How Important is the Free T3:Reverse T3 Ratio? - Dr. Westin Childs | Thyroid & Health Supplements That Work

How Important is the Free T3:Reverse T3 Ratio?

As a thyroid patient, you're probably well aware of some of the more advanced tests available to assess thyroid function including things like reverse T3 and sex hormone binding globulin

But, are you aware of the calculation (not lab test) of the free T3:reverse T3 ratio?

This calculation can give you a lot of extended information about your thyroid and can/should be used by many of you. 

This article will walk you through why it's important, where it falls short, how to calculate it, and more. 


What is the Free T3:Reverse T3 Ratio?

If you are new to the thyroid world then these terms may not even make sense to you. 

Don't let that scare you away as they really aren't that complicated to learn or to understand. 

Let's start with the basics:

What is the free T3:reverse T3 ratio?

This ratio is calculated when you divide these two values together. 

What's important for you to understand here is that this ratio is CALCULATED based on two other lab tests: the free T3 and reverse T3

You need to know it's a calculation because otherwise, you are going to be confused when you look at lab requisition forms and you don't find it anywhere on there. 

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In order to get this ratio, you need to do a little bit of math. 

But, don't freak out because the math is easy and we will talk about the calculation a little bit more below. 

But, before we do, here are just a few of the reasons you would want to do this math:

  • This ratio helps you understand how well your body is converting T4 into T3 - read more about this here if you haven't heard about this conversion process. 
  • It helps you understand if your current medication and dose are working for you - If your medication is working then your free T3 should be normal and your reverse T3 should be low/normal. 
  • It helps put your symptoms into context - Many people find that they are confused because of their so-called 'normal' lab tests and this ratio can explain how you can have a normal TSH but still not be getting enough
  • It can help you understand how to adjust your medications - many patients do well on a combination of both T4 and T3 medication but most doctors only prescribe T4. This can be a problem for certain individuals and you can identify if you fit into this category with the help of this ratio. 
  • It can help you understand other factors which you should be treating/evaluating - factors such as gut health and inflammation can all influence this ratio. If your ratio is off then it can help direct you to other problems which may be influencing your thyroid. 
  • It's one of the most sensitive measures of thyroid function - the difference between your free T3 and your reverse T3 can identify thyroid dysfunction very early on and becomes abnormal long before other measures of thyroid function such as TSH and free T4. 
free t3 and reverse T3 are more sensitive than TSH to your health status

What Does it Tell you?

Clinically speaking, this ratio is supposed to tell you how much free T3 you have RELATIVE to reverse T3 in your serum. 

This is important because you can think about these two thyroid metabolites (T3 and reverse T3) as antagonistic to one another. 

The more free T3 you have (within normal levels) the more likely your cells are to become activated by thyroid hormone. 

The more reverse T3 you have in your body the more likely it is to BLOCK the positive effects of free T3 on your cells. 

This occurs because both free T3 and reverse T3 compete for the same cellular thyroid receptors (1). 

The main difference between them is that when free T3 is sitting on the thyroid receptor that good things are happening in the cell. 

If reverse T3 sits on the receptor it does nothing for the cell except block the positive effects of T3. 

So, as a patient, you want your free T3 levels to be much higher relative to your reverse T3 levels. 

As long as you have sufficiently high free T3 levels you can make the assumption that your cells are being activated by thyroid hormone appropriately. 

And your free T3:reverse T3 calculation simply tells you how likely this is to occur. 

As a patient, you then have two things to worry about:

Factors which positively influence your free T3 level <-- you do want this to happen. 

And factors which increase your reverse T3 level <-- you don't want this to happen. 

It turns out that there is a single mechanism which controls both levels at the same time. 

This mechanism is referred to as T4 to T3 conversion or peripheral thyroid conversion. 

This conversion process is incredibly important because it's not under conscious control by you as the patient. 

Instead, it's under physiologic control based on the environment that your cells are in. 

The more readily your body converts T4 into T3 the better. 

There are several factors which influence this conversion process in a negative way including things such as calorie restriction (2) (this is why cutting your calories is so bad if you have thyroid disease!), chronic infections (3) (some of the same infections that trigger Hashimoto's), inflammation (4) (from any cause), gut disruption (many thyroid patients suffer from GI issues), nutrient deficiencies (which is why I include certain nutrients in my thyroid conversion boosting supplement), and more. 

These factors, if present in your body, will PUSH the conversion of T4 into reverse T3 instead of free T3. 

natural thyroid supplements version 2

The more severe the factors the more severe the disruption to thyroid conversion. 

