Abbott Laboratories’ reference range normalizes lower Free T3

abbott laboratories

In late 2018, in Ontario, Life Labs changed its reference range for Free T3 (Triiodothyronine) thyroid lab tests.

In our closed Canadian Thyroid Support Group where patients share their lab results and therapy progress over time, this particular Free T3 test has been behaving strangely so far, especially comparing changes from the old assay to the new.

Let’s clarify what’s at issue here and why it matters so much to our health.

LifeLabs’ Free T3 test is behaving erratically in ways that go beyond the shift to lower numbers.

  • Our main issue is NOT the lower digits of our test results — we know that Free T3 assay kits may yield slightly different results.  So that’s not the issue.
  • The issue IS the new lower reference range, which is used as a guide to medical interpretation, and therefore it can result in serious therapeutic misinterpretations with effects on our long term health.
  • AND we do not believe the reference range has been properly validated for accuracy at lower levels of Free T3. The change in results do not accord with patients’ dose changes or clinical status.

So, I did research to find out what might be going on.  Has Abbott’s laboratory assay kit been tested independently?

I found out some surprising things about

  • Industry biases built into Abbott’s manufacturer-recommended reference range
  • Confirmation of an industry-biassed reference range in independent research
  • Society-wide medical neglect of this important indicator — the poor calibration of Free T3 immunoassays in general — that only worsens the tragedy of patients’ suffering associated with misinterpretation of their lowered Free T3 status.

I provide the details of this analysis here.

Why the Free T3 test matters so much

We realize that many doctors have been taught to believe that Free T3 measurement is unnecessary in hypothyroid therapy. Please see this separate blog post with 24 scientific references that proves that this belief is not only incorrect but harmful to patients:

Abbott’s economic self-interest

Abbott Laboratories manufactures Synthroid, well known as the most popular brand of Levothyroxine (L-T4) medication in North America.

Synthroid induces a low T3:T4 ratio that is exaggerated among those with less thyroid gland tissue and higher doses. (1-2)

Therefore we should question whether Abbott’s “recommended” ranges are accurate, because they are motivated to minimize and normalize the lower T3 levels caused by their bestselling drug Synthroid.

Is Abbott’s reference range biased? Likely YES.

We discovered an independent research paper that reported data showing the Abbott lab test could be very biased toward normalizing our lower Free T3 to a significant degree.

The LifeLabs Ontario laboratory now states that it uses Abbott Architect, a laboratory immunoassay kit, according to the Free T3 page on their website. (3)


Fillee et al, a team of three researchers from Belgium compared three assays including the Abbott Architect in 2012. (4)

The new ref ranges of 2.6 – 5.8 pmol/L at LifeLabs are very close to the assay manufacturer’s (Abbott’s) recommended reference ranges shown in Table 3 below, 2.6 – 5.7. The difference is only 0.1 at the upper end.


Fillee et al’s team tested all three brands of assay kits on their own group of 68 healthy control subjects.

Table 2 data below showed that Fillee et al’s team yielded a much narrower reference range for Abbott Laboratories’ test kit than the range Abbott recommended. (4)


Comparing the two tables yields these differences in red, showing Abbott’s has the largest discrepancy at the low end:

Roche Elecsys:

  • MFG says 3.1 – 6.8, Filee says 3.07 – 5.92.  Low cutoff difference -0.03

Beckman DxI 800:

  • MFG says 3.8 – 6.0; Fillee says 3.97 – 5.74.  Low cutoff difference +0.13

Abbott’s Architect:

  • MFG says 2.6 – 5.7; Fillee says 3.05 – 5.27.  Low cutoff difference -0.45

A difference of -0.45 may not seem like much until you see the data in graphs displayed below.

Graphs reveal that two of the three Free T3 assays that Fillee et al studied — the Architect and the Elecsys — tended to shift even “normal” values upward into the upper 2/3 of the reference range — as shown below, where plots on a Z-test enable fair comparison based on Fillee et al’s 95% reference values.

Healthy controls


Be aware that Fillee et al have drawn the red circles and black lines based on their OWN “corrected” reference ranges for the assay kits, NOT the manufacturer’s recommended reference ranges, so the red circles minimize the bias in Abbott’s recommended -0.45 pmol/L lower cutoff.

As you can see in the graphs indicated by the blue circles, healthy controls, when tested by these two assays, rarely yield values in the lower 1/3 to 1/4 quarter of the range, and yet the 95% reference range dips lower.

