic panel, and sometimes CRP. It frequently does not include ferritin separately (only iron studies or FBC), vitamin D, free T4 (only TSH), vitamin B12 and folate, HbA1c, or hs-CRP. Ask your GP to confirm which markers were measured.
Request specific tests that may have been missed
If you have symptoms consistent with the grey zones described above, you can ask your GP to test for specific markers. Requests for ferritin, vitamin D, B12, and HbA1c are all clinically reasonable requests, particularly if you can describe symptoms that map to potential deficiency. Your GP may or may not agree to all of them under NHS funding — being specific about symptoms improves the chance of these being requested.
Consider a comprehensive private blood test
If your GP is unable to run the specific markers you need, or if you want a complete baseline picture of all 10 biomarkers described in this guide, a private comprehensive panel is the most efficient route. Helvy's panels include the markers most commonly missed on NHS tests — ferritin, vitamin D, free T4 alongside TSH, B12 and active B12, HbA1c, hs-CRP, and testosterone — with results interpreted in the context of optimal, not just reference, ranges.
Look at trends, not just snapshots
A single set of results is a snapshot. Retesting at 90 days allows you to see whether levels are improving (in response to dietary changes or supplementation) or declining (suggesting an underlying issue that needs attention). Trends matter more than individual readings.
Know when to push for GP referral
If your symptoms are significantly impacting your quality of life, if multiple borderline biomarkers are identified, or if you have risk factors for underlying conditions (family history of thyroid disease, autoimmune conditions, coeliac disease), this is worth returning to your GP to discuss further investigation or specialist referral.
What Helvy Tests That Your GP Probably Didn't
Helvy panels are designed around the biomarkers most commonly found in the grey zone — the markers that NHS routine testing frequently misses or measures inadequately.
Ferritin
Not just FBC/iron stores — specific ferritin measurement
Vitamin D
Full 25-hydroxyvitamin D, not estimated
Free T4 + Free T3
Not just TSH — the actual hormones your cells use
Active B12
Holotranscobalamin — the biologically available fraction
hs-CRP
High-sensitivity CRP for low-grade inflammation
HbA1c
3-month blood glucose average for metabolic insight
Total testosterone
For men — in the context of symptoms and age
Magnesium
Serum magnesium as part of complete metabolic picture
Helvy testing is complementary to NHS care — not a replacement. Results are interpreted in the context of optimal ranges and your symptoms, and our reports flag grey-zone findings that may warrant discussion with your GP or a specialist.
GP Blood Test vs Helvy — What You Get
| What You Get | GP Blood Test (NHS) | Helvy |
|---|---|---|
| Full blood count (FBC) | Yes | Yes |
| Ferritin (specific) | Sometimes | Yes — always |
| TSH | Yes | Yes |
| Free T4 + Free T3 | Rarely (only if TSH abnormal) | Yes |
| Vitamin D | Sometimes | Yes — always |
| Vitamin B12 (total) | Sometimes | Yes |
| Active B12 (holotranscobalamin) | No | Yes |
| HbA1c | If diabetes risk flagged | Yes — always |
| hs-CRP (high-sensitivity) | No (standard CRP only) | Yes |
| Testosterone | Men only, if requested | Yes (men's panels) |
| Magnesium | Only if clinical indication | Yes |
| Results interpreted vs optimal ranges | No | Yes |
| GP consultation included | Yes | No — complement to GP care |
| Results turnaround | 1–2 weeks typically | 2–5 working days |
| Trend tracking over time | Limited | Yes — dashboard tracking |
Helvy panels are not a substitute for clinical assessment. Always discuss your results with your GP or a healthcare professional.