Early in my career, I use to see patients who were fresh off of the hCG diet come into my office and present with reverse T3 levels in the 40's to 50's ng/dL. 

These levels are incredibly high with respect to the standard reference range which typically goes from around 5 to 25 ng/dL. 

A reverse T3 level that high will almost always be accompanied with a very low free T3. 


Because, in the case of calorie restriction, the body is trying to slow down the metabolism to match the reduced calories that you are consuming. 

It does this by blocking thyroid hormone which would normally act to increase your metabolism, increase the number of calories that you burn, and increase your heart rate. 

It blocks this process which slows down your metabolism, causes weight gain, lowers your heart rate, and lowers your body temperature.  

Calculating your Free T3: Reverse T3 Ratio

Remember when I said you would have to do some math to get this value? Well, we are on that part. 

Don't freak out though because it's very easy to calculate. 

All you need to do is take your free T3 and divide it by your reverse T3 (see the example below). 

The only somewhat challenging part can be ensuring that your units are correct. 

This is typically only an issue if you are getting your lab test outside of the United States but for purposes of this article you need to make sure that your units are as follows:

  • Free T3 (sometimes referred to as triiodothyronine, free on lab tests) should be in pg/mL
  • Reverse T3 (sometimes referred to as triiodothyronine, reverse on your lab tests) should be in ng/dL

If your reverse T3 and free T3 are not in the proper units you can use a calculator such as this one to transfer them to the correct units. 

Once your units are correct then you can simply do the calculation. 

So, what kind of value should you be looking for?

In general, you want your calculated value to be greater than 0.20. 

If your calculated value is less than 0.20, such as 0.15 or something similar, then this might be an indication that you have an issue with thyroid conversion or you are on the wrong medication. 

From here you may want to favor therapies which tend to increase your T3 (such as those found here) over therapies which tend to increase your T4. 

Let's look at a practical example (from one of my patients):

free t3 example
reverse t3 example

In this example, we have our value of 2.8 for our reverse T3 and a value of 14.4 for our reverse T3. 

Because they are in the proper units (free T3 is in pg/mL and reverse T3 is in ng/dL) we can divide 2.8 by 14.4 to get the value of 0.194. 

This value is less than our 0.20 cutoff that I mentioned previously and so this patient may have an issue which needs to be corrected. 

In this situation, I would take a deep dive into the history and other lab tests of this patient to determine if they are indeed converting thyroid appropriately or if they have issues which may be blunting this process from occurring. 

If the units are not correct then you would want to convert them before doing the calculation. 

Remember, though, that this calculation is not perfect and the ratio that works for your body may be different from what others need. 

So don't be alarmed if you find that your free T3:reverse T3 ratio is less than 0.2 and you are feeling amazing. 

If you are feeling poorly, however, then this data point can definitely help put you on a different path and help you. 

How helpful is it? 

The question really becomes:

How helpful is calculating the free T3:reverse T3 ratio and is it something that you should look at every time you order your lab tests?

The answer is not as straightforward as you might think. 

While calculating this ratio is certainly helpful, I wouldn't consider it vital in every single case. 

Certainly, however, if you looked at the data you would find that some consider this ratio to be the single most important thyroid test (or calculation, depending on how you look at it). 

But I just don't see this to be true in clinical practice. 

One reason for this is that this ratio can be manipulated based on the type of thyroid medication that you are taking and the time of day that you get your lab tests drawn. 

This can result in you forcing your ratio into an "optimal" range and yet still feeling the symptoms of hypothyroidism. 

The reason for this has to do with how volatile your serum free T3 level can be. 

For instance, if you take any medication which contains T3 and then check your thyroid lab tests within a few hours, you will find that your free T3 level skyrockets up. 

It will rocket up to high levels but it will eventually fall down over the next 24 hours. 

So, depending on when you test your free T3, and when that compares to when you took your T3 medication, you can artificially adjust your free T3:reverse T3. 

But this doesn't necessarily mean that you are doing the right thing or even helping your situation. 

So, what does this mean for you as the patient?

It means that the free T3:reverse T3 ratio is something that you will want to keep an eye on it, but I wouldn't base or adjust your medication solely on this factor. 

When you get into advanced thyroid lab testing you can also check other factors such as your sex hormone binding globulin and total T3. 

Both of these lab tests tend to be better markers of long-term T3 status in the body. 

Your Doctor and the Free T3:Reverse T3 Ratio

Based on how important of a calculation this ratio is, you might be surprised to find out that your doctor may have never heard about it. 

