Optimal vs Conventional Reference Ranges: What "Normal" on Your Blood Test Really Means
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You feel exhausted, your hair is shedding, and your blood test comes back "normal." Or your ferritin is 15, and nobody mentions it because it sits just inside the lab's range. Both situations are common, and both come down to the same question: what does "normal" on a lab report actually mean? In this guide I'll explain how reference ranges are made, what optimal ranges are, where they're genuinely useful and where the evidence is thinner, using two examples I see all the time: ferritin and TSH.
Reference range vs optimal range: A reference range is the span of results a lab expects in most apparently healthy people, usually the middle 95%. It's a statistical description, not a target. An optimal range is a narrower band that some practitioners, especially in functional medicine, use to flag results drifting away from ideal before they cross the lab's line. Optimal ranges aren't standardized, so they work best as an extra lens alongside your lab's range and your doctor's interpretation, not as a replacement.
How reference ranges are made
Most reference ranges are built the same way. A lab tests a group of apparently healthy people, classically at least 120, and takes the middle 95% of their results as the range.1 Labs can also adopt published ranges from other labs or manufacturers, or use consensus limits set by expert bodies such as the American Diabetes Association.2
That method has three consequences most patients never hear about:
- One in 20 healthy people falls outside the range by design. Because the range covers 95% of a healthy group, 5% of healthy people will have an "abnormal" result on any single test.1
- Ranges vary between labs and regions. Test methods, equipment, the population sampled and even geography all shift the numbers. Studies of healthy people in six Asian cities found real biological differences between cities, which is one reason universal ranges are hard to set.1,2
- Some ranges aren't based on healthy averages at all. Cholesterol is the classic example. The 97.5th percentile in the general population sits around 280 to 300 mg/dL, but the recommended upper limit of 200 mg/dL is roughly the population median. It was chosen because of cardiovascular outcomes, not because it's typical.1 Experts call these decision limits, and they're the closest thing conventional medicine has to an "optimal" range.
What "normal" can't tell you
A reference range is a frame of reference for comparison, which is why the word "normal" is slowly being replaced by "reference."1 Guidance for providers from the American Society for Clinical Laboratory Science makes three points worth knowing as a patient:2
- A result inside the range may not rule out disease, especially early on.
- A result outside the range may not mean disease.
- A result that shifts within the range may be an early sign of a problem.
That last point is the one most people miss, and it's why tracking your results over time matters so much. A marker that has crept up steadily for five years tells a very different story from one that has sat in the same place, even if both are technically "normal." I cover how to set that up in my guide to keeping a lab results tracker.
Reference ranges also only go so far in accounting for you. Many labs report ranges by sex, and some by age, and certain results are calculated with age built in, like eGFR for kidney function. But a range can't adjust for your diet, a medication you take or a condition you live with.1
What are optimal ranges?
Optimal ranges, sometimes called functional ranges, are narrower bands meant to reflect the levels associated with good health and function, rather than simply the absence of disease.3 The idea is that a result drifting toward the edge of the reference range is worth a conversation before it crosses the line.
When I trained as a Nutritional Therapist, Dr. Dicken Weatherby's book Blood Chemistry and CBC Analysis was our go-to reference for functional ranges. His company, Optimal DX, now builds those ranges into software used by practitioners. Just like labs, functional practitioners don't all agree on optimal ranges. What I value about Weatherby's work is that Optimal DX explains how its ranges are derived, references the research behind them and updates them as new evidence comes in. I've since completed the Optimal DX Academy and use their software in my own bloodwork reviews. I'm a paying customer: Optimal DX doesn't pay me or sponsor this article.
Three things are important to understand from the start:
- Optimal ranges aren't standardized. Different practitioners, books and software use different numbers for the same marker, so it's worth knowing where a range comes from and whether it's referenced.
- Optimal ranges aren't individualized, but their interpretation should be. The same result can mean different things for different people depending on symptoms, history, age and medications. That's the practitioner's job.
- No single marker is diagnostic. No one biomarker should be treated as a stand-alone diagnostic test. Each needs to be read as part of a pattern when evaluating how the body is functioning and the risk of disease, and ideally over time, looking for trends toward or away from dysfunction. Optimal DX makes the same point: functional blood chemistry analysis isn't diagnostic.4
Used that way, as context rather than a verdict, optimal ranges can be genuinely helpful. Two examples show both their value and their limits.
Example: ferritin
Ferritin reflects your iron stores, and it's one of the clearest examples of a range that has been too wide. For years, a ferritin of 15 ng/mL was widely treated as the cutoff for iron deficiency. That threshold, still used by the WHO for non-pregnant women, comes from expert opinion based on lab methods used decades ago.5 So a menstruating woman with a ferritin of 15 to 25, tired and losing hair, was often told her iron was fine.
