Can FT3 be Used to Manage Liothyronine (T3) Thyroid Treatment?

Many doctors and thyroid patients think that the FT3 thyroid lab test can be used to assess whether the dosing of Liothyronine (T3) is at the right level for the individual.

Testing FT3 can be useful if the thyroid patient is on Levothyroxine (T4-Only), or on T4 with a tiny bit of T3. In these cases, the FT3 level is relatively stable over twenty-four hours. Testing FT3 within 12 hours or so of taking any T3 would be ok in this circumstance of low T3 doses (< 10 mcg T3).

However, the situation changes dramatically if the patient is on mostly T3 therapy (with some or no T4 medication). These are the patients for whom this article is relevant.

Free T3, or FT3 for short, is the laboratory test of the bio-available level of T3 within the bloodstream at the time of the test. Remember, only FT3 can enter the cells and be biologically active.

On T3 therapy, this FT3 level is far from stable. It is about as stable as a rollercoaster!

After taking a dose of T3 medication, the T3 absorbs very quickly into the body. It absorbs far faster than T4 based medication. At 2.5 hours after a dose, the FT3 reaches peak levels in the bloodstream – which means most of the T3 dose has been absorbed:

The research study included this chart that shows how rapidly FT3 rose after the Liothyronine (T3) dose and how FT3 fell after the dose:

Important note: the volunteers in this study were euthyroid, i.e. normally they were not on any thyroid medication. They had a working thyroid gland and so their FT3 would never crash to extremely low levels 10-20 hours after the T3 dose. Many of us would see a more severe drop off of FT3 to a level that would not be good!

The study still illustrates the peaks and troughs of FT3 induced by Liothyronine dosing. But does that matter?

FT3 changes too dramatically following T3 dose administration to use it to manage medication dosage. The important word here is ‘manage’.

Many thyroid patients, including me, can sense a T3 dose has been taken within 20 minutes of taking it. So, we know that T3 begins to be digested and absorbed into the bloodstream within 15-30 minutes. It reaches peak FT3 levels in the blood at about 2.5 hours and will gradually decline in the bloodstream after this. All the T3 will be absorbed from the T3 tablet within 3-4 hours. By then, the level of FT3 in the bloodstream will be lower, because much of it will have entered the cells.

So, FT3 levels fluctuate. For those on T3 therapy, the length of time between the last T3 dose and the blood draw for an FT3 test is the biggest single factor in what level of FT3 shows up in results. If this varies, the FT3 result will vary. Testing FT3 within a few hours of a T3 dose will show a much higher level of FT3 than the trough level that will be reached from the longest gap a thyroid patient has between two T3 doses. For this reason, I prefer to see people testing FT3 at trough levels versus peak levels – it is a fairer test and more stable than testing anywhere close to peak levels.

However, the higher the amount of T3 in a dose that the person is on, the longer it will take to fall. For example, I know many people who would have over the range FT3 if they tested FT3 only 12 hours after a T3 dose. But at 18-24 hours they may well be be in-range for FT3 (and have fewer issues with a doctor who was reviewing their results). Individuals do vary a lot though – even absorption rates vary, so some people will be quicker for FT3 to fall and some will take longer. Do I believe that some people need T3 doses that take them over the FT3 range at some number of hours after a dose of T3 – yes! This again makes using the FT3 range when on T3-Mostly or T3-Only very difficult.

Note: It can be helpful to test FT3 in order to check that FT3 does change with changing T3 dosage and to check if the T3 is absorbing. However, I do not think that testing FT3 is going to show that a thyroid patient on T3 therapy is adequately treated. The following explains why I think this.

Here is a strong statement and I really do believe this:

For those on mostly T3 therapy, blood levels of FT3 do not reflect how patients feel and how well they are treated/medicated!

The T3 thyroid receptors at the cell nuclei are where most of the action of thyroid hormone occurs. The T3 needs to arrive at the receptors for the action of thyroid hormone to make any real difference to how our cells work. I am not going to go into how this all works but it involves something called ‘gene transcription’ that is enabled by the T3 arriving at the T3 thyroid receptors. T4 cannot do this. It has to be the T3 thyroid hormone – the only biologically active thyroid hormone. The right level of gene transcription ensures our cells all perform the particular functions that they are designed for. Muscle cells have a different function to brain cells etc.

What is the most important thing that a T3 dose needs to achieve?