The Free T3 results in the lower quadrant and below are rare only for the Architect and Elecsys assays.

  • Comparisons with the third assay (the DxI 800 assay) yielded graphs with a more even distribution of results throughout the reference range region (graphs not shown here).

Most importantly for thyroid patients, the few Free T3 values below reference unfairly skew the 95% range of these assays to normalize the lower-range values that could fall in the rather unpopulated gap.

These outlying low FT3 values found among “healthy” control groups could have been obtained in patients whose FT3 was temporarily lowered by chronic illness or medications, so we doubt whether all the conditions that alter FT3 were excluded.

Implications based on Fillee’s “control group” data

The control group graph visually demonstrates why it is extremely important to eliminate from control groups ALL patients who are on calorie-restricted or protein-restricted diets, exhausting exercise, chronic fatigue syndrome, and even depression — these common health states are associated with unusually lower Free T3 levels in people with healthy thyroid glands — even while they simultaneously maintain a normal TSH and T4 level. (5)

A person with a healthy thyroid gland can recover from abnormally low T3 states easily and quickly — simply by increasing thyroidal stimulation and output (raising the so-called “thyroid thermostat” to recover metabolic rate). (6)

However, a thyroid patient on a static dose of thyroid hormone who has no thyroid gland tissue CANNOT raise their low Free T3 by increasing thyroidal secretion — thyroid patients can be medically imprisoned within this hypoT3 region for years or decades if their doctor considers it “normal.”

Fillee’s test of Abbott and Elycsys on Thyroid disease patients

Triangle = hypothyroidism. Circle = Graves’ disease.


(Again, remember that Fillee et al have drawn the red circles and black lines based on their OWN “corrected” reference ranges for the assay kits, NOT the manufacturer’s recommended reference ranges.)

In the graphs above, at least some of these patients must have been on thyroid therapy.

  • Graves’ disease patients normally have a FT4 and FT3 above range.  Since “circle” data points representing Graves’ appear below reference, they are likely either post-thyroidectomy or post-RAI therapy and now on T4-monotherapy.
  • Hypothyroidism in an untreated state does not present with a FT4 above reference, yet we see triangles above the FT4 reference. Overtreatment with T4-monotherapy for severe hypothyroidism may cause FT4 to rise above reference and simultaneously depress FT3 to the lower third or below reference.

Now consider the graph above in light of the relative lack of data points in this region among the “healthy” control group (the previous graph with our blue circles).

  • There are many more “thyroid disease” patients in the lower 1/4 of the reference range, and lower, despite being on thyroid therapy. Treatment has not normalized many patients’ FT3 status.

Now consider Free T4 compared to Free T3.

  • The Free T4 test has no distortion for the Architect assay in the lower end of reference.  Since Free T4 levels are now used confirm a diagnosis of overt versus subclinical hypothyroidism when TSH is above reference range, it is in the interests of Abbott Laboratories to ensure that the lower end of Free T4 are accurate and not biased by a lower cutoff that could defer thyroid therapy.
  • The Free T3 graph on the right shows distortions for Architect’s assay in the lower end — it yields higher test results for these patients that move the data points closer to the red “normal” circle.

Therefore, Abbott Architect places Low T3 in hypothyroid patients closer to the reference range than the Elecsys, even after Abbott’s skewed reference range is corrected.

Some of the outlying low FT3 values would appear NORMAL if the red circle were edited to match Abbott’s recommended reference ranges (which were skewed at the lower end by 6-7%).

Implications based on the “thyroid disease” data

This data underlies the importance of testing our Free thyroid hormone assays on a very large number of treated thyroid patients (not just control subjects) and comparing that data with a different Free T3 assay and with truly healthy controls.

To create a good “thyroid disease” graph to contrast with your “control” graph, you must enroll subjects who are all being treated on the same standard therapy modality (i.e. L-T4-monotherapy) while their TSH is “normalized.”  Additionally, researchers should sub-classify patients’ data by an index shown to be relevant in research, such as thyroidal function (2) — this could be based on thyroid gland ultrasound and T4-dose per kg body weight — in order to prevent falling into the error of “Simpson’s Paradox” caused by grouping together disparate patients’ data:

  • ASK: How much functioning thyroid gland tissue do they have left? In an autoimmune thyroid patient, has their gland entirely atrophied or fibrosed as shown by ultrasound? Are they on a high dose of L-T4 per kg body weight?

