What Your GP Won't Explain But Dr Becky Will: The Truth About "Normal" Blood Test Ranges
You've had the blood test. The letter or app notification says everything is "normal." And yet you're still tired by 3pm, still not sleeping properly, still not quite yourself. If that sounds familiar, it isn't a sign that anything was done wrong , it's a sign that GP testing and personalised testing are simply designed to answer different questions. Understanding that difference starts with what "normal" actually means on a lab report.
"Normal" is a statistic, not a health target
When a laboratory sets the reference range printed next to your result, it isn't measuring what's optimal for you. It's measuring where the middle 95% of a local reference population happens to sit. By definition, that means the range is built to include people who are unwell, borderline, or simply unlike you in age, sex, lifestyle, or ethnicity , as long as they fall within the statistical middle. Two different labs, using two different local populations, can and do publish two different "normal" ranges for the same test.
This isn't a controversial claim , it's how reference ranges are built by design. Laboratory guidance is explicit that a value inside the range doesn't guarantee good health, and a value outside it doesn't guarantee disease; the range is a statistical tool, not a diagnostic verdict.
A real example: thyroid function
Take TSH (thyroid-stimulating hormone), one of the most commonly requested GP tests. The standard reference range most UK labs use is roughly 0.4–4.0 mIU/L. If your result falls anywhere in that range, it will be reported as normal.
But NICE guidance itself recommends that for most people being treated for an underactive thyroid, TSH should be controlled to the lower half of that range , around 0.4–2.5 mIU/L , because that's where thyroid function tends to work best. In other words, a reported "normal" result of 3.8 mIU/L is genuinely, correctly normal by GP screening standards, while also sitting close to the point where guidelines suggest thyroid function may already be under strain. Both things are true at once , it simply depends on what question is being asked of the result.
What this means for you're 40+ blood test
From your early 40s onwards, the body changes faster than most annual check-ups are designed to track: hormones shift, metabolism slows, nutrient absorption becomes less efficient, and recovery takes longer. A general practice blood panel does an important job well , it's built to flag disease and catch red flags early, using a wide, population-based range, and that safety-net function matters enormously. What it isn't designed to do is explain why you feel flat, foggy, or simply "off" while every marker still lands inside that range , because that was never its purpose.
That's where the kind of testing I do at Anew Health adds something different, rather than something better. Alongside your GP's screening, I look at where each marker sits within the range, how your markers relate to one another, and what that pattern means for how you actually function day to day , not just whether you're statistically average for your area.
Feeling unwell while being told you're "fine" isn't in your head. It usually means the two of you are looking at the same result through two different, equally valid lenses, one built for screening, one built for optimisation.
If you're 40 or over and your GP results keep coming back "normal" while you don't feel it, a deeper, personalised look at your blood work is a sensible next step alongside your regular GP care.
Dr Becky Coates Owner, Anew Health
P.s If you’re unsure on where to start, Anew Health covers a range of options that can help you start to feel more YOU again.
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