Thyroid Laboratory Test Results and Ranges: Can They Really be Used for All Thyroid Medications? 

I have written about this topic in my books, especially The Thyroid Patient’s Manual, and in various blog posts. I am going to speak very frankly and from 35 years of experience, research and working with thyroid patients (huge numbers who have got well as a result of these conversations and from reading my books).

Let’s start with the ‘belief system’

Doctors and endocrinologists are trained to believe that all people must conform to thyroid lab results that fit with the laboratory reference ranges. The ranges are based on an assessment of mostly healthy people and only some thyroid patients.

The reference ranges are ‘normal ranges’ and this is a statistical term that can also be written as ‘Gaussian range’. This means that most people fit somewhere within it. The spread of a population of tested patients will range from only a tiny number at the bottom of the range, with far more around the middle of the range and some at the very top of the range. This is why a ‘normal range’ is often described as a ‘bell shaped curve’. The important thing here is the range represents a distribution of people. Therefore, having results somewhere within the reference range for something important like FT3 does NOT mean you are correctly treated. You have to be in the right place within the reference range for you.

So, thyroid patients have to be in the right spot in the reference range for them. See this post that backs this up with a research paper: https://paulrobinsonthyroid.com/research-shows-free-t4-and-free-t3-ranges-for-individual-thyroid-patients-are-less-than-half-as-wide-as-the-wide-population-laboratory-ranges/

The belief system that is currently being used by most doctors though is that a thyroid test result within the range means the thyroid patient is ‘normal’ because the range is a ‘normal range’. This is really very incorrect.

In addition to the above, most doctors and endocrinologists do not think that the thyroid patient’s Free T3 level is crucial. Most of the time, doctors do not even test FT3 – they test TSH only, or TSH and FT4. If they test FT3 at all, then having it fall inside the ‘normal range’ is ‘good enough’.

However, having an FT3 result which is at a good point for the individual is critical. Why is this? T3 is the active thyroid hormone. TSH and FT4 do not mean much in terms of the thyroid patients health but Free T3 (FT3) does. Only T3 is the active hormone at our cell nuclei and only T3 helps to improve symptoms. Two more important blog posts:
https://paulrobinsonthyroid.com/t3-is-the-biologically-active-thyroid-hormone/
and
https://paulrobinsonthyroid.com/only-free-t3-ft3-tracks-changes-in-symptoms-during-thyroid-treatment-research/

So, we start with a belief system amongst doctors and endocrinologists that having thyroid test results within the reference ranges means ‘normal’. Even low in the range or mid-range FT3 is considered fine. Changing thyroid medication dosages in any way that might make TSH low (suppressed) or not keeping FT4 and FT3 in range is seen as totally unacceptable. In fact, doctors and endocrinologists often call patients in to see them to adjust their thyroid medication if their lab test results fall outside the ranges.

The belief system extends to completely embracing the lab testing method where the results are looked at and then the thyroid medication is adjusted almost entirely based on manipulating thyroid results somewhere within the reference ranges. It is almost like a game. The focus is rarely highly focused on the patients signs and symptoms and on the Free T3 thyroid hormone.

Sadly, doctors and endocrinologists during meeting after meeting with the thyroid patient convince him/her of these beliefs. Over time, the thyroid patients come to accept the same belief system. The thyroid patients often feel that the appointments with their doctor or endocrinologist to review the latest set of lab test results are actually going to help them. Thyroid patients often come to think that they actually need these lab tests and regular reviews to ensure that they are properly treated.

Frequently, it is only when thyroid patients have been sick for many years do they begin to question all of this. It is a very, very sad state of affairs and it continues as I write this. The belief system has infected a great many of the medical profession and it has also been passed on to thyroid patients. Hopefully, some of the thyroid patients realise this at some point.

The belief system is flawed. That is the first thing I needed to say.

So, what can we say about TSH, FT4 and FT3 values and their reference ranges during thyroid treatment?

Let’s start with TSH.
I make it very clear in The Thyroid Patient’s Manual that, once someone is on thyroid treatment, the use of laboratory test results is not the same as during diagnosis. We know from research that during thyroid treatment, it is acceptable for TSH to be close to zero/suppressed. Most doctors still mistakenly believe that this means the patient is hyperthyroid or thyrotoxic – this is simply not true. If the patient shows no clinical presentation (symptoms or signs) of being hyperthyroid and as long as FT3 is not over the very top of the reference range, a suppressed TSH is acceptable and not a concern.

See points 1, 6, 11, 12, 14 and 15 within this list of research studies:
https://paulrobinsonthyroid.com/collection-of-research-papers-for-easy-access-research/
also
https://paulrobinsonthyroid.com/suppressed-tsh-on-thyroid-treatment-does-not-mean-the-patient-is-hyperthyroid-research/

I also make it clear in The Thyroid Patient’s Manual that only the FT3 lab test value correlates to patient symptoms, i.e. if the thyroid treatment can be adjusted so that symptoms improve, it is the FT3 that has invariably risen. So, during treatment FT3 ought to rise and as it rises, without high Reverse T3 (rT3), the patient should feel better.

