The Free T3 (FT3) Laboratory Reference Range May NOT be Applicable for Thyroid Patients Taking: T3-Only, NDT, or T4/T3 Medications

Why is it foolish to rigidly apply the FT3 lab reference range to people on T3-Only, and probably for those on T4/T3 and NDT too?

Almost all doctors think the FT3 reference range should always be abided by, i.e. an FT3 result should always be in range. Most patients agree with this approach too, as it is what they are often told.

Often the effect of doing this is that patient’s T3 medication is never increased to a therapeutic dose, or it may even be reduced if FT3 is above the top of the range. So, why is this not ok?

Well, there are some extremely good reasons for this.

The argument for this came about through a discussion I had with a thyroid researcher. I have never seen this written about anywhere else before.
However, it is logical and is holding many people back from getting well.

Let us start with people on T3-Only, as the argument is more obvious in this case.

Well, it is simple, but not so easy to describe in a few words.

The FT3 lab range is created from healthy people or people on T4 therapy. The FT3 range is clearly based on blood measurements.

However, healthy people or those with normal levels of FT4, have T4 to T3 conversion constantly going on inside their cells. All of our cells do some level of conversion of T4 to T3. They either make D1 or D2 deiodinase enzymes in order to do this conversion. The liver and kidneys make D1 (the liver uses D1 to clear a lot of rT3) and the rest of the tissues make D2 (which is actually more efficient at converting T4 to T3). The T4 to T3 conversion occurs inside the cells.

Much of this intracellular converted T3 is never returned to the bloodstream. So, these healthy, or T4-treated, people have the measured FT3 in the blood, PLUS some good amount of extra FT3 being constantly converted within the cells. The FT3 they have in the blood is moving into the cells on a constant basis, PLUS they have the hidden FT3 from conversion.

So, the FT3 lab range only represents the blood level of FT3 and does NOT include the extra, sneakily converted FT3 that is additionally present in the person’s cells.

Now, on T3-Only therapy thyroid patients often do not have much T4 left (I have virtually zero). So, we get limited, or no, extra FT3 from conversion! Obviously, some of the FT3 is moving into the cells, but we lack the extra intra-cellular conversion that is going on constantly in healthy people or those on T4 medications.

This is critical to be aware of. 

If those on T3-Only get limited by a doctor saying we cannot go above the FT3 lab range, we are likely to not be getting enough T3!

So, this situation sometimes needs to be compensated with a higher FT3 than the lab range. Mine is sometimes 2 – 3 points above the top of the range (depending on when the blood draw is taken relative to the last dose of T3).  I am NOT hyperthyroid, or thyrotoxic, in any way, which is what most doctors would interpret from an FT3 above the reference range. This also implies that for someone on T3-Only, they often need far higher T3 doses than when T3 is used in conjunction with T4. The T3 dosage might be higher than doctors or other thyroid patients might suspect. Practical experience suggests 40 to 80 mcg of T3 is fairly typical for most people when they use it as a T3-Only therapy. A few people need a little less and a few need more. I have personally been taking 60 mcg of T3 for well over ten years and I can happily cope with higher levels without any issues.

Consequently, when patients who are on T3 therapy contact me and say they still do not feel well, but their lab test results say their FT3 is at the top of the range, I am not at all surprised that they do not feel great.

T3 therapy needs to use the patient’s symptoms, and some key signs, like body temperature, heart rate, blood pressure in order to find the optimal T3 dosing. Symptoms are especially critical. If a patient has no hyper symptoms at all, has normal body temperature, has not got an elevated heart rate and has good blood pressure, they are extremely unlikely to be hyper – regardless of an FT3 result that may be a little over the top of the reference range.

On T3-Only, multiple sets of these measurements over the day, before and 2-3 hours after each T3 dose, provides good information, that virtually guarantees there is no hyperthyroidism, or thyrotoxicity, present if the measurements are all normal. To be even more secure a doctor could occasionally run an ECG, and blood calcium, or other actual measures of true body function. 

Lab tests are of almost no value when on T3-Only / T3 therapy.

This topic is thoroughly discussed within the Recovering with T3 book.

The above argument applies but to a lesser extent to T4/T3 therapies and NDT. The top of the reference range may be too restrictive for some of these thyroid patients also. This is because their balance of FT3 and FT4 may be different from what they had/needed before they had hypothyroidism. This argument becomes more relevant the more T3 is in the T4/T3 combination being used.

I have worked with many thyroid patients who have never recovered until they reached near zero rT3, near-zero FT4 and higher than range FT3.

I have also written about this last point in The Thyroid Patient’s Manual and in both Recovering with T3 and The CT3M Handbook.

In summary, the laboratory tests and their reference ranges are of little value on T3-Only therapy. Perhaps the only exception is that through lab testing you can find out if TSH is suppressed, and whether you are clearing rT3.

An FT3 result has little diagnostic value when on T3-Only therapy, apart from knowing that it is rising, as the T3 is increased (i.e. there are no absorption issues). FT3 should not be used to restrict the level of T3 medication for a thyroid patient on T3-Only, and perhaps also if they are taking T4/T3 or NDT.

The above knowledge is in all my books, but I have not seen anyone else write about it… yet.

It is not surprising that the FT3 reference range is unhelpful for those on T3-Only or T4/T3 therapy. The lab reference ranges were never developed with these therapies in mind, or based on collections of data from patients who are successfully treated whilst on these therapies.

I hope you find this interesting and useful.

Best wishes,


p.s. If any of you are planning on discussing the ideas presented here to your own physician, I recommend going slowly, carefully and politely. This article may challenge a lot of long-held beliefs. So, softly-softly might be the best way to approach things. I mainly wrote this for thyroid patients to provide insightful background information.

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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