When “Normal But Unwell” Needs Urgent GP Attention
While grey-zone biomarkers explain a great deal of unexplained unwellness, some symptoms require urgent clinical assessment regardless of blood test results. These are red flags that should prompt you to return to your GP without delay:
Seek urgent GP assessment if you have:
- !Unexplained weight loss of more than 5% of body weight in 3 months
- !Drenching night sweats not explained by menopause or temperature
- !Persistent fever or recurrent unexplained infections
- !Blood in your stool or urine
- !Progressive neurological symptoms — weakness, numbness, vision changes, coordination problems
- !Unexplained lumps or swelling in lymph nodes, neck, armpit, or groin
- !Persistent cough of more than 3 weeks without explanation
- !New or significantly changed pattern of headaches
These symptoms need investigation regardless of what previous blood tests showed. A normal blood test result from six months ago does not rule out new pathology. If you have any of these symptoms, your GP needs to assess you clinically — not just order another blood test.
Evidence-Based Next Steps for Each Grey Zone
If your results sit in the grey zones described above, there are evidence-based steps your GP may recommend or that are generally supported by NHS guidance. These are not personal recommendations — always discuss with your doctor before starting any supplementation or changing your diet significantly.
Low-normal ferritin (12–50 µg/L)
- Discuss your ferritin result and symptoms with your GP
- Dietary increases: red meat, legumes, dark leafy greens, vitamin C with iron-rich foods
- Your GP may recommend iron supplementation if symptoms are significant
- Avoid tea and coffee with iron-rich meals (tannins inhibit absorption)
- Retest ferritin at 90 days to confirm response
Borderline TSH (2.5–4.2 mU/L) with symptoms
- Request free T4 measurement alongside TSH at your next test
- Request thyroid antibody test (TPO antibodies) — positive antibodies indicate Hashimoto's
- Your GP may recommend a repeat TSH in 6–8 weeks to check for progression
- Ensure adequate selenium and iodine in your diet (Brazil nuts, seafood, dairy)
- Discuss symptom burden with your GP — some GPs will refer to endocrinology if symptoms are significant
Sub-optimal vitamin D (25–75 nmol/L)
- The NHS recommends 10 mcg (400 IU) vitamin D3 daily for all adults in autumn and winter
- Your GP may recommend higher doses (1,000–4,000 IU) if levels are significantly below 75 nmol/L
- Take vitamin D3 with K2 for optimal calcium metabolism
- Increase sun exposure when possible (arms and legs, 15–30 minutes midday, spring to autumn)
- Retest at 90 days to confirm response to supplementation
Low-normal B12 (200–500 ng/L) with symptoms
- Discuss symptoms with your GP — neurological symptoms should always be investigated
- Your GP may recommend a therapeutic trial of B12 supplementation if grey-zone result with symptoms
- Dietary sources: meat, fish, eggs, dairy (vegans and vegetarians are at higher risk)
- Consider active B12 (holotranscobalamin) testing for more accurate functional assessment
- B12 injections may be considered if absorption is suspected to be impaired
Borderline HbA1c (36–42 mmol/mol)
- NICE NG28 supports lifestyle intervention in people at risk of type 2 diabetes
- Reduce refined carbohydrate and sugar intake
- Increase physical activity — 150 minutes of moderate exercise per week
- Prioritise protein and fibre at meals to reduce post-meal glucose spikes
- Your GP may refer to the NHS Diabetes Prevention Programme if you are at risk
Low-normal magnesium (0.7–0.85 mmol/L)
- Dietary sources: dark leafy greens (spinach, kale), nuts and seeds, legumes, dark chocolate
- Your GP may suggest magnesium supplementation if symptoms are significant
- Magnesium glycinate or magnesium malate are considered better-absorbed forms
- Alcohol and high-stress states increase magnesium excretion
- Retest at 90 days if supplementing
How Often to Retest
A single blood test result is a snapshot in time. The most useful information comes from tracking trends across multiple tests — seeing whether borderline biomarkers are improving, stable, or declining over months.
START HERE
Baseline
Establish your baseline across all key biomarkers. Identifies which areas need attention.
90 DAYS
First retest
Long enough to see meaningful changes in ferritin, vitamin D, HbA1c, and B12.