If it's so important why don't more doctors know about it?

I think the main reason has to do with the fact that most doctors are content looking at the TSH and Free T4 values

From their perspective, there really isn't any other reason to look beyond those tests because that is how they are trained. 

But that doesn't make this ratio any less important, especially for those who have more atypical types of cases. 

In fact, the more atypical case that you have, the more likely you are to benefit from extended thyroid lab tests and calculations. 

And by atypical I am referring to those patients who don't respond well to T4 medications such as Synthroid or levothyroxine, to those patients who are taking medication but still symptomatic, and so on. 

These types of patients benefit greatly from calculating this ratio. 

The good news is that you don't really need your doctor to calculate this test but you do need him/her to order the requisite lab tests on your behalf. 

Once you have the lab tests though you can simply use the guidelines above to calculate it. 

As always, my recommendation is to find a doctor who is willing to work with you but if that isn't possible given your circumstances then you can still go about calculating it yourself. 


My personal opinion is that you should not only be aware of the free T3:reverse T3 ratio but that you should also understand how to calculate it and at least calculate it on some of your lab tests. 

Believe it or not, however, I don't actually calculate this ratio myself on all of the patients that I treat. 

This is not because I don't think it's important but more because I can generally see the trend of the free T3 and reverse T3 and overall thyroid pattern without doing the actual calculation. 

But, understanding the importance of this metric is very important, especially for those who are just starting on their thyroid journey. 

Now I want to hear from you:

Have you calculated your free T3:reverse T3 ratio?

What did your results tell you?

Was it what you were expecting? 

What do you think could be contributing to your result?

Leave your questions or comments below! 

References (Click to Expand)

free t3 reverse t3 ratio

Dr. Westin Childs

About Dr. Westin Childs

Hey! I'm Westin Childs D.O. (former Osteopathic Physician). I don't practice medicine anymore and instead specialize in helping people like YOU who have thyroid problems, hormone imbalances, and weight loss resistance. I love to write and share what I've learned over the years. I also happen to formulate the best supplements on the market (well, at least in my opinion!) and I'm proud to say that over 45,000+ people have used them over the last 4.5 years. You can read more about my own personal health journey and why I am so passionate about what I do here.

14 thoughts on “How Important is the Free T3:Reverse T3 Ratio?”

  1. I am curious as to whether you think my labs are indicative of a serious thyroid problem: Ft3 = 2.7, Rt3= 36.8, ratio = .07.
    I’m have been on 30mg of armour (divided dose) daily since 1999. Have hypo symptoms recently–fatigue, sleep, morning headaches. Age 74. Thank you!

    • Hi Leigh,

      Labs are only part of the equation when considering thyroid disease. You also need to assess clinical symptoms and put them into context. If the symptoms fit AND the lab tests are abnormal, then those two things have strong positive predictive value for the presence of thyroid disease.

  2. Hi, I just went to an endocrinologist to get more intensive blood work to find out why I am getting more and more nodules on my thyroid. Had 2 and over the last 7 years I now have 8 or 9. She sent me for lab work. TSH is 1.88, T4 Free is 0.9, T3 Total is 96 and Thyroid Peroxidase Antibodies is 1. I wanted to use your calculator to find out if I am within what ranges and now I see Free T3, Reverse T3 and Thyroglobulin Antibody on your information pages. I was not tested for this? Why would I not be tested if this matters? And Should I ask this doctor to perform these two additional tests or just go with the information I now have. I feel lousy and I am tired of going for yearly ultra sounds given by my primary doctor only to find out that I am not getting anywhere on preventing nodes from growing or multiplying. I am 68 and feel 80. So discouraged because I believed going to a specialist would give me the proper tests to find out what I could do. My primary just says I am on the border line …maybe. Am waiting for Endocrinologist to get back to me. Are more tests needed to get an answer and should I press this doctor to give me those tests or just look for someone else?

    • Hi Josette,

      There isn’t a strong link between thyroid function and thyroid nodules which is probably why your doctor didn’t order those tests. Most people with thyroid nodules do NOT have issues with thyroid function (this is the general rule). In addition, ordering these tests do not tell you WHY you have the nodules. To do that you will probably need to biopsy them (with genetic testing) and get a very comprehensive and environmental exposure history.

      I should also point out that most doctors are not concerned with the “why” behind people have thyroid nodules. They are more interested in if they are cancer or not.

