Why Free T3 & Total T3 are the Most Important Thyroid Lab Tests

The free T3 and total T3 lab tests are among the most important for evaluating thyroid function in your body. 

These tests give vital information as to how your body is processing thyroid hormone

And they give you information about the hormone which is responsible for activating your cells and turning on your genes. 

These basic levels can be assessed and used in conjunction with other thyroid lab tests to give you a very clear picture of thyroid function in your body. 

This post will teach you everything you need to know about free T3 as a lab test and how to understand your levels...

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What Does Free T3 Tell You About your Thyroid?

There are many ways to assess thyroid function in your body and many different lab tests. 

You are probably already familiar with TSH or thyroid stimulating hormone

This lab test gives you an idea as to how responsive your thyroid is to your brain and if that connection is working properly

But what does free T3 tell you about your thyroid?

Free T3 gives you an idea as to how ACTIVE your thyroid is at the cellular level. (1) 

The measurement of free T3 is telling you how much free and active thyroid hormone your body as available to work with. 

how the body creates free t3

Free T3, after all, is the single most important thyroid hormone in your body. 

It's the hormone that is responsible for attaching onto the surface and the nucleus of your cells and making genetic changes to those cells. 

This simple process is what makes your thyroid increase your energy, manage your weight, regulate your cycle and so on. 

But what happens if you don't have a sufficient amount of free T3 in your body?

You WILL experience the symptoms of hypothyroidism. 

Why?

Because free T3 does all of the heavy-lifting when it comes to your thyroid. (2)

If you aren't familiar with the thyroid conversion process or why it's so important for your body to go through this process please see this short video below which will bring you up to speed: 

TSH vs Free T3, Total T3 & Free F4

Like most people, you are probably more familiar with the TSH as a marker for testing your thyroid. 

And you wouldn't be alone in this understanding because Doctors also love to use this test. 

But is it the single best test to assess thyroid function in your body? Did you know that there are many other thyroid function tests

The answer may surprise you. 

It turns out that in healthy individuals the TSH is probably a great way to assess thyroid function in the body. 

It's also probably the best way to quickly glance at thyroid function, through lab tests, to determine if the thyroid is "normal". (3)

But where it fails is when you use it as the only source to help manage patients who are already taking thyroid medication

We know, from recent studies, that the TSH fails to predict free T3 and free T4 levels in patients who are taking thyroid medication. (4)

That's important, but what exactly does it mean?

It means that if you are taking thyroid medication (such as Levothyroxine or Synthroid) that you may have a completely normal TSH, but that doesn't mean that your free thyroid hormone levels are "normal". 

This may explain why many patients on these types of medications don't feel optimal despite having "normal" thyroid levels. 

That's where free T3 steps in. 

Free T3 can, and should, be used in conjunction with the TSH to ensure that you are getting a sufficient amount of ACTIVE thyroid hormone to your cells. 

And this way of looking at hormone shouldn't be surprising to you or Doctors. 

Why?

Because for virtually all other hormone levels in the body, we ALWAYS check for the active hormone!

If I asked you if your testosterone was "normal" wouldn't you actually want to know how much testosterone is floating around in your blood? (5)

Well, the TSH is NOT that marker. 

The TSH is a proxy marker for how well the pituitary is functioning but it does NOT tell you how much hormone is in the body. 

When you break it down in this way it just becomes obvious that we would want to look at the active thyroid hormone (free T3 and free T4) but this logic is lost on many patients and doctors. 

Avoid falling into this trap! The free T3 can give you seriously helpful information about how your thyroid is functioning. 

Download my Free Resources:

Foods to Avoid if you have Thyroid Problems: 

I've found that these 10 foods cause the most problems for thyroid patients. Learn which foods you should absolutely be avoiding if you have thyroid disease of any type. 

How to Calculate "Optimal" Free T4, Free T3, & Reverse T3 Ratio: 

Calculating these ratios is important because it can help you determine if your efforts are on the right track and whether or not your medications are working. 

Download more free resources on this page

Optimal Ranges for Free (What's a "Normal" Range?)

But what is a "normal" or "healthy" range? What levels should you look for when you test for this simple lab test?

This is a simple yet very important question. 

The answer is that it varies based on which lab company is drawing and running your bloodwork. 

And that happens for this reason:

Each lab company creates their own "reference ranges" that your result is compared to. 

