How optimal blood test ranges work: Thriva’s approach

Standard reference ranges tell you whether your levels fall within the range seen in 95% of the general population. 

While it can help you spot clear abnormalities, it doesn’t tell you much about where your levels sit in relation to long-term health and longevity. That’s where optimal ranges come in.

In this article, we explain what optimal ranges are and how we use them at Thriva.

What are normal blood test reference ranges?

Normal reference ranges are the values used to interpret your blood test results. They’re how your GP decides whether something is “normal” or “abnormal.” These ranges are typically based on where 95% of a sample population falls.

Some ranges are set based on evidence linking specific levels to disease risk. Cholesterol reference ranges, for example, are influenced by their association with heart disease.

Why “normal” blood test results aren’t always enough

There are a few reasons why a “normal” result doesn’t always mean you’re in the clear.

Reference ranges don’t always account for the relationship between specific levels and disease risk or life expectancy. Your vitamin D might be “normal” at 55 nmol/L, but the evidence linking higher levels to reduced all-cause mortality suggests you’d benefit from being higher.

The populations used to set these ranges aren’t always ideal either. They can include people who aren’t entirely healthy, or who are at higher risk of future disease. 

For example, a 2007 study found that standard TSH reference ranges were likely skewed by undiagnosed thyroid dysfunction, meaning the “normal” range was wider than it should have been.

And the research behind some ranges relies on small sample sizes, which means they may not reflect where you’d actually want to be.

How do optimal blood test ranges differ?

Optimal ranges go a step further and focus on the reference ranges associated with better health outcomes. Rather than just flagging whether you’re within a “normal” population range, they’re based on evidence linking specific levels of a biomarker to a reduced risk of disease or improved life expectancy.

At Thriva, our clinical team systematically reviews the latest scientific literature and clinical guidelines to identify these ranges. They’re continually updated as new evidence emerges.

Optimal vs normal ranges: Vitamin D, thyroid, and cholesterol

Vitamin D

Traditional guidelines suggest that levels below 25 nmol/L indicate deficiency, and levels above 50 nmol/L are considered sufficient. That range is based primarily on the relationship between vitamin D and bone health.

But vitamin D is also important for muscle and immune function. A 2013 analysis of US national health data found that the lowest risk of all-cause mortality was associated with levels between 75 and 100 nmol/L, which is well above the “sufficient” threshold. 

A 2025 review of dose-response meta-analyses confirmed this range, finding the lowest risk for most health outcomes at 25(OH)D levels between approximately 50 and 100 nmol/L.

TSH (thyroid function)

TSH, the hormone that regulates thyroid function, has a standard reference range of roughly 0.4–4.0 mIU/L in most UK labs. However, a 2023 meta-analysis of over 134,000 people, published in The Lancet Diabetes & Endocrinology, set out to define the optimal healthy ranges of thyroid function. They based this on cardiovascular disease and mortality risk rather than just statistical norms.

If your TSH is 3.5, your doctor will likely say it’s normal. But outcome-based evidence from large populations suggests it may be worth paying attention to, particularly if you have symptoms like fatigue or weight changes.

Cholesterol

Standard cholesterol reference ranges are primarily designed to flag cardiovascular risk. But a 2019 prospective cohort study of 12.8 million Korean adults found a U-shaped association between total cholesterol and all-cause mortality, meaning both very low and very high levels were linked to higher mortality. The relationship also varied significantly by age and sex.

This is a good example of why optimal ranges need to account for more than a single disease outcome.

A few things to keep in mind

Optimal ranges are associated with better health outcomes, but that doesn’t mean pushing every biomarker into the optimal zone will automatically improve your health. The relationship between levels and outcomes can be more complex.

What’s optimal for one person may depend on their age, sex, existing conditions, and lifestyle.

That’s why we don’t suggest targeting optimal levels for every biomarker as a goal in itself. They’re better used as guides. They can be helpful for identifying areas worth paying attention to alongside your overall health and lifestyle. 

Our doctors can help you interpret your results in context and work out what actually matters for you.

Takeaway

Normal reference ranges are useful for spotting clear abnormalities, but they don’t always give you the full picture. 

Optimal ranges offer a more refined view, showing you where your results sit in relation to the latest evidence on disease risk and longevity. Think of them as a more ambitious benchmark, not a rigid target.

Frequently asked questions

What does “normal” mean on a blood test?

A “normal” result means your levels fall within the reference range used by the lab, which is typically the range where 95% of the general population sits. It rules out obvious abnormalities but doesn’t necessarily mean your levels are ideal for long-term health.

Are optimal ranges the same for everyone?

Not always. While optimal ranges are based on population-level evidence, what’s optimal for you can depend on your age, sex, health history, and lifestyle. They’re a useful starting point, but interpreting them in context is important.

Should I worry if my results are normal but not optimal?

Not necessarily. Optimal ranges highlight areas where there may be room for improvement, but a result in the normal range isn’t cause for concern on its own. If something is consistently at the edge of a range or you have symptoms, it’s worth discussing with a doctor.

How does Thriva calculate optimal ranges?

Our clinical team reviews the latest scientific literature and clinical guidelines to identify ranges associated with the lowest risk of disease and improved life expectancy. These ranges are continually updated as new evidence becomes available.