What matters most is that the T3 receptors in the cell nuclei have had enough T3 so that the cell nuclei can continue to function well for some time. To do this, the cells do not need to have stable levels of FT3 in the blood. This last point is very important. This is why, for many people, 3 doses of T3 per day work really well. In most cases, it is better to multi-dose the T3 because of the rapid nature of how fast it peaks in the blood and then declines again. Many thyroid patients need to multi-dose T3 so that the T3 gets to the thyroid receptors in several reasonable sized pulses over 24 hours.

Taking T3 in multi-doses, when the doses are the right size for the individual, is like rapid charging of a battery with each T3 dose. The ‘battery’ then has enough charge for quite a long time. Even after the ‘rapid charging’ is complete, there is still enough T3 left within the bloodstream and within the cells for some trickle charging to continue for some hours.

An example of this last point is that I take a CT3M dose at about 3:00 am in the morning and no more until 11:00 am – a gap of 8 hours. This gap will be in the presence of peaking FT3 at about 5:30 am and a steady decline in FT3 all the way to 11:00 am. I have often completely forgotten to take any T3 until the early afternoon and still not noticed any degradation in how I feel. This is because my T3 receptors in the cell nuclei have ‘had a good feed’ of T3 with the previous dose.

I hope this goes some way to explaining why, when we do exercise, we do not simply ‘run out of T3’. Once we have enough T3 at the thyroid receptors in the cells, the effect of the T3 will continue to enable the cells to work well for a considerable amount of time.

Let me put the above a different way. On T3-Mostly or T3-Only therapy we know that TSH is often very low or suppressed and that low TSH on thyroid treatment does not mean someone is hyperthyroid. We also know that those people on mostly T3 are likely to have a below range or even near zero FT4 and consequently near zero rT3. So, we already have most of the thyroid labs that don’t comply with the reference ranges. Why should FT3 be any different? Why would anyone assume that someone on T3 ought to comply with an FT3 reference range designed for healthy people or those on T4? It does not make any real sense.

I have worked with thousands of patients over the past 17 years who never get well unless they are on individual doses of T3 that are large enough to make an impact on their symptoms and signs. This often takes them over the FT3 reference range if the FT3 blood test is done too close to the last T3 dose. Frequently these people need to leave 24-hours after the last T3 dose if they are to have any hope of getting an FT3 result within the FT3 reference range. Basically, the FT3 test becomes far less useful the higher the T3 doses are.

So how can we tell if a thyroid patient has had enough T3?

To assess if a thyroid patient has had the right T3 dosage, what we need to know is whether their cell nuclei have had enough T3 in order for the cells to run at the right metabolic rate. There is no test for this right now and I cannot foresee a time in the near future when such a test will exist.

For a thyroid patient on T3 therapy, trying to test whether they are on the right dosage of T3 based on a highly unstable level of FT3 is a flawed and misleading exercise. It is also flawed to attempt to assess T3 dosage based on the pituitary hormone TSH.

However, while a thyroid patient’s T3 dosage is being adjusted, it can be very helpful to have occasional FT3 tests just to check that the T3 medication is being absorbed. More T3 medication ought to induce some level of increase in FT3 (as long as the same length of time from the last T3 dose to the time of the blood draw is maintained each time). However, FT3 is not going to inform anyone about whether the correct dosage has been achieved, and neither will TSH or FT4.

On T3 therapy when the patient is on T3-Only, or T3-Mostly with only a little T4 medication, the FT3 level could be in range, top of the range or even over the range. It is impossible to conclude whether the patient is under-dosed or over-dosed from any of these results. If the patient on T3-Only or T3-Mostly therapy has suppressed TSH or extremely low FT4, it is not possible to conclude that the patient is hyperthyroid, or requires the addition of T4 medication.

However, on the positive side once a thyroid patient feels really well, knowing what their FT3 lab result is after a fixed time from the last T3 dose can provide some future value, if for instance, their symptoms worsen and they need to get some clue as to why this is. Testing the FT3 again with the same period of time of the last T3 dose could provide a much needed clue.

The current laboratory ranges are not fit for purpose for patients on T3-Only or T3-Mostly thyroid medication

The current laboratory ranges are designed for healthy people on no thyroid medication or for those on T4 monotherapy. These people have FT3 in the bloodstream but they also have on-going intra-cellular conversion of T4 to T3. They have more T3 than shows in the bloodstream than those patients on T3-Mostly or T3-Only.

The lab ranges are not designed for patients on T3-Only or T3-Mostly therapy, so why would anyone in their right mind attempt to apply the lab ranges in this case?

