The Gap Series · Blood Chemistry · Clinical Investigation
Reference ranges tell you where 95% of the tested population falls. They don't tell you where optimal function begins. The gap between those two things is where most people who have been told they're fine are actually living — and where functional investigation does its most important work.
This is part of The Gap — a series of clinical essays on what medicine misses and what functional investigation offers instead.
I've been doing this for a long time. And one of the things that never stops surprising me — even now, even after thirty-seven years — is how often the answer to "why do I feel this way?" is sitting right there in a blood test result, correctly labelled as normal, and completely missed.
Not missed because the doctor wasn't paying attention. Missed because of a foundational assumption built into the way we interpret laboratory results — an assumption so embedded in conventional medicine that most clinicians never think to question it.
The assumption is this: that normal and optimal are the same thing.
They are not.
When a laboratory establishes a reference range for a blood marker — TSH, ferritin, vitamin D, fasting glucose, anything — it does so by measuring that marker in a reference population and calculating where 95% of values fall. The outer 2.5% at each end become the boundaries of "abnormal."
This is a reasonable statistical approach. But the reference population is not a population of optimally healthy people. It's a population of people who presented for blood tests — people who had some reason to be tested. People who were already symptomatic enough to see a doctor, or who were undergoing routine screening in a context where chronic disease is common.
The result is that your "normal" TSH range is derived from a population that includes people with subclinical hypothyroidism, people on thyroid medication, people who are obese, people who are chronically stressed, and people who are simply not well. The average of an unwell population is not a target for optimal function.
It's a target for being statistically average.
The conventional reference range for TSH is typically 0.4 to 4.0 mIU/L, depending on the laboratory. A result of 3.8 is reported as normal. A result of 4.2 is flagged as abnormal and may trigger a prescription.
But TSH is a pituitary hormone, not a thyroid hormone. It's the signal the pituitary sends when it detects insufficient thyroid hormone in circulation. A high TSH means the pituitary is shouting louder because the thyroid isn't responding adequately. A TSH of 3.8 and a TSH of 1.2 are both "normal" by conventional standards — but they represent very different physiological states.
Research has consistently shown that TSH values in the upper half of the conventional normal range are associated with symptoms of hypothyroidism — fatigue, cold intolerance, weight gain, hair loss, cognitive slowing, low mood — and that many patients feel significantly better when TSH is maintained between 1.0 and 2.0. The American Association of Clinical Endocrinologists proposed in 2003 that the upper limit of normal should be revised to 2.5. The proposal was not adopted.
A woman with a TSH of 3.6, fatigue she can't explain, hair falling out in the shower, and a resting body temperature that never quite reaches 37 degrees will be told her thyroid is normal. She may be told to eat less, sleep more, and consider whether she might be depressed.
The conventional lower limit of the ferritin reference range is typically around 12–15 µg/L for women. A ferritin of 14 is normal. Technically. Statistically.
But clinical research, and consistent clinical observation across thirty-seven years, suggests that symptoms of iron insufficiency — fatigue, brain fog, poor exercise tolerance, restless legs, hair loss — often persist until ferritin is above 50–70 µg/L, and that many women don't feel well until ferritin is in the 80–100 range.
The gap between "not anaemic" and "iron-replete enough to function optimally" can span 60 or 70 points on a scale where everything above 12 is labelled normal. That gap is where millions of women are living. Tired, foggy, losing hair, being told their bloods are fine.
The NHS defines vitamin D deficiency as below 25 nmol/L. Insufficiency as 25–50 nmol/L. "Adequate" as above 50 nmol/L.
But research on vitamin D's role in immune function, mood regulation, cancer prevention, cardiovascular health, and musculoskeletal function suggests that optimal circulating levels are considerably higher — likely in the 100–150 nmol/L range for most people.
A level of 52 nmol/L is technically adequate by NHS standards. It is also, by the evidence on biological function, suboptimal for a significant proportion of people. In Scotland, where I practice, meaningful sun exposure is limited to perhaps four months of the year under good conditions. A population-level vitamin D insufficiency — functional rather than deficient by conventional definition — is not an edge case. It's the norm.
The difference between "normal" and "optimal" is the difference between a person being told they're fine and a person being given an accurate map of what's actually happening in their body.
The functional approach to blood chemistry doesn't replace conventional reference ranges — it contextualises them. A result of 3.8 for TSH and a result of 1.2 for TSH are both normal. But they are not the same, and they do not deserve the same response.
What I'm looking for when I review blood chemistry is not whether values fall within the statistical range of a reference population. I'm looking for where values sit within the range that supports optimal biological function — and I'm looking at the pattern of values together, not each marker in isolation.
Because biochemistry is not a list of independent variables. TSH doesn't exist in isolation from ferritin, which doesn't exist in isolation from cortisol, which doesn't exist in isolation from vitamin D, which doesn't exist in isolation from gut absorption, which doesn't exist in isolation from what someone is eating, how they're sleeping, and what kind of stress load they're carrying. The body is a system. The blood test is a snapshot of that system at a moment in time. Reading it well requires understanding both.
If you've been told your bloods are normal and you still don't feel well, the question worth asking is not "what's wrong with me?" It's "where within the normal range do my results actually sit — and is that range derived from an optimally healthy population?"
The Randox blood chemistry panel we use as part of the TDG programme measures over 140 markers — not because more is always better, but because the pattern matters. A TSH of 3.6 means something different when ferritin is 14 and vitamin D is 48 than it does when those markers are optimal. Context is everything.
And "normal" is not a context. It's a statistical artefact. Don't mistake it for a destination.
Blood Chemistry Options · TDG Five-Test Programme · Talk to the Clinical Concierge
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