What Are Optimal Thyroid Levels?

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Originally published on 29th May 2016
Last updated on 14th October 2021

It’s a term I, and so many other sources for thyroid information, use a lot. What Are “Optimal Thyroid Levels”?  

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When your doctor runs a test and you receive the results, your levels will either fall inside or outside of the range. Ranges are usually given in brackets besides the test result.

However, as thyroid patients, it is important that we advocate for ourselves when it comes to where in range they fall. After all, it is not just about falling within the range. We are looking for something more specific.

‘Optimal levels’ are a more specific section within the given range, which is often very wide and much less specific in comparison.

Optimal Matters 

You’ll find that many thyroid advocacies and progressive medical practitioners agree that when testing a thyroid panel (or “Thyroid Function Test”), a TSH less than 2 or 2.5, and a Free T3 in the top quarter of the range, with a Free T4 mid-range or a little higher is considered optimal. Antibodies as low as possible are also seen as optimal as this signals Hashimoto’s being more under control or ‘in remission‘. 

These optimal levels mean that a lot of thyroid patients say they feel most well when their TSH is 2.5 or below, for example. Or their Free T3 is at 17.5 or above on a scale of 10-20.

For you, you may feel best somewhere else within range, but you should try to find out and maintain it at what you feel best at. This is the definition of optimising your thyroid treatment.

Since a lot of us didn’t have our thyroid levels tested before we developed a thyroid condition and began to feel unwell, we often don’t know what our levels were when we felt well. So you must work with your doctor to find where your own individual optimal levels are.

What Are Some References That I Can Use When Speaking to My Doctor About Wanting to Reach Optimised Thyroid Levels?

Blood Test Kit

References for TSH:

In 2002, the National Academy of Clinical Biochemistry (NACB) issued new guidelines for the diagnosis and monitoring of thyroid disease, which reported that the TSH reference range was too wide and actually included people with thyroid disease, thus making it inaccurate.

When more sensitive screening was done, which excluded people with thyroid disease, 95% of the population tested had a TSH level between 0.4 and 2.5. As a result, the NACB recommended reducing the reference range to those levels. [1]

This report backs up the optimal TSH level of being below 2.5.

Based on the NACB’s findings, in January 2003, so just a few months later, the AACE (American Association of Clinical Endocrinologists) made this announcement:

“Until November 2002, doctors had relied on a normal TSH level ranging from 0.5 to 5 to diagnose and treat patients with a thyroid disorder who tested outside the boundaries of that range. Now AACE encourages doctors to consider  treatment for patients who test outside the boundaries of a narrower margin based on a target TSH level of 0.3 to 3. AACE believes the new range will result in proper diagnosis for millions of Americans who suffer from a mild thyroid disorder, but have gone untreated until now.”

Furthermore, the third National Health and Nutrition Examination Survey (NHANES III) screened 17,353 subjects from 1988 to 1994 and excluded those with diseases or factors known to affect thyroid function too. In the resultant ‘normal’ population of 13,344 subjects, 95% had TSH levels that fell between 0.3 and 2.5, which is almost identical to the findings of the NACB above, again, backing up a TSH below 2.5. [2]

New NHS England guidelines from November 2018 also clearly state that TSH should be between 0.4-1.5 when treated with Levothyroxine medications. [3]

Interestingly, a 2018 study said that:

Hypothyroid symptom relief was associated with both a T4 dose giving TSH-suppression below the lower reference limit and FT3 elevated further into the upper half of its reference range. [4]

For thyroid patients on T3 containing thyroid medication, such as T3 synthetic Liothyronine or natural desiccated thyroid medications, it is worth noting that the higher amount of T3 in these (compared to the ratio of T4 to T3 in a human thyroid gland) has a suppressive effect on TSH. Therefore, your TSH may be suppressed (below the bottom of the range), yet your Free T3 and Free T4 remain within range and not close to overmedication or hyperthyroidism. [5]

This is what I personally experience – when my Free T3 and Free T4 sit within range (and no where near the top of the range even), my TSH remains suppressed. And in the past, even when my Free T3 and T4 have fell below the bottom of their ranges, my TSH still remains suppressed. Thyroid patient advocacy Stop The Thyroid Madness report that this happens to a large amount of those on T3 medications. [6]

References for Free T3:

Regarding aiming to have a Free T3 higher in range, a 2018 study reported that:

“Hypothyroid symptom relief was associated with both a T4 dose giving TSH-suppression below the lower reference limit and FT3 elevated further into the upper half of its reference range.”[7]

A 2018 study concluded that:

“Hypothyroid symptom relief was associated with… FT3 elevated further into the upper half of its reference range… Residual hypothyroid complaints in LT4-treated patients are specifically related to low FT3 concentrations. This supports an important role of FT3 for clinical decision making on dose adequacy, particularly in symptomatic athyreotic patients.” [4]

Moving Forward

It is important to understand that different labs/doctors use different ranges, so you must interpret your results individually; don’t compare them to anyone else’s. You must use your lab’s reference ranges.