That changed in September 2026. The American Society of Hematology's new guideline recommends diagnosing iron deficiency at a ferritin of 30 ng/mL or lower in adults, including menstruating and pregnant individuals, and 50 ng/mL or lower in high-risk groups such as people with heavy periods.6 ASH notes that some people previously told they weren't iron deficient may now meet the criteria, especially if their ferritin was close to the old cutoff.6 For people with inflammation, where ferritin rises regardless of iron stores, the thresholds are higher again.7
Some functional practitioners aim higher still, often around 50 to 70 ng/mL for menstruating women. There's some support for going beyond the bare minimum: hematologists writing about bleeding disorders consider a ferritin goal above 50 ng/mL prudent for anyone who menstruates.8 One study found that a ferritin of 70 to 79 ng/mL was associated with the lowest mortality and lower levels of inflammatory markers.9 As an observational study, it shows a link rather than cause, and too much iron is harmful too. The practical takeaway: a ferritin of 15 isn't fine, and it's worth asking your doctor about the new thresholds if yours is under 30, or under 50 with heavy periods.
Example: TSH
TSH, the thyroid-stimulating hormone, shows the opposite problem: the "right" upper limit is genuinely contested. Most labs set it somewhere around 4 to 5 mIU/L, depending on the lab and population.10 Some experts have proposed lowering it to 2.5, because most people without thyroid disease sit below that.11 Optimal DX goes further, using 1.0 to 2.0 mIU/L as its optimal range.12
Supporters of a narrower range point to studies like these:
- In the Whickham Survey, which followed adults in northern England for 20 years, a TSH above 2 mIU/L was linked to a higher chance of later developing hypothyroidism, especially when thyroid antibodies were present.13
- In a screening study of 24,765 adults, people with TSH in the upper part of the normal range had more body fat, higher blood pressure, blood fats and insulin than those with a TSH between 0.47 and 1.48.14
- In US national survey data, all-cause mortality was lowest with a TSH of roughly 1.2 to 1.95, with higher risk at both the low-normal and high-normal ends.15
These are association studies: they show a link, not proof that lowering TSH into a narrower range improves outcomes.
Others push back. TSH naturally rises with age: in one large US survey, the 97.5th percentile was 3.56 mIU/L for people in their twenties but 7.49 for people in their eighties.10 Lowering the upper limit to 3.0 when screening older adults with vague symptoms has been estimated to label an extra 22 to 28 million Americans as having subclinical hypothyroidism.11 The French Endocrine Society has gone the other way for older patients, suggesting an upper limit of age divided by 10 for people over 60, so 8 mIU/L at age 80.11
So for TSH, there's no single optimal number that fits everyone. A TSH of 3.8 could deserve a closer look in a 30-year-old with symptoms, and be perfectly appropriate in an 80-year-old. This is exactly where the trend over time, your symptoms and your other thyroid markers matter more than any one range.
Where optimal ranges help, and where to be careful
| Where they help | Where to be careful |
|---|---|
| Prompting an earlier conversation when a result drifts toward the edge of the range | Numbers vary between sources, so always note where an optimal range comes from |
| Giving you a goal to work toward with diet and lifestyle changes | A result outside an optimal range isn't a diagnosis, and shouldn't cause alarm on its own |
| Catching cases where the reference range has lagged behind the evidence, as with ferritin | Chasing an optimal number with supplements can cause harm, as with iron |
| Making trends easier to spot when tracked over time | One range rarely fits everyone, as TSH and age show |
How to use optimal ranges yourself
- Always record your lab's reference range. It's what your doctor will use, and it's the baseline everything else builds on.
- Add an optimal range as a second layer, and note where it comes from: a guideline, your practitioner or a specific source.
- Use the same lab where you can. Repeat testing at one lab makes your results more comparable over time.4
- Watch the trend, not just the latest number. A steady drift toward or away from your targets tells you more than any single result.
- Discuss it with a professional. Bring your results and the ranges you're using, especially before starting supplements.
Track both ranges in one place
My Blood Test Tracker Plus Optimal and Plus Charts & Optimal let you record your lab's reference range and your own optimal range for each of 450+ biomarkers in Excel or Google Sheets. Results are color-coded against both, and since version 6.0, results sitting at the very edge of the conventional range are flagged as borderline too, so you can see at a glance where a marker sits and how it's moving over time. The ranges are left blank on purpose, so you can use the ones from your lab reports and the sources you and your practitioner trust.