It makes no scientific or logical sense to attempt to apply the ranges developed for healthy or T4 medicated patients to those patients who are on T3-Only or T3-Mostly.

If there was a need to have lab ranges for those on T3-Only or T3-Mostly thyroid medication, then this population of people on T3 therapy would have to be studied and ranges would need to be developed specifically for this usage of thyroid medication. Any range developed for patients on T3-Only or T3-Mostly therapy is definitely likely to be a lot higher at both the bottom and top end of the reference range. Force fitting the current labs for use with T3-Only or T3-Mostly patients cannot work and is likely to leave the majority under-medicated.

None of the current lab ranges are useful in assessing whether the T3 dosage is too high or too low or just right in T3-Only or T3-Mostly therapy – not TSH, not FT4 and not even FT3. These lab results are only useful in how they change as the T3 is being adjusted. The changes in TSH, FT4 and FT3 only show trends during T3 dosage adjustment. They cannot be used to assess the adequacy, inadequacy or excess of the T3 medication. The absolute values of the thyroid lab results cannot answer the question of whether the patient is on the right T3 dose or not.

Thyroid patients who have a doctor or endocrinologist who simply will not accept an FT3 result over the top of the reference range, even though they are on T3 therapy, need to be particularly careful about when the lab test is done after the last T3 dose.

So, how can someone assess whether their T3 therapy is dosed correctly for them? The answer is not simply by ‘going how they feel’. This is far too vague and it is too easy to be misled by what you think is going on. We need a better, more objective approach.

So, what’s the way to assess whether the T3 therapy dosage is right or not?

I recommend the use of both Symptoms and Signs. Symptoms are subjective assessments of things like energy level, mental clarity, digestive system, skin and hair condition etc. However, Signs are more objective measurements. Signs include things like body temperature, heart rate, blood pressure, cholesterol level, calcium level etc., The Signs are numbers and they take the guesswork out of assessing T3 doses if the right ones are recorded at the right times.

Symptoms and Signs assess the response of a thyroid patient’s body to the T3. This is actually the closest measurement we have to how the cell nuclei are responding to the T3.

My book Recovering with T3 has a full protocol for doing this.

I have a blog post in this website that briefly explains one method of tracking symptoms and signs:

When I was first trying to get well, I got too confused trying to assess my T3 dosage based on how I felt (my symptoms alone). I gave up trying to do it that way after 6-12 months. This is when I began to create the method that is now written about within the Recovering with T3 book.

It is the pattern of the SIGNS before and after doses that most helpful. Subtle changes in these can be informative. This approach stops people from going on hunches and also alerts them to dosing that is too high.

If someone were trying to manage T3 doses using FT3, I would say that this is a recipe for disaster, unless it is only a small amount of T3 combined with T4.

Symptoms and signs need to be used. The cellular batteries need to be ‘recharged’ and only symptoms and signs can tell us if this has actually happened. After a T3 dose, ‘trickle charging’ of the cellular batteries should occur. Perhaps, at some time in the future, technology will provide better support for the dosing of the biologically active thyroid hormone T3. However, we are not there yet.

This blog post was about the problems of trying to use the FT3 laboratory test to manage T3 treatment. I have also written about why the FT3 lab test range is flawed for those people on T3 therapy. I am listing this blog here for completeness:

Here is a summary of the points made in this blog post:

  • If a doctor or endocrinologist is planning to test your FT3 level, then any patient on T3 treatment needs to be careful about how long they leave after the last T3 dose before the blood test. I often recommend 18-24 hours to try to ensure that the FT3 result is in range. Ideally, the patient would test this privately first and see if this gap provides the desired result. Having FT3 over range runs the risk of the patient’s T3 dosage being reduced or even stopped.
  • A T3 dose peaks rapidly in the blood and FT3 takes many hours to slowly lower.
  • The size of the T3 dose determines how high the peak of FT3 is and how long it takes for FT3 to fall within the FT3 reference range.
  • Small T3 doses e.g. 10 mcg or so, may allow FT3 to fall back within the FT3 reference range within 12-15 hours. But this varies by person and cannot be guaranteed 100%.
  • Larger T3 doses that some patients require may take 18-24 hours or even longer for FT3 to fall back within the FT3 reference range.
  • For those thyroid patients who wish to test their own thyroid labs. privately for their own viewing, testing after 12-18 hours of the last T3 dose is completely fine. The FT3 result would not be being used in this case to potentially reduce or stop the patient’s T3 prescription.
  • Knowing that FT3 changes when the T3 dosage is adjusted is helpful and it can show that the T3 medication is absorbing (and of good quality).
  • However, The FT3 reference range is not designed for those on T3-Mostly or T3-Only therapy. Both the low end of the FT3 reference range and the top end of the FT3 reference range are too low. The peaks of FT3 due to T3 doses are high. In addition, these patients lack the on-going, hidden extra T3 that is constantly being converted within the cells from T4 that healthy people have (or those on T4-Only meds have). The blog post that I referred to above this summary explains this in more detail.
  • The bottom line is that it is not possible to use the FT3 test result to determine if a T3-Mostly or T3-Only thyroid patient is on the correct T3 dosing regime for them. It can be used carefully to see how FT3 changes with different doses as long as the interval from the dose to the blood test is the same and the laboratory testing it is also the same. Symptoms and signs need to be used to really assess how the T3 dosing is performing.