These key methodological adjustments will help the data set reveal the following disjoint between those with higher Free T3 and lower Free T3 on the standard therapy:

  • ASK: Where do Free T3 levels fall among T4-monotherapy patients ENTIRELY dependent on T4-T3 conversion alone? (Lower Free T3 is more common if they are a poor converter) (2)
  • ASK: Where do Free T3 levels fall among T4-monotherapy whose FT3 is BOOSTED by direct T3 from remaining thyroid gland secretion? (higher Free T3 is more common, less likely to be symptomatic on standard therapy) (2)

Why lower Free T3 test results matter

“The consequences of untreated or wrongly treated hypothyroidism can be significant, ranging from nonspecific symptoms to life-threatening manifestations.” (Welsh and Soldin, 2016) (7)

An unknown percentage of treated thyroid patients are suffering poor health and/or chronic hypothyroid symptoms such as low body temperature, low heart rate, depression and debilitating fatigue.

Yet these symptoms are indeed classic hypothyroid symptoms, and these symptoms and even more are associated with Free T3 levels in the lower half of reference range and lower. (8)

We have known since 2003 that in patients with overt untreated hypothyroidism (TSH >20) that various markers of tissue hypothyroidism (clinical score, ankle reflex time, creatine kinase, total cholestero) correlate very strongly with Free T4 and Free T3 hormone levels, whereas these marker of tissue hypothyroidism have no correlation or only a weak correlation with TSH. (9)

Yet today, if the patient’s TSH is “normalized” and Free T4 is high normal, it is likely to be wrongly assumed that their symptoms must arise from other health conditions, not low Free T3.

Sadly, a TSH-focused doctor is

  • more likely to dismiss (or not even bother to test for) lower Free T3,
  • less likely to raise a suffering thyroid patients’ hormone dosage,
  • more likely to dismiss the option of T3-based hormone therapy in patients who are incapable of converting T4 medication sufficiently. (1, 2)

Dismissal of lower T3 test results may divert doctors’ attention to order other tests for other health problems. In the end, this can cost more health care dollars.

A final word on the “inaccuracy” of Immunoassay methods

The only Free T3 test known to be 100% accurate is “liquid chromatography tandem mass spectrometry (LC/MS/MS)” Free T3 test. (10)

We don’t have access to the LC/MS/MS Free T3 test methodology in Canada — at least not that we could find by searching online. It’s currently offered in the US for $200 US a test (LabCorp and QuestDiagnostics).

Welsh and Soldin’s (2016) article is titled “How reliable are free thyroid and total T3 hormone assays?” (7) claims that the low reference cutoffs of Free T3 immunoassays are generally too low, making it falsely appear that a person is not hypothyroid in their Free T4 and Free T3 when indeed they are, when blood is tested by the gold standard of ultrafiltration LC-MS/MS assay.

“2/3 of individuals classified as subclinical by immunoassay have FT4 or FT3’s below the 2.5TH percentiles and are therefore now classified as clinically hypothyroid”

Spencer et al’s Figure 2 D shows how 14 immunoaassay Free T3 tests differ according to their mean / average result, and the target reference mean is between the dotted lines.


As explained by Spencer et al, 2017, “Currently, most FT4 and FT3 immunoassays display significant negative or positive biases that exceed the intra-individual biological variability.” An international council is actively working with the industry to recalibrate their free hormone immunoassays against “reference measurement procedures” (RMP). “However, although recalibration to the RMP has been shown to greatly reduce between-method biases, implementation of a global re-calibration effort has been delayed by practical, educational and regulatory complexity.

Let’s more carefully assay the immunoassays!

Despite all the controversies and weaknesses of immunoassays, John Midgley, a developer of Free thyroid hormone assays and a researcher of hypothyroidism, claims that immunoassay methods can be very accurate as a methodology. (11-12)

Research in thyroid science continues to rely largely on immunoassay methods in most studies that assess Free T3 and Free T4.

The Free T3 and Free T4 immunoassays reported in methods sections of research reports yield strong enough results for coefficients of variation and their range of accuracy.

It’s just that we must design and calibrate them properly for clinical laboratories, not just research laboratories.

Our immunoassays CAN be reasonably reliable and cost-effective tools, but they are not being calibrated in light of the international gold standard and not being assessed on local test subjects where climate and diet might affect thyroid function.

We simply can’t rely on biased manufacturer-recommended reference ranges.

So, let’s get on it then!  Assay the assays!

