Why the FT3 Lab Reference Range may Not be Applicable for People on T3-Only, NDT, T4/T3 Therapy

Why is it foolish to rigidly apply the FT3 lab reference range for 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 isn't this 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 logicall and is holding many people back from getting well.

Let's 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 intra-cellular  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 FT3 and does NOT include the extra, sneakily converted FT3 that is additionally present in the person's cells.

Now, on T3-Only therapy we don't have much T4 left (I have 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 going on constantly. 

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 usually 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 hyper or thyrotoxic in any way, which is what most doctors would interpret from a FT3 above the reference range. This also implies that for someone on T3-Only, they often need far higher T3 doses than when is is used in conjunction with T4 - higher than people mght expect. 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.

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

T3 therapy needs to use the patients symptoms, and some key signs, like body temperature, heart rate, blood pressure in order to find the optimal dosing. Symptoms especially are critical. If a patient has no hyper symptoms at all, has normal body temperature, has not got an elevated heart rate out of the typical range and has good blood pressure, they are extremely unlikely to be hyper - regardless of an FT3 result which 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, provide 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 a blood calcium or other actual measures of true body function. 

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

This is all discussed within the 'Recovering with T3' book (Updated Edition 2018).

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 people also. This is because their balance of T3 and T4 may be different to what they needed when they were well, and before they had hypothyroidism. This argument becomes more relevant the more T3 is in the T4/T3 combo being used.

It is worth noting that I have seen many thyroid patients who have never recovered until they reached zero rT3, near zero FT4 and higher than range FT3.

I have also written about this point in 'The Thyroid Patient's Manual' and in the Updated Edition 2018 of both 'Recovering with T3' and 'The CT3M Handbook'.

Basically, the lab ranges are of of little value on T3 therapy, with perhaps the only exception being that 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 it is rising as the T3 is increased (i.e. there are no absorption issues), and should not be used restrictively when someone is on T4/T3 and NDT also.

The above knowledge is in all my books now, 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. It simply was never developed with these therapies in mind. 

I hope you find this interesting and useful.

Best wishes,

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.