FT4 is not correlated to symptoms at all – so tracking FT4 with the view that if it goes up the person must feel better is a flawed concept. FT3 is the only thyroid lab correlated to symptoms and as such is important to track – see point 5 in the same list of research studies I provided above.

The thyroid labs are useful to track but the patient’s response in terms of symptoms and signs should be the most important thing a doctor tracks. The clinical presentation of the patient is more important than thyroid labs.

So, the focus really needs to be on the clinical presentation of the patient – symptoms and signs are more important than the lab test results. Only FT3 tracks symptoms and TSH can be suppressed. Do not assume that FT4 changes mean the treatment is working. Thyroid lab test results are useful to look at during treatment but they should not be the main guide during it – the clinical presentation of the thyroid patient (signs and symptoms) has to be placed front and centre.

Those on T4 treatment that are responding well to T4, can, of course, use the current lab tests and lab ranges, with the caveats explained above in the ‘belief system’ section.

However, once T3 is added things change. T3 is added to T4 treatment in some cases, because T4 medication often does not resolve symptoms. See the research papers 3, 8, 9, 12, 15 and 16 within the collection of studies contained within the link above.

As the T3 aspect of the combination is increased, frequently the T4 content has to be reduced in order to avoid excess rT3 and to actually increase FT3. See: https://paulrobinsonthyroid.com/more-t4-t3-thyroid-medication-might-not-always-raise-patients-ft3-levels-in-thyroid-hormone-treatment/

As this happens TSH can get low or suppressed, FT4 can fall close to the lower end of the reference range (or even below it). As more T3 is added to thyroid medication, FT3 is so influenced by the time the last T3 medication was taken that it becomes a far less useful measure (other than to know that the T3 medication raised FT3). In order to be able to compare FT3 test results, thyroid patients ought to always leave the same amount of time between the last T3 dose and the blood draw.

However, be aware that any lab tests done within 12 hours of the last T3 dose can risk a high FT3 result. This is why I often recommend an 18-24 hour gap from the last T3 dose and the blood draw to avoid a doctor decreasing the needed T3 medication due to a short-term raised FT3. It is also best to do a private blood test first to be certain that your guessed interval with no thyroid meds prior to the blood draw is actually enough to keep your doctor happy with the TSH and FT3. Until the knowledge of how T3 (even in T4/T3 or NDT therapy) can change the situation with the lab test results is much better in the medical community, this is the type of thing that patients need to do to get well and stay well:
https://paulrobinsonthyroid.com/can-ft3-be-used-to-manage-liothyronine-t3-thyroid-treatment/

It is possible as the patient is given more T3 and potentially less T4 that both FT3 and FT4 do not conform to the reference ranges any more, i.e. they fall outside of them. TSH might also be very low. The lab tests themselves and any attempt to adjust thyroid medication to ensure that the results fit into the reference ranges can at this point result in extremely poor choices being made. As more T3 is used and less T4 is used, there comes a point where the value of even doing the laboratory testing of thyroid levels becomes questionable.

I will not discuss T3-Mostly or T3-Only therapy here as this can be even more extreme. I will refer you to another blog post: https://paulrobinsonthyroid.com/can-ft3-be-used-to-manage-liothyronine-t3-thyroid-treatment/

So, can thyroid lab test results and ranges be applied with all treatments?

My answer to this is, “No!” The laboratory reference ranges have to be used as a guideline. Lab test results that fall within these ranges do not mean a thyroid patient is correctly treated. Lab test results that fall completely outside the lab test ranges do not mean the thyroid patient is incorrectly treated. Signs and symptoms need to guide the treatment.

Thyroid patients need to put less trust in these highly trusted lab test results and references ranges. The medical profession – endocrinologists and doctors – need a massive upgrade in their knowledge, understanding and treatment practices. Many of them feel handcuffed by guidelines but this has to change.

All the thyroid hormones still have to be available for clinicians to use in the treatment of their patients. It is just that one has to be careful not to worship the lab results in the same way once T3 is added to treatment. Seeing changes occur in labs is a good thing – it means the way that the levels are changing is working. But trying to fit people on T4/T3 or T3-Only into the same lab result straight-jacket using the same reference ranges is not going to work.

Doctors who always insist on keeping all thyroid lab test results within the reference ranges will actually ensure that some patients remain sick. Some thyroid patients can only get well through not conforming to the rigid ranges that are designed for healthy people or those on T4 medication. This is the bottom line.

Much is not right in current thyroid treatment practice. I hope this provides more insight to those of you who are trying to make sense of labs and lab ranges.

Here are 4 other blog posts that add to the information above:
https://paulrobinsonthyroid.com/biochemical-bigotry-enforcing-normalised-thyroid-lab-test-results-is-harmful-to-health/
and
https://paulrobinsonthyroid.com/chasing-ideal-thyroid-hormone-lab-test-results-is-the-road-to-nowhere-for-thyroid-patients/
and
https://paulrobinsonthyroid.com/the-ft3-laboratory-reference-range-may-not-be-applicable-for-thyroid-patients-taking-t3/
and
https://paulrobinsonthyroid.com/thyroid-treatment-focused-on-laboratory-test-results-and-reference-ranges-is-failing-thyroid-patients/

Best wishes, Paul
(First written in July 2019. Completely re-rewritten in November 2023 – to clarify and be more open about my views)