There is a question I ask almost every new client, usually within the first ten minutes of a consultation. "Have you ever been told your test results are normal?" The answer is almost always yes. Sometimes it's been said by three different GPs over five years. Sometimes it was followed by: "so there's nothing wrong." And sometimes the client had just given up asking.
I then ask them to tell me what normal means. And that question — which sounds obvious — is usually where the conversation starts to get somewhere useful. Because most people have no idea. They assume normal means what it sounds like it means: fine. Healthy. Within the range you should be in. Operating as intended.
It doesn't mean any of those things.
Normal, in the context of a lab reference range, means statistically common in the population used to derive the range. That is a very specific, very limited claim — and it is substantially different from saying you are functioning optimally.
How reference ranges are built
To understand the problem you need to understand how the reference range gets onto the page in the first place. The process is roughly this: a laboratory takes a large sample of people, measures a specific marker, plots the distribution of results, and then draws lines at the 2.5th and 97.5th percentile. Everything between those lines is "normal." Everything outside is flagged.
This is a statistical exercise. It describes what is common. It says nothing whatsoever about what is optimal for function, for energy production, for hormone synthesis, for immune regulation, or for any of the other things you might actually care about.
There are two further problems with the populations used. First, the people who have their blood drawn at a GP surgery or hospital are not a random sample of healthy humans — they are, by definition, a population with health concerns. The reference ranges for many markers are therefore derived from people who are already unwell to varying degrees. Second, most reference ranges are not age-stratified or sex-stratified in the way clinical reality demands. The ferritin range for a 22-year-old athlete and a 68-year-old woman post-menopause is not the same clinical question — but the same number might appear on both their reports with the same green tick.
"A reference range tells you whether your result is statistically common. A functional optimal range tells you whether your result supports the physiology you're trying to achieve. These are different questions."
What functional optimal ranges actually represent
Functional medicine uses different thresholds — not to be contrarian, but because the research supports them. Functional optimal ranges are derived from studies that correlate specific marker levels with specific physiological outcomes: symptom resolution, disease prevention, tissue function, hormonal capacity. They ask a different question than the lab reference range does.
The lab asks: is this result within the statistically common range?
The functional range asks: is this result high enough — or low enough — for the physiology to work properly?
These are not always the same answer. In fact, for several of the most clinically important markers, they diverge significantly. Here is a comparison of conventional reference ranges versus functional optimal ranges for some of the markers I assess routinely:
| Marker | Conventional "Normal" | Functional Optimal | The gap |
|---|---|---|---|
| Ferritin (women) | >12 µg/L | 70–100 µg/L | Up to 88 µg/L |
| Vitamin D (25-OH) | >50 nmol/L | 100–150 nmol/L | Up to 100 nmol/L |
| TSH (thyroid) | 0.4–4.5 mIU/L | 1.0–2.0 mIU/L | Range 3× narrower |
| Fasting glucose | <5.6 mmol/L | <5.0 mmol/L | Pre-resistance missed |
| Homocysteine | <15 µmol/L | <7.0 µmol/L | Up to 8 µmol/L |
| hs-CRP (inflammation) | <5.0 mg/L | <1.0 mg/L | 5× more sensitive |
| Free T3 (thyroid) | 3.1–6.8 pmol/L | 5.0–7.0 pmol/L | Lower half = symptoms |
Every result in the "gap" column represents a clinical zone where the conventional range says nothing is wrong — while the research literature and clinical experience both say something is consistently wrong. That zone is where a significant proportion of persistent, unexplained symptoms live.
The ferritin story — in detail
I have written about ferritin specifically in another piece, but it is worth revisiting here because it illustrates the problem so precisely. The conventional lower threshold for ferritin — the point at which the lab flags a result as low — sits around 12–15 µg/L in most UK laboratories. The functional optimal range for women is 70–100 µg/L. The gap between 12 and 70 is enormous in physiological terms.