A Free T4 at 14, with a range of 9-19, is mid-range for example, but a Free T4 at 11 is mid-range for a range of 7.5-14.5. So both are considered optimal readings.

You must look at your result in comparison to the given range, usually stated in brackets, beside it. Where does it fall?

Work With Your Doctor 

I always encourage thyroid patients to get their test results printed off, for ease of reference and comparison as you try things to correct your thyroid levels and reach good health.

Always work with a doctor on evaluating and reevaluating your thyroid hormone levels, keeping in mind your symptoms and overall health as well. We’re all individuals and there is no ‘one size fits all‘, but there are obviously health risks if your thyroid hormone levels are both too low or too high for an extended period of time. You can present the above information to your doctor.

It’s important to remember that as we’re all individual, optimal levels can look different for different people too.

Have you found your ‘optimal thyroid levels’?

You can click on the hyperlinks in the above post to learn more and see references to information given.

Be Your Own Thyroid Advocate Book Girl HoldingLearn more about how to make the most of your health with hypothyroidism. The book Be Your Own Thyroid Advocate: When You’re Sick and Tired of Being Sick and Tired builds on this article in detail and explains how to thrive with thyroid disease.

References:[1] http://www.aacc.org/sitecollectiondocuments/nacb/lmpg/thyroid/thyroid-fullversion.pdf

[2] https://www.ncbi.nlm.nih.gov/pubmed/11836274

[3] Pages 8 and 12 – https://www.sps.nhs.uk/wp-content/uploads/2018/11/RMOC-Liothyronine-Guidance-v2.0-final-1.pdf

[4] https://www.thieme-connect.de/products/ejournals/abstract/10.1055/s-0043-125064

[5] https://www.ncbi.nlm.nih.gov/pubmed/15705921/

[6] https://stopthethyroidmadness.com/tsh-why-its-useless/

[7] https://www.ncbi.nlm.nih.gov/pubmed/29396968

Further reading:


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12 thoughts on “What Are Optimal Thyroid Levels?

  1. Hi Rachel,

    I read the above article on Optimal Levels for thyroid and clicked on the links to studies you’ve noted at the bottom of article and apparently they aren’t active links or “hyperlinked” as you mentioned. I’d like to print out out a couple of the studies for my endo. I’m a thyca survivor and have been trying to get her to not focus so much on my TSH! My FT-4 is under range and FT-3 is not even midrange on a low dose mix of T-4 @50mcg T-3 @15mcg, yet she is always concerned about my TSH being suppressed. I’ve been dealing w/chronic fatigue and fibromyalgia symptoms for YEARS…finally fired first Endo after reading your book and onto second one. Please let me know if you can activate links so I can print out. Thanks!

  2. Excellent post, Rachel. I find this beyond frustrating and it really makes diagnosis and getting patients help so difficult, even when symptoms are pretty significant it means they get ignored. It seems ridiculous the range can vary between counties in the UK and across different countries. I said this to an old GP years ago and she agreed, but none since have been understanding about the difficulties with thyroid levels. Also worth noting for smoking that TSH can be suppressed by smoking… Another spanner in the works for accurate diagnosis, especially when it relies on TSH and a range that’s often around 5.5+ for hypothyroid diagnosis.

  3. I’m on combo t4 100mcgs plus 15mcgs t3.
    My latest blood results are:
    Tsh <0.03(0.35-4.74)
    Ft4 13(9-19)
    Ft3 3.6(2.6-5.7)

    These results stay the same at all my blood tests.

    The blood is drawn 24 hours after last dose of both t4 and t3.

    I suffer a lot with muscle pain in the upper part of my back and don’t know if this is anything to do with my levels??
    Are you able to reply and advise please

    1. Hi Rosemary, unfortunately I cannot provide medical advice or guidance. However, some patients seem to feel best with their FT3 higher up in range as the article above talks about.

  4. My recent thyroid results are 0.82 range is 0.35-5.

    I have Pernicious Anaemia, low vitd/calcium.

    I have difficulty loosing weight is my main concern.

    I am on 125 of levo, but doctor said it can go up to 150.

    Any ideas on how I can give my slow weight loss a boost!?

    1. That sounds like just your TSH has been tested. Free T3 and T4 should also be tested and getting them optimal also helps many people with losing weight among other thyroid symptoms.

  5. Hi Rachel I’ve been on levothyroxine for over a 4 years 125mcg to 150mcg iam it feels like it’s getting worse with anxiety and fatigue I do a 9 hour a day as a security guard but it’s getting so difficult to stay in employment,but iam going to see an endocryolist in October what would be the main thing I should I ask him many thanks fazil in north Yorkshire x

  6. You can certainly see your enthusiasm in the way you write. The world hopes for even more passionate writers like you who aren’t afraid to mention how they believe. Especially about thyroid problems!

  7. Had private blood test cause not getting anywhere with GP ..been diagnosed 2002 hypothyroidism however I can never stay stable ..either over or under…Currently taking levo…..results tsh 9.11 t4 total 124.0 free t4 20.70 free t3 3.31 b12 18.1 please could you explain what’s going on I’m a bit of a novice….

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