These reference ranges are based upon the results of OTHER people local to your area. (6)

And they often include people of ALL ages and of varying health status. 

That means the reference range is compiled of a bunch of both healthy and unhealthy people which can skew the range. 

When you look at your results you want to be compared to HEALTHY individuals who are around your same age

You don't want to be compared to someone who is in their 80's with multiple health conditions. 

So how do you do this?

By looking at the reference range and ensuring that your result falls within the top 50% percentile of that reference range

This isn't a perfect way to do it, but it is much better than potentially comparing yourself to unhealthy people. 

Let's use this as an example:

low-free-t3

This example shows a free T3 of 2.3 with a reference range of 1.7 to 3.7. 

If we break down the difference between 3.7 and 1.7 we get 2.0. 

That means that the top 50% of the "healthy" range is between 2.7 and 3.7 (3.7 being the top end of the healthy range). 

That also means that the bottom 50% of the "less optimal" range is between 1.7 and 2.7 (1.7 being the bottom end of that range based on the reference range). 

So you want your result to ideally fall between 2.7 and 3.7 (in this example!)

You can apply this same methodology to your lab test. 

Is having a High Free T3 Dangerous? (Symptoms of High Free T3)

Just like having a low free T3 is potentially dangerous so is having a high free T3. (7)

In a perfect world, you want your lab test in just the "perfect" zone (the top 50% of the reference range listed above). 

What you will find, however, is that your free T3 will vary based on when you take your thyroid medication, (8) the type of thyroid medication you are taking and when you check your blood work. 

To prevent abnormalities in your blood work you will want to follow these basic rules:

#1. Make sure to test your blood work about 24 hours after you take your thyroid medication

This will help prevent abnormal highs when testing. 

You want to look at the lowest level of thyroid hormone that is in your body prior to your next dose. 

Checking at hour 24 (assuming you take your thyroid medication every 24 hours) will show you the lowest that your free T3 will be right before you take your medication. 

If this value is in the 50% of the reference range then you know that throughout the day you are maintaining at least that level

#2. Don't check your blood work right after you take your thyroid medication

This will give you a value which is falsely elevated and not indicative of what your levels will be throughout the day. 

If you still have a high free T3, despite following these rules, then you might be in danger of taking too much thyroid hormone. 

If you do, you will most likely notice it as you become symptomatic. 

Symptoms that may accompany a high free T3 include: 

  • Jittery sensation
  • Heart palpitations
  • Sweating
  • Hot flashes
  • Tremors of the hands
  • Diarrhea
  • Anxiety
  • Rapid heart rate

If you are experiencing any of these symptoms and you have a high free T3 then you may need to adjust your dose. 

Symptoms of Low Free T3

Perhaps more common than high free T3 is the condition of having a low free T3. 

Most people, due to a variety of reasons, tend to be undertreated as opposed to overtreated. 

Undertreatment of hypothyroidism usually results in low free T3 levels which can be identified when these levels are in the bottom 50% of the reference range (see example above). 

Symptoms associated with this condition often mimic those associated with hypothyroidism. (9)

Why?

Because free T3 is the hormone responsible for all of the benefits of thyroid hormone. 

So it makes sense that low levels may result in these symptoms. 

Symptoms of low free T3 include: 

  • Weight gain
  • Fatigue
  • Constipation
  • Cold intolerance
  • Slow heart rate
  • Lower than normal body temperature
  • Brain fog
  • Depression
  • Menstrual problems

If you have these symptoms and low free T3 on lab testing then you may need to make adjustments to your medication regimen. 

This may include using supplements or medications designed to help improve your free T3

What to do to Raise your Free T3 (Medications + Supplements)

There are two main ways that you can improve your free T3. 

The first is through the use of medications and the second is through the use of targeted supplements. 

Let's discuss medications first:

Low free T3 when you are already taking thyroid medication is usually an indication that your body is having trouble activating the thyroid hormone. 

In addition, it may be a sign that you are either not absorbing your medication or that your dose is not high enough. 

Either way, you may need to make changes to your medication regimen. 

The easiest way to increase T3 levels is by adding medications which contain this hormone. (10)

Medications such as liothyronine and Cytomel can be added to your regimen which will directly increase T3 levels. 

The problem with this approach is that it requires your Doctor to be on board. 

It can be difficult to convince a physician to provide these medications as they are usually not comfortable using them (even though they are safe!). 