  3. Hi there,

    I have a ratio of 0.197

    T3 being 3.7498 and reverse t3 being 19

    T4 = 15 pmol/L
    T3= 5.76 pmol/L

    My insulin in 170 pmol/L

    I am on Erfa ( 1.5 grains in morning and 1 grain in afternoon).

    I am struggling to lose weight and at a loss what to do next.

    What would you suggest?

  4. I was diagnosed with Hashimoto’s thyroiditis, hypothyroidism about a year ago. My MD will only test for TSH, So I ordered a “Thyroid Panel” from a private testing company.

    This what I got back: TSH 0.08, Free T4 1.20, and Free T3 2.46: That’s it-no Reverse T3 or T4. (it was not offered for testing.)

    Could you enlighten me as to how I can get those tests? I live in a remote mountain town in Colorado. Thank you [email protected].

  5. My calculation is free T3 was 4.3, Reverse T3, 14.0, the ratio is .31. My TSH is 10.65. I still cannot get to feeling better, in fact, I have a nodule on the front of the right side of my neck. A US needle biopsy was done Atypical cell, nonconclusive, more test are scheduled. I do not want to lose my thyroid!

    Katherine Reighley

    • Hi Katherine,

      Typically, we don’t biopsy thyroid glands based on thyroid lab results so your thyroid nodule and your abnormal thyroid lab tests are probably unrelated (except that you have two different conditions in the same gland).

  6. Thank you Dr Childs for all of the information you provide, it is invaluable. I have been struggling with thyroid disease my whole life (45+ years) because I have never been able to get the help I need in Australia, or the UK when I lived there. No follow up after surgery as a teen, I became severely hypothyroid and extremely ill. I finally borrowed a medical book from the local library and made the diagnosis myself – then asked a GP to please do a blood test. Started on thyroxine at 21. Have also been diagnosed with fibromyalgia. Having reached rock bottom with no quality of life, I started researching and first found Dr John Lowe’s research. I only fairly recently found you. I am currently looking for a suitable doctor I can access (who doesn’t just treat with Levothyroxine) as the local specialist doesn’t even test for T3. I finally managed to get a GP to agree to an RT3 test, and as I suspected, it is off the chart! My last RT3 was 950 pmol/L with the ref range in Australia being (140-540). Of course the GP didn’t know what to do with the results and sent me away with no advice. My last Free T3 was 4.4 pmol/L with the ref range in Australia being (2.6-6.00). Doctors always want to reduce my Levothyroxine based on my TSH which since surgery has been negligible, usually around 0.01mIU/L. After repeatedly becoming even more hypothyroid from doing this, I stopped following this advice.

  7. Dr Childs-
    I have been on NP Thyroid or Naturthroid for 4 years now at a dose of 0.5 gr. The main reason to begin was excessive hair loss, which I still have. I now wear a wig. My TSH and Free T3, Free T4 have always been normal, yet my Free T3 hangs at the very bottom of the normal range. My naturopath has always told me to test without taking my throid meds that same day. Last year I began taking my thyroid meds on the day of testing and scheduling my appointment 3-4 hours subsequent to have a direct comparison each time. The last time drawn I began taking a thyroid conversion supplement (6 weeks prior) to see if it would cause an increase in Free T3. I made it to a level of 3.0 pg/mL, which is the highest level I have ever seen. Previous high was 2.8. Is this my “skyrocket” number that you refer to in your article? If so, it’s not that high. I still think I have a significant conversion issue. My Reverse T3 fluctuates and was lastly 15 ng/dL. So my ratio calculates out to be 0.20, which is the lower threshold of your normal range. Still wondering if I need an additional T3 only med. How can I get a consult with you?

  8. Dr. Childs.
    I used the calculations for theT3/RT3 Ratio from this site.
    I entered the results from a recent lab test into this converter making sure I had the correct units listed on the lab results.
    Using RT3 of 15.8 ng/dl and 2.2 pg/ml, my result was a ratio of 13.9.
    Using your method I get .139. Your value is moving the decimal point two places to the left from what I get using the calculator mentioned above.
    I don’t understand the 2 different values. What am I missing?

  9. I’m a 63 yr old female, feeling everyday very fatigued, tired, brain fog, and hair loss.
    My Free T3 is 2.3 pg/mL – Rev T3 is 14 ng/dL giving me a figure of 0.164 when divided. So below the 0.20 cut off.

    I do have small intestinal bacteria overgrowth going on, constipation, hiatal hernia and hormones out of balance. I am working on getting rid of the SIBO with herbs. I don’t know what to do next with all of this. What would you suggest I do about my thyroid?


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