Want a practitioner's eyes on your results?
In my Functional Bloodwork Audit, I review your recent blood tests through a functional lens, looking at patterns across markers rather than single results, and send you a prioritized plan. My calendar is on a short pause right now, but you can join the waitlist on that page and I'll be in touch as soon as spots reopen.
Frequently asked questions
Are optimal ranges evidence-based?
It depends on the source and the marker. Some optimal ranges draw on outcome research, for example studies linking fasting glucose below about 90 mg/dL with the lowest long-term cardiovascular risk.16 Others rely more on clinical experience. Because optimal ranges aren't standardized, look for sources that explain and reference how each range was set, and update it as new research appears.
Why is my lab's range different from someone else's?
Each lab sets or verifies its own reference ranges based on its test methods, equipment and population, and ranges also vary by region, age and sex. That's why it's important to record the range printed on each of your reports, and to compare results from the same lab where possible.
If my result is in the normal range, does that mean I'm healthy?
Not necessarily, and the reverse is also true. A result inside the reference range doesn't rule out a problem, especially early on, and a result outside it doesn't always mean disease. A shift within the range over time can be an early sign worth discussing, which is why tracking your results is so useful.
Should I ask my doctor to use optimal ranges?
You can certainly raise them, especially if a result has been trending in one direction or sits near the edge of the range and you have symptoms. Frame it as a question rather than a demand, bring your results over time, and be open to your doctor's reasons. A practitioner trained in functional blood chemistry can also help you interpret optimal ranges in context.
The bottom line
Reference ranges tell you how your result compares with most healthy people at one lab. They don't tell you whether your result is ideal for you, and they don't show where it's heading. Optimal ranges can fill some of that gap, as long as you treat them as context rather than a verdict, know where they come from, and look at the trend over time. Ferritin shows how much a range can matter. TSH shows why no single range fits everyone.
References (16)
- 1ReviewBoyd JC. Defining laboratory reference values and decision limits: populations, intervals, and interpretations. Asian J Androl. 2010;12(1):83-90. Source ↗
- 2Professional bodyAmerican Society for Clinical Laboratory Science (ASCLS). Laboratory patient safety tips: using a laboratory reference interval (range) for result interpretation. Information for providers. 2019. Source ↗
- 3IndustryOptimal DX. The optimal range. Source ↗
- 4IndustryOptimal DX. Optimal takeaways for optimal ranges. Source ↗
- 5Population studyComparison of current World Health Organization guidelines with physiologically based serum ferritin thresholds for iron deficiency in healthy young children and nonpregnant women using data from the Third National Health and Nutrition Examination Survey. Source ↗
- 6Clinical guidelineAmerican Society of Hematology. ASH sets new standards for diagnosing iron deficiency. Press release, September 16, 2026. Source ↗
- 7Clinical guidelineAmerican Society of Hematology. Diagnosis of iron deficiency: 2026 guideline summary. Medscape. Source ↗
- 8Expert commentaryIron deficiency and iron deficiency anemia in inherited bleeding disorders: common, underrecognized, and undertreated. Res Pract Thromb Haemost. 2025. Source ↗
- 9Observational studyDePalma RG, et al. Optimal serum ferritin level range: iron status measure and inflammatory biomarker. Metallomics. 2021;13(6):mfab030. Source ↗
- 10Clinical summaryCleveland Clinic Consult QD. Identifying subclinical hypothyroidism. Source ↗
- 11ReviewMost elderly patients with subclinical hypothyroidism do not need to be treated. Cleve Clin J Med. 2025;92(4):221. Source ↗
- 12IndustryOptimal DX. Why ODX optimal blood chemistry ranges are preferred for prevention.
- 13Cohort studyVanderpump MP, et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clin Endocrinol. 1995;43(1):55-68. Source ↗
- 14Cross-sectional studyChang YC, et al. High TSH level within normal range is associated with obesity, dyslipidemia, hypertension, inflammation, hypercoagulability, and the metabolic syndrome: a novel cardiometabolic marker. J Clin Med. 2019;8(6):817. Source ↗
- 15Cohort studyInoue K, et al. Association of subclinical hypothyroidism and cardiovascular disease with mortality. JAMA Netw Open. 2020;3(2):e1920745. Source ↗
- 16Cohort studyBancks MP, et al. Long-term absolute risk for cardiovascular disease stratified by fasting glucose level. Diabetes Care. 2019;42(3):457-465. Source ↗
This article is for educational purposes only and isn't a substitute for personalized medical advice. Optimal ranges are not diagnostic. Please discuss your results, and any supplements, with a qualified healthcare provider. See my full medical disclaimer.
