Using the Percentile of the Reference Range for Thyroid Hormone Blood Test Results Can Often Be Revealing

This is a very simple blog post about thyroid blood test results that I was prompted to write as a couple of thyroid patients seemed to find this information useful. It is basic maths and I have always found it quite useful.

What is the Percentile of the Range for any result?

The percentile of a range is how large the result is when expressed as a percentage of the entire width of the reference range. As an example, for a reference range of 1 to 4, a result of ‘4’ is 100 percentile (100%), a result of ‘1’ would be 0 percentile (0%) and a result of ‘2’ would be 50 percentile of the reference range (50%).

How Percentiles be Useful for Thyroid Patients?

When patients get thyroid blood test results like Free T4 (FT4) and Free T3 (FT3), it can be very obvious that they are low in one of the results, or high in some cases. However, over time the results might seem to change but it can be very hard to spot whether the changes are significant or not. Using percentiles can clarify where the test result is in the reference range. It is also significantly easier to see if the thyroid hormone has actually improved or deteriorated over time.

Note: Free T3 is the bio-available measure of T3, the active thyroid hormone. FT3 is the T3 in the bloodstream which is not bound to protein (thyroid binding globulin). Total T3 can be measured also, but this includes the bound T3 and as such it is not very useful.

Sometimes, a thyroid patient needs to change the laboratory that is being used to do the test. This almost always means that the reference range will be slightly different. It can then be very hard to see if the test result is better, worse or the same as with the previous laboratory. Using percentiles makes the comparison a lot easier, although different laboratories have different assays (the actual testing method in the lab.) and this can also change the result.

Very often a doctor or endocrinologist will look at the laboratory test results and tell the thyroid patient that all is well, using the famous phrase, “Your results are in range”, or even more annoyingly, “Your results are normal”, even though you feel like death!  Using percentiles can quickly show very low results, especially for the active hormone FT3.

So, using percentiles can be helpful and insightful.

How is a Percentile Calculated?

Let us use some actual thyroid test results to make this more realistic. Here are some results from an actual thyroid patient, who converts quite well and is on a combination of T4 and T3:

TSH 0.001 mIU/L
FT4 18.20 (9.01-19.05) pmol/L
FT3 5.40 (2.62-5.70) pmol/ L

I do not use percentiles for TSH. I do not pay much heed to TSH when it is not high. You can read plenty about my disregard for low TSH in my other blog posts and in my books.

I do use percentiles for FT3 especially, as it is the active hormone. I use it for FT4 and for Reverse T3 (rT3) if it is available.

Let’s go straight to FT3. The bottom of the FT3 reference range is 2.62, and the top of the fT3 reference range is 5.70 in this case. The length of the reference range is known as the Reference Interval and this is 5.70 – 2.62 = 3.08 reference interval for FT3.

We also need the amount above the bottom of the range that the actual FT3 result is 5.40 – 2.62 = 2.78.

So, all we do now is divide the amount of FT3 above the bottom of the range by the FT3 reference interval x 100%.

FT3 percentile of the range = 2.78 / 3.08 x 100% = 90.25%.

Clearly, this very simple formula also lends itself to being put into a spreadsheet for those thyroid patients that are tracking their thyroid labs over time with medication changes.

For the above results, the FT4 reference interval is  19.05-9.01 = 10.04.

The amount above the bottom of the range of the FT4 result was: 18.20 – 9.01 = 9.19.

So the FT4 percentile is 9.19 / 10.04 x 100% = 91.5 %.

Clearly, if there is an rT3 result, then the rT3 percentile can be calculated in the same way.

For completeness, you can be below the range for a thyroid test result, i.e. a negative percentile, and it is also possible to be above the range and have a > 100% percentile.

For those of you who are more visual, here is a very simple graphic explaining the above:

How can Percentiles be used?

I have already mentioned that it is far easier to see if the thyroid lab results have changed by using percentiles.