EVERY 6–12 MONTHS
Maintenance
Monitor stability and catch any emerging trends before they become significant problems.
OCTOBER AND MARCH
Seasonal
For vitamin D and iron, seasonal testing catches the annual low point (October–November) and shows recovery (March–April).
Trends matter more than individual readings. A ferritin rising from 18 to 45 μg/L over 90 days of dietary improvement tells you something important. So does a ferritin falling from 55 to 30 μg/L without explanation — that needs investigation. Single snapshots, taken months apart with no baseline for comparison, provide limited actionable information.
Frequently Asked Questions
Can blood tests come back normal and you still be ill?
Yes. NHS reference ranges are designed to detect established disease, not to identify optimal function. Many people sit at the lower end of ‘normal’ ranges for biomarkers such as ferritin, vitamin D, B12, and TSH — levels that are technically in range but associated with real symptoms including fatigue, brain fog, and low mood. The gap between ‘no diagnosable disease’ and ‘actually feeling well’ is where millions of people in the UK live.
What should I ask my GP if blood tests are normal but I feel unwell?
Ask for your exact numbers, not just the interpretation. Ask which specific tests were run and whether ferritin, vitamin D, free T4, and B12 were included. Compare your results against the optimal ranges in this guide. If multiple markers are in the grey zone, ask for a follow-up discussion about the cumulative significance of borderline findings.
What blood tests should I ask for if I'm always tired?
The most commonly missed tests for fatigue are: ferritin (iron stores, not just haemoglobin), vitamin D, vitamin B12, free T4 alongside TSH, and HbA1c. A full blood count, kidney function, and liver function tests are also useful to exclude other causes. If you are a man and fatigue is severe, testosterone is worth including. If you have joint pain or a family history of autoimmune disease, an ESR and ANA screen may also be appropriate.
Can you have a thyroid problem with normal blood tests?
Yes. Subclinical hypothyroidism — where TSH is elevated but within the upper end of the NHS range (2.5–4.2 mU/L) and FT4 is at the lower end of normal — is associated with fatigue, weight gain, brain fog, and cold intolerance in some people. The BMJ review of subclinical hypothyroidism confirms that symptoms can occur without TSH exceeding the threshold for treatment. Additionally, some people feel symptomatic at TSH levels the NHS would consider normal if their FT4 and FT3 are at the low end of the range.
What is the difference between normal and optimal blood test ranges?
‘Normal’ in an NHS context means within the population-based reference range — statistically, your result falls within the range of 95% of the reference population. ‘Optimal’ refers to the level at which research suggests you are likely to feel well and support long-term health — this is typically a narrower range within the lower end of ‘normal’ for some markers (like TSH) and the upper end for others (like ferritin, vitamin D, and B12). Optimal ranges are derived from functional medicine research and clinical evidence, not diagnostic statistics.
Should I get a private blood test if my NHS results are normal?
If your NHS results are ‘normal’ but you feel consistently unwell, a comprehensive private panel can provide useful additional information — particularly if NHS testing didn't include ferritin, vitamin D, free T4, active B12, or hs-CRP. Private testing also allows results to be interpreted against optimal ranges, provides trend tracking, and doesn't require GP referral. It is complementary to NHS care, not a replacement for clinical assessment.
How often should I retest if I feel unwell but blood tests are normal?
A baseline comprehensive test is the starting point. If results show grey-zone biomarkers and you make dietary or lifestyle changes, retesting at 90 days allows enough time to see meaningful changes in ferritin, vitamin D, HbA1c, and B12. For ongoing monitoring, every 6–12 months is appropriate if results are stable. Seasonal testing for vitamin D (October and March) is particularly useful given the UK's sun exposure patterns.
KNOW YOUR NUMBERS
Know your numbers. Understand your body.
Helvy panels test the biomarkers most commonly missed by NHS routine testing — and interpret your results against optimal, not just reference, ranges. Understand what your numbers actually mean for how you feel.