Society has to start caring more about the welfare of millions of suffering female over-40 hypothyroid patients. Only this can motivate them to get to work on improving the accuracy of testing lower concentrations of Free T3.


1. Gullo, D., Latina, A., Frasca, F., Le Moli, R., Pellegriti, G., & Vigneri, R. (2011). Levothyroxine Monotherapy Cannot Guarantee Euthyroidism in All Athyreotic Patients. PLoS ONE, 6(8).

2. Midgley, J. E. M., Larisch, R., Dietrich, J. W., & Hoermann, R. (2015). Variation in the biochemical response to l-thyroxine therapy and relationship with peripheral thyroid hormone conversion efficiency. Endocrine Connections, 4(4), 196–205.

3. LifeLabs. (n.d.). LifeLabs – Test Information Directory – FREE TRIIODOTHYRONINE. Retrieved January 6, 2019, from

4. Fillée, C., Cumps, J., & Ketelslegers, J.-M. (2012). Comparison of three free T4 (FT4) and free T3 (FT3) immunoassays in healthy subjects and patients with thyroid diseases and severe non-thyroidal illnesses. Clinical Laboratory, 58(7–8), 725–736.

5. Chatzitomaris, A., Hoermann, R., Midgley, J. E., Hering, S., Urban, A., Dietrich, B., … Dietrich, J. W. (2017). Thyroid Allostasis–Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming. Frontiers in Endocrinology, 8.

6. Peeters, R. P., Wouters, P. J., van Toor, H., Kaptein, E., Visser, T. J., & Van den Berghe, G. (2005). Serum 3,3′,5′-Triiodothyronine (rT3) and 3,5,3′-Triiodothyronine/rT3 Are Prognostic Markers in Critically Ill Patients and Are Associated with Postmortem Tissue Deiodinase Activities. The Journal of Clinical Endocrinology & Metabolism, 90(8), 4559–4565.

7. Welsh, K. J., & Soldin, S. J. (2016). How reliable are free thyroid and total T3 hormone assays? European Journal of Endocrinology, 175(6), R255–R263.

8. Larisch, R., Midgley, J. E. M., Dietrich, J. W., & Hoermann, R. (2018). Symptomatic Relief is Related to Serum Free Triiodothyronine Concentrations during Follow-up in Levothyroxine-Treated Patients with Differentiated Thyroid Cancer. Experimental and Clinical Endocrinology & Diabetes, 126(09), 546–552.

9. Meier, C., Trittibach, P., Guglielmetti, M., Staub, J.-J., & Müller, B. (2003). Serum thyroid stimulating hormone in assessment of severity of tissue hypothyroidism in patients with overt primary thyroid failure: cross sectional survey. BMJ, 326(7384), 311–312.

10. Spencer, C. A. (2017). Assay of Thyroid Hormones and Related Substances. In L. J. De Groot, G. Chrousos, K. Dungan, K. R. Feingold, A. Grossman, J. M. Hershman, … A. Vinik (Eds.), Endotext (Originally published 2000; Updated February 20, 2017). South Dartmouth (MA):, Inc. Retrieved from

11. Midgley, J. E. (1993). The free thyroid hormone hypothesis and measurement of free hormones. Clinical Chemistry, 39(6), 1342–1344. Retrieved from

12. Midgley, J. E. M., & Christofides, N. D. (2009). Point: legitimate and illegitimate tests of free-analyte assay function. Clinical Chemistry, 55(3), 439–441.

5 thoughts on “Abbott Laboratories’ reference range normalizes lower Free T3

  1. Free T3 and Free T4 tests are very different in their quality. I don’t as initial inventor rate the Abbott tests as very good. It is difficult to produce a robust FT4 test and 10 times more difficult to produce a good FT3 test. Elecsys in my opinion is better. I spent a large amount of time testing every possible variation of thyroid status before the test (now Vitros ECi -the best in the business) was launched. The other companies have paid lip service by simply measuring the common situations – its the rare ones that really examine test validity. Oh and BTW I don’t rate the Soldin work either. There are many potential errors in their method. In my opinion the Vitros ECi FT3 method is the only valid one on the market.

    John E Midgley

    1. Dear John Midgley, as a follower of your work and reader of several of your articles on assays, I’m very honored that you’ve replied. Thanks for your thoughts on the level of difficulty to create a good FT3 test, and your praise for Elecsys and Vitros ECi. Your expert opinion weights a lot.

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