My dad had a heart attack at 57. When they ran all the standard tests, his cholesterol was 4.0 mmol/L — low, by any measure. The kind of cholesterol level the guidelines would celebrate. And yet: heart attack. Triple bypass. His low cholesterol was never discussed. Never questioned. Never examined as potentially problematic.
What was also never discussed: his chronic dehydration, his mineral status, his stress hormone levels, his homocysteine. Nobody ran those tests. He had "normal" cholesterol, so the metabolic picture was assumed to be fine. It wasn't.
That moment — watching a man with excellent cholesterol numbers go through triple bypass surgery — is part of why I've spent 37 years questioning what "normal" actually means in clinical practice.
The problem with normal ranges
When your GP runs a blood panel and reports that your results are "normal," what they mean is that your results fall within the reference range for that marker. That reference range is derived from the general population — specifically, from the results of a large group of people who had that test run.
Here's the catch: that population includes people who are unwell. It includes people in the early stages of disease who haven't been diagnosed yet. It includes people who are metabolically suboptimal but not yet sick enough to flag. The "normal" range is built from a snapshot of the population as it currently exists — which, in the context of modern chronic disease rates, is not a particularly healthy population.
A ferritin of 15 ng/mL is technically within the normal range at most UK laboratories. It is also low enough to cause significant fatigue, impair thyroid hormone conversion, and compromise immune function. Your body is not optimal at ferritin 15. It is surviving at ferritin 15. There's a meaningful difference, and standard ranges don't make that distinction.
Normal vs optimal — the gap where symptoms live
Functional medicine uses a different set of reference points — not based on the sick population, but based on research into where markers should sit for optimal physiological function. These aren't invented numbers. They come from decades of research into where people feel well, where disease risk is lowest, and where the body's systems run most efficiently.
The gap between "normal" and "optimal" is substantial for many markers. And the symptoms you're experiencing — the fatigue, the brain fog, the weight that won't shift, the mood fluctuations, the gut problems — frequently live in that gap.
The five tests that aren't on a standard GP panel
The standard NHS blood panel — even a reasonably comprehensive one — typically checks haemoglobin, white cell count, kidney function, liver function, thyroid (usually TSH only), and cholesterol. It's a disease screen. It's designed to catch things that need immediate medical intervention.
What it doesn't check: fasting insulin (which rises 10 years before fasting glucose does), Free T3 (the active thyroid hormone, as opposed to TSH which is the pituitary signal), ferritin with full iron panel, homocysteine, magnesium, zinc, the diurnal cortisol curve, gut pathogen load, neurotransmitter metabolites, mitochondrial function markers. None of these are exotic tests. They're simply not part of the disease-detection framework.
Functional medicine runs these markers because it's asking a different question. Not "is this person sick?" but "how well is this person's biology functioning, and where are the gaps?"
Why your GP isn't to blame
I want to be clear about this, because it matters. GPs are working within a system that is designed for disease management, not optimal health. They have 10-minute appointments, NHS reference ranges, and clinical guidelines that govern what they can test and prescribe. Most GPs I know are thoughtful, caring professionals who are doing their best within significant structural constraints.
The gap isn't usually incompetence. It's a difference in what the system is built to look for. Conventional medicine is designed to answer "does this person have a diagnosable disease?" Functional medicine is designed to answer "why does this person feel the way they feel, and what does their biology actually need?"
Those are different questions. They require different tools.
A client came to me having spent over £5,000 on supplements over three years. Her cupboard looked like a pharmacy. She'd tried everything recommended by various practitioners, online protocols, and well-meaning friends. Her GP had run standard bloods twice. Normal. She'd been told her thyroid was fine, her iron was fine, her cholesterol was fine.
We ran comprehensive testing. Found H. pylori infection she didn't know she had. Found ferritin of 14 — technically normal, clinically inadequate. Found flat diurnal cortisol indicating HPA axis exhaustion. Found food sensitivity to gluten and dairy she'd been consuming daily. We addressed each in sequence. Within four months, her energy had returned. Her brain fog cleared. The £5,000 of supplements hadn't worked because nobody had identified what actually needed to be addressed.
What "comprehensive" testing actually looks like
The TDG (Test, Don't Guess) programme runs five tests simultaneously: advanced blood chemistry using functional reference ranges across 150+ markers, GI-MAP DNA-based stool analysis, DUTCH Plus comprehensive hormone testing, Organic Acids Test (which gives a cellular energy and neurotransmitter metabolite picture), and food sensitivity panel covering 200+ foods.
These five tests, read together and cross-referenced against each other, tell a story that no individual test can tell alone. Low ferritin on blood chemistry looks different in the context of H. pylori infection on GI-MAP (the infection is causing the deficiency). Flat cortisol on DUTCH looks different alongside elevated Krebs cycle markers on OAT (the mitochondria are struggling in the context of adrenal insufficiency). Elevated quinolinic acid on OAT plus low serotonin metabolites makes sense when the GI-MAP shows significant dysbiosis (the gut can't synthesise adequate serotonin precursors).
The cross-referencing is where the clinical picture emerges. A single test gives data. Five tests give answers.
The question to ask your GP
If you've been told your tests are normal and you're not satisfied with that answer, there are specific questions worth asking. What reference ranges are you using, and are they population-based or research-based optimal ranges? Has fasting insulin been tested, or just fasting glucose? Has Free T3 been tested alongside TSH? What is the actual ferritin number — not just "normal," but the number?
You may find your GP is helpful with some of these requests. You may find the system isn't set up to accommodate them. Either way, knowing the difference between normal and optimal is the starting point for making sense of your own health data.
The bottom line
"Your tests are normal" means your results fall within the range of a population that includes a lot of unwell people. It means you don't have a diagnosable disease on the markers tested. It doesn't mean your biology is functioning optimally. It doesn't mean there's nothing to find. And it definitely doesn't mean you should feel the way you feel.
After 37 years of clinical practice — including running comprehensive testing on thousands of clients whose standard tests were "normal" — I've yet to encounter a case of persistent, significant symptoms where comprehensive functional testing found absolutely nothing. The symptoms are messages. Testing is how you read them.