Ferritin is not just an iron storage marker. It is a cofactor in thyroid hormone conversion, a prerequisite for normal dopamine synthesis, and — at very low levels — an acute phase reactant that can actually mask iron deficiency by rising in the presence of inflammation. A woman with a ferritin of 18 µg/L, experiencing fatigue, hair loss, brain fog, and poor thyroid conversion, will receive a normal result on a standard iron panel. Her ferritin is technically above the lab's threshold. It is nowhere near the level at which the physiology runs well.
This is not a fringe position. The research correlating functional ferritin levels with symptom resolution is extensive. It is simply not reflected in the reference range, which was built for a different purpose.
TSH — the thyroid problem in miniature
The conventional TSH range spans 0.4 to 4.5 mIU/L. That is a tenfold variation between the low end and the high end — treated by standard reporting as equally "normal." A TSH of 0.5 and a TSH of 4.2 sit in the same green zone on most lab reports. Clinically, they represent significantly different thyroid states.
Research — including work from the British Thyroid Foundation — suggests that optimal TSH for most people sits between 1.0 and 2.0 mIU/L. A result of 3.8 is technically "normal" but is associated with increased hypothyroid symptoms, reduced Free T3 conversion, elevated cholesterol, and impaired energy metabolism in a significant proportion of the population. The patient with TSH of 3.8 will be told there is nothing wrong with their thyroid. They may live with that answer for years.
This matters particularly because TSH is often the only thyroid marker run in primary care. It is a pituitary output measure — an indirect signal about thyroid demand. The actual thyroid hormones — Free T4 and Free T3 — are frequently not tested at all. Free T3, the active form that enters cells and drives metabolism, is the most clinically relevant thyroid marker for most symptoms. It is also the least likely to be on the standard panel.
Inflammation — the threshold problem
High-sensitivity CRP measures systemic inflammation. The conventional upper limit is typically 5.0 mg/L. The functional optimal is below 1.0 mg/L. A result of 2.4 mg/L sits in what cardiologists call the "intermediate cardiovascular risk zone" — meaningfully elevated, associated with increased all-cause mortality in longitudinal studies, and implicated in driving thyroid conversion failure, insulin resistance, and HPA axis disruption simultaneously.
A result of 2.4 mg/L passes through a standard lab report entirely unremarked. It is below the conventional threshold. It is not normal in any functional sense.
The most common clinical pattern I see is a client who has had repeated normal results, is on no medication, and has been told their health is fine. When I look at their results through a functional lens, I typically find: ferritin in the 15–40 range, vitamin D between 55–75 nmol/L, TSH between 2.5–4.0, hs-CRP between 1.5–3.5, and homocysteine between 9–13. None of these are conventionally flagged. All of them represent meaningful functional insufficiency. The cumulative effect — across thyroid conversion, energy production, inflammation, and methylation — is often exactly the clinical picture the client is describing.
The tests were right. The interpretation framework was the problem.
What to do with this
If you have blood results that have been dismissed as normal but your symptoms tell a different story, the first and most useful thing you can do is look at the actual numbers rather than the green ticks. A result can be within the lab's conventional range and simultaneously below the functional threshold for normal physiology. These are not contradictory findings. They are two different answers to two different questions.
The second thing worth knowing is that the markers most commonly involved in this gap — ferritin, vitamin D, thyroid hormones, hs-CRP, homocysteine, fasting insulin — are all measurable, all correctable where deficient, and all more informative when interpreted together than in isolation. A low ferritin in the context of elevated hs-CRP and a TSH of 3.5 is a coherent clinical story. Each marker alone might pass unremarked. Together they point in a clear direction.
The third thing is that getting those markers properly interpreted — against functional optimal ranges, in the context of your symptoms and history — is what the Blood Chemistry Health Audit does. You send your results. I interpret them the way they should be interpreted. Seven days. Written report.
The information is usually already there. It just hasn't been read properly yet.
Have results that came back normal when you know something isn't right?
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