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The Clinical Translator — Answer-Ready Facts

Lab Reference Ranges Weren't Built on Every Body

A lab result reads as normal only in relation to a reference range, and many of those ranges were built on a narrower population than the person reading the result.

When researchers rebuild a test's reference range by age, sex, and race, diagnoses change. One 2025 study found nearly half of thyroid diagnoses shifted once that correction was made.

Real Biology, a Separate Design Problem

Some of this is real biology. Ancestry-linked differences in specific biomarkers exist and repeat across independent studies, not as noise in the sampling. The problem sits somewhere else: a reference range built almost entirely from one population, then handed to everyone as if it were universal, or a correction factor folded into a diagnostic equation without the data to justify it. A single lab value can carry both problems at once, a genuine population difference sitting inside a threshold that was never properly validated for the person actually being tested.

The Research Behind The Pattern

Thyroid testing shows the pattern clearly. An urban outpatient study of more than 22,000 patients found the median TSH for Black patients ran lower than for white patients, 1.18 compared with 1.54 mIU/L, a difference large enough to change who gets flagged for disease (Boucai and Surks, 2008). A 2025 study went further and rebuilt the reference range itself by age, sex, and race using data from more than 8,000 adults, then compared the results against the standard range already in clinical use. Correcting for those three factors reclassified 48.5% of people previously flagged with subclinical hypothyroidism as within normal range, and 31.2% of people flagged with subclinical hyperthyroidism the same way (Li et al., 2025).

Hematology carries the same problem in the reverse direction. Reference ranges for hemoglobin and iron measures have historically been reported separately by race, a practice two hematologists called for eliminating outright in The Lancet Haematology, arguing that defining pathology by a social construct like race validates the false idea that Black and white bodies differ physiologically in ways they don't (Weyand and McGann, 2021).

Not an Accusation. A Design Limitation.

The range was never built to represent every body reading the result. That is not an accusation against any single lab. It is a design limitation baked into how reference ranges get made, and it is why a second opinion on the range itself, not just the number, is sometimes the more useful question.

Keep Verifying

Does this mean my labs are wrong?

Not wrong. Incomplete. A lab value is accurate as a measurement. Whether it gets flagged as normal or abnormal depends on the reference range it's compared against, and that comparison can be built on a narrower population than yours.

Which labs are most affected?

Thyroid function and hematology (hemoglobin, iron measures) are the two with the clearest published evidence so far. A broader 2018 study found that more than half of all lab tests with currently fixed reference ranges actually differ by race and ethnicity in healthy people, so the list isn't limited to those two (Rappoport et al., 2018). Glycemic markers and kidney function carry a similar pattern and are covered in more depth in the fuller piece linked below.

How do I know if this applies to me?

If your labs have come back "normal" while your symptoms haven't resolved, checking whether the range itself accounted for your age, sex, and race is one of the first things worth verifying, not the last.

This is explored further in an upcoming piece for Brainz Magazine.

Verify What Your Labs Might Be Missing

If your labs read normal but something still isn't, the free Success Tax Assessment is a starting point for identifying what a standard reference range might be missing in your specific case.

Take the Success Tax Assessment

Danielle Izaak-Lewis, Clinical Nutrition Strategist · Last updated September 10, 2026.

References

  1. Boucai L, Surks MI. (2008). Reference limits of serum TSH and free T4 are significantly influenced by race and age in an urban outpatient medical practice. Clinical Endocrinology. https://doi.org/10.1111/j.1365-2265.2008.03390.x
  2. Li Q, et al. (2025). Thyroid Function Reference Intervals by Age, Sex, and Race: A Cross-Sectional Study. Annals of Internal Medicine. https://doi.org/10.7326/ANNALS-24-01559
  3. Weyand AC, McGann PT. (2021). Eliminating race-based reference ranges in haematology: a call to action. The Lancet Haematology. https://doi.org/10.1016/S2352-3026(21)00106-X
  4. Rappoport N, et al. (2018). Comparing Ethnicity-Specific Reference Intervals for Clinical Laboratory Tests from EHR Data. Journal of Applied Laboratory Medicine. https://doi.org/10.1373/jalm.2018.026492