However, it is critical to be aware that only changes in FT3 affect symptoms in a meaningful way. If FT3 stays the same but FT4 goes up, it is unlikely that you will experience improvement. FT4 is not a proper thyroid hormone and it does not connect to the thyroid receptors within our cells. Only FT3 is the active hormone and tracks symptoms:

The other thing to be aware of is that having results that fall within the reference range is not sufficient to feel well. Every thyroid patient needs to have results that fall in the right part of the reference range for them, individually, in order to feel well:

So, tracking FT3, FT4 and rT3 results and their percentiles can be very helpful.

I prefer NOT to generalise, as thyroid patients are individuals and therefore have different needs. But in general, thyroid patients do better with an FT3 result that is in the upper half of the FT3 reference range or even the upper quartile. So, in general, they do better with FT3 > 50% of the range and often >75% of the range or even nearer the top of the range. Frequently, this also has to be matched by an rT3 percentile that is NOT high.

Anyway, I never thought to write this blog post previously, but in the hope that it may help some people, I have gone ahead and done so.

Best wishes, Paul

Paul Robinson

Paul Robinson is a British author and thyroid patient advocate. The focus of his books and work is on helping patients recover from hypothyroidism. Paul has accumulated a wealth of knowledge on thyroid and adrenal dysfunction and their treatment. His three books cover all of this.

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  1. Francis on 7th May 2023 at 8:35 am

    Great article, I will now calculate my percentages.. I’m on T3 only and generally sit outside the reference range, I feel better outside the range than I do within the range.

    • Paul Robinson on 7th May 2023 at 12:03 pm

      I assume you mean above the range Francis.

      Many on T3-Only are like this if they do the blood test within 12-18 hours of the last T3 dose.

      But it is dose-dependent. FT3 causes huge spikes in FT3 level and as such is nearly impossible to use as a tool to assess the T3 dosage – other than the T3 is genuine and is being absorbed. The chart included in the research article here speaks volumes to this point:

      Best wishes, Paul

  2. Diogenes on 8th May 2023 at 12:12 pm

    I agree that the FT4/FT3 ratio is a useful tool in diagsosis and treatment. But for maximum value, the healthy ratio (and numbers) could be a basic number to help decisions. Eg, if a patient presented with symptoms, then it may be that a ratio higher than usual could indicate nonthyroidal illness (inhibition of deionase activity), but with a lower ratio, subclinical hypothyroidism ( the thyroid is losing the ability to make T4, but continues to produce T3, right up until the end of its working existence.

    • Paul Robinson on 8th May 2023 at 1:38 pm

      Agreed. I was just trying to give those people who only tend to look at the absolute numbers and don’t even realise how low they are something slightly more useful. I wasn’t actually talking about the ratio at all Diogenes. I was only attempting to suggest that an FT3 percentile might be more valuable for some people to track than just the absolute value of FT3.

      I’ve already posted a blog post based on some of your work on FT4/FT3. For some thyroid patients, this is still too complicated for them to work with and they’d need help from someone (ideally their doctor but this won’t happen I know), for them to make use of this.

      Best wishes, Paul

  3. Laura on 8th July 2023 at 11:39 pm

    Would this apply to undiagnosed patients on no thyroid treatment? I.e. if a patient’s FT3 is within the lower quartile, and we all have our individual ranges, is there an argument that FT3 should be 50% or 75% of the range? (ie a scientific argument/literature an endocrinologist might listen to..)

    My FT4:FT3 ratio is not abnormal, but FT3 is only 25% of range.

    • Paul Robinson on 9th July 2023 at 9:51 am

      Hi Laura,
      Some people can be fine on 25% of the FT3 range. If your FT4, TSH are all in range and you have no raised TPOAb or TGAb autoantibodies and no hypothyroid symptoms then I wouldn’t worry about it.

      On thyroid treatment and with some loss of thyroid tissue it is more worrying to see low FT3 levels.

      Best wishes, Paul

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