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 may refer to at least one blog post with its link during this discussion.

The questions I would like to discuss and try to answer are:

1) Can the TSH, FT4 and FT3 values within their reference ranges be used to guide thyroid treatment with T4, T4/T3, NDT, and T3-Only?

2) Even if they cannot be used to guide/lead thyroid treatment, can they still be used in some form for all the different thyroid medications?

Let us start with 1 – Can TSH, FT4 and FT3 within their reference ranges be used to guide thyroid treatment?

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 now 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:

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. 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 answer to the question is “No, not alone – patient symptoms and signs are more important, only FT3 tracks symptoms and TSH can be suppressed. Do not assume that FT4 changes mean the treatment is working.”

Let me now discuss question 2.
Can the lab ranges still be used in some form for all the different thyroid medications – T4. T4/T3, NDT and T3-Only?

This may be an even more contentious question for some people. The list of medications includes T4/T3, NDT, and T3-Only, and some doctors believe that T4-Only is all that is ever needed. Thyroid patients know that T4-Only often fails to resolve symptoms and that all the treatments need to be available. I certainly believe this latter statement.

See the research papers 3, 8, 9, 12, 15 and 16 within the collection of studies contained within the link above. I am not even going to get further into this aspect of the discussion as it is obvious to me – T4-Only therapy frequently fails to resolve patients’ symptoms.

All the thyroid hormones have to be available for clinicians to use in the treatment of their patients.

So, can the laboratory reference ranges still be used?

Well, we already know that an individual has personal reference ranges for FT4 and FT3 that are less than half as wide as the wide population ranges.

This means that just because someone has FT4 and FT3 in the reference range (potentially the middle) they may still be inadequately treated.

Let me talk a little about T3-Only therapy for a moment. In a healthy person, all the cells have ongoing T4 to T3 conversion going on inside them. This is enabled by good quality deiodinase enzymes. Much of this converted T3 is used by the cells and is not returned to the bloodstream. So, when a person has an FT3 test (measured in the blood), the FT3 measured does not account for the extra converted T3 within the cells. So, a healthy person or someone on T4-Only meds have more FT3 than shows up in an FT3 blood test, as we cannot measure what is in the cells.  So, for a person on T3-Only therapy, the current FT3 reference range is wholly inadequate, as many people on T3-Only need enough T3 medication to account for both blood T3 and cellular converted T3. The top of the FT3 reference range for people on T3-Only is too LOW. See this blog post for a better description:

For people on T4-Only therapy, the top of the lab ranges for FT4 and FT3 are going to be fine to work with. This is true as long as the caveats I have already stated are taken into account, i.e. that actual individual ranges are less than half as wide, FT3 is the only lab to track symptoms, and a suppressed TSH is acceptable (if the person is not hyper and FT3 is not exceeded). Most importantly, patients’ symptoms and signs are the most critical things to focus on.

Now, for T4/T3 or NDT therapy the answer might not be entirely the same as for T4-Only. NDT has a 4:1 ratio of T4 to T3. This is not the normal human thyroid produced T4:T3 ratio, which is more like 8:1. So, in some cases, the top of the FT3 range might be a little too low for some people – similar to the case with T3-Only therapy but not as extreme. For T4/T3 treatment it depends on how much T3 is being given. As the T3 amount increases the same issue might begin to occur as with T3-Only – the top of the reference range for FT3 might need to be exceeded. As long as the patient is not hyperthyroid and symptoms and signs are all healthy I would see no issues with that on T4/T3 or NDT. This is not backed up by a specific research study right now but I believe it to be true through logical deduction (and experience).

This situation is entirely reasonable. The FT4 and FT3 population lab reference ranges are constructed from the blood samples of healthy people or people on T4-Only therapy. Why would anyone expect the same lab ranges to be totally transferrable to people who take T3 in combination or T3-Only? That does not follow at all – yet this is how most doctors attempt to use them. This is a big flaw in current practices.

So, my answer to question 2 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.

Actual patient individual ranges will be less than half as wide as the population ranges. As medications that include T3 are used the top of the FT3 reference range might be too limiting. Certainly, for T3-Only therapy, the top of the FT3 reference range is likely to be far too limiting. For some patients on NDT and T4/T3, the top of the FT3 reference range might also be a little limiting.”

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.

Best wishes,


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.

Like this post? Then why not share or print it using the buttons below:

Leave a Comment