I hope that you found this interesting and informative.

Note: all of this is explained in the Recovering with T3 book. See:

Best wishes,


(Updated in September 2023)

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. ShelleR on 25th July 2020 at 6:42 pm

    Excellent article, very helpful and would be a good one to print and take to your practitioner!

    • Paul Robinson on 26th July 2020 at 9:29 am

      Thank you so much for the feedback!
      Best wishes,

  2. Mike Wellard on 26th July 2020 at 7:38 pm

    Hi Paul,

    Do you not think there is any value in taking the 24 hour urine test to assess how much T3 is being utilised by the body.

    I was treated by Dr Peatfield and when I first saw him my blood tests showed T3 levels at the top end of the range but I still felt ill. Dr Peatfield recommend the 24 hour urine test which revealed that my T3 utilisation was at the very bottom of the range – a complete contrast to the blood test results.

    Dr Peatfield had diagnosed me with mild adrenal insufficiency and after a few months treatment I took the urine test again which revealed a much higher T3 utilisation.

    • Paul Robinson on 27th July 2020 at 9:27 am

      Mike, this blog post is all about the use of FT3 when on T3 therapy and in that situation the best measure of T3 effectiveness is still symptoms and signs. Clearly, testing cortisol is often essential and this can make any T3 work more effectively.
      Best wishes,

  3. Anneli Tähtinen on 14th October 2020 at 10:46 pm

    Hi Paul,
    I have a high heart rate and the body temperature is normal or a little low but still FT3 is low. I have been on T3 only for 4,5 years whereafter the lab test dropped from the upper range to low. My dosage has been 43,75 mcg all the time and now in the summer My doctor prescribed some NDT in addition to T3: The range was low for a year and now after taking a small dose of NDT it is gradually rising but not good at all yet Maybe my symptoms are caused by adrenal issues.I’ve been having a lot of stress all the time. I took the Dutch-test recently because of my hormonal imbalances and let’s see if there is something to be found.. I haven’t received the result yet.
    Now my doctor wants me to take only the morning dosage of thyroid medication for 15 days and then make again the test TSH,FT3 and FT4 but I should take the NDT dosage in the morning and the test at 11 o’clock. I don’t know what is the point here.

    • Paul Robinson on 15th October 2020 at 3:06 pm

      Hi Anneli,
      I would need a lot more information that this to have a really informed answer.
      However, blood test results are not the best way to manage T3 treatment. My Recovering with T3 book has a safe and good protocol for managing T3. Low FT3 is usually not a healthy situation.
      In terms of the heart rate, this could be caused by low cortisol for sure, but equally low T3 levels can also cause a strain on the heart and in some people high heart rate and high BP can result.
      It would be best to test cortisol.
      Best wishes, Paul

  4. Bente Seljestokken on 20th November 2020 at 11:43 pm

    I really want to by your book, but I wonder what you recon a small dose T3? I’m on 35 to 40 mcg together with 100 mcg T4.

    • Paul Robinson on 21st November 2020 at 10:12 am

      Hi Bente,
      Dosing of T4 and T3 is unique to each person. It depends on symptoms and signs and on some key lab test results.
      I usually recommend people begin by reading my The Thyroid Patient’s Manual book as this covers the right lab testing and how to use symptoms and signs. It also discusses various thyroid treatments including T4 / T3.
      It is impossible to give useful advice based on so little information and it would be unwise of me to attempt to do so.
      Good luck!
      Best wishes, Paul

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