If you fall into this category then you may need to take the supplement route

Certain supplements, available over the counter, can be used to help improve thyroid function and thyroid conversion. 

These supplements may help your body by providing the necessary vitamins and nutrients involved in the thyroid conversion process. 

My personal recommendation is to use a supplement such as this which contains zinc, selenium, Vitamin A and guggul, all of which are designed to help this process. 

The great thing about supplements is that they can be combined with thyroid medications, they are easy to get and may help a great many people. 

Conclusion

Hopefully, you appreciate the value that testing free T3 can give you when looking at your thyroid. 

Free T3 is available as an easy and routine blood test which is covered by all insurances (I've ordered it thousands of times and have never had an issue so don't buy that excuse!). 

Testing your free T3 will help you to understand if your body activating thyroid hormone in your body and should be used in conjunction with other lab tests such as TSH and free T4. 

Whenever you make any changes to your treatment regimen make sure that you test afterward!

This will help guide your treatment and help you to know what is working and what isn't. 

Now I want to hear from you: 

Have you tested your free T3?

Do you know if you have a low free T3 or high free T3?

Do you feel that your body is converting or activating your thyroid hormone?

Why or why not?

Leave your comments or questions below and I will do my best to answer! 

References (Click to Expand)

This post was most recently updated on August 23rd, 2019

Dr. Westin Childs

Dr. Westin Childs is a Doctor of Osteopathic Medicine. He provides well-researched actionable information about hormone-related disorders and formulates supplements to treat these disorders.He is trained in Internal Medicine, Functional Medicine, and Integrative Medicine. His focus is on managing thyroid disorders, weight loss resistance, and other sex hormone imbalances.You can read more about his own personal journey here.

25 thoughts on “Why Free T3 & Total T3 are the Most Important Thyroid Lab Tests”

  1. So grateful for your expertise. I’ve learned so much about thyroid disease from reading your posts.
    I feel the worst of my life!! Just had labs. T3 was at the high end of reference range at 178. T4 free was low, 0.79. TSH was very low, 0.019. Thyroid antibodies was normal, 16. Reverse T3 was low end of normal at 9.7. Dr took me off levothyroxine and started me on Armour Thyroid, 60 mg a day. Been on this for about 3.5 weeks so far. I feel really lousy with the entire realm of typical hypothyroid symptoms on an extreme level. My doctor is certainly open to different medications to better regulate but perhaps I should take vitamin supplements you mentioned? I understand you cannot diagnose nor blatantly override another doctor!! It’s just been going on 2 years and my thyroid isn’t getting straightened out. My mother has ovarian cancer and I’m the only one she has to care for her and it’s becoming a personal internal struggle because I feel so bad and she’s the most important one right now!! Any advice would be appreciated.

    • Hi Amy,

      As far as I can tell there is very little downside to using supplements (or at least a trial). They either work and help you, which is great, or you get no benefit from them and they don’t harm you in the process. If you can’t get help from your current provider then using other therapies such as diet, supplements and so on is a reasonable next step.

      • Hello. I purchased and received today your thyroid adrenal reset complex and was wondering if I take 2 capsules at the same time or if it’s better to take like one in the morning and another in the evening? Thank you for a reply if possible!!

        • Hello!

          Yes, most people take 2 capsules per day. Some people find that dose to be too stimulating so they use 1 capsule per day but most do fine on 2 caps per day. Hope this helps!

  2. Hi Dr. Childs, I’m struggling with thyroid symptoms for a few years now. Changed from Synthroid to Naturethroid b/c of severe hair loss, scalp pain, weight gain, fatigue, constipation and depression. Naturethroid got rid of some issues like weight gain and some fatigue and depression but the weird hair loss and severe scalp pain never stopped. I learned at my zinc levels were low at 65 so I now eat 5 oysters a day and it reduces # of hair strands from falling but scalp hurts. My FT3 level with fasting and 24 hours after last dose shows up as 2.3 which I believe is the issue?? I’m also experiencing fatigue again which got better a couple months after I started NDT but is now returning. I’m thinking of adding SRT3 and reducing Naturethroid (I’m on 1.25 grains and multi dose that), my question is how do I properly add SRT3 compounded med without giving myself more hair issues and heart palps? Thank you Dr. Childs.

  3. Hi Dr. Childs:
    So I’m taking 20mcg of Cytomel. 10mcg in the am 10mcg in the afternoon. Just had labs done T3 not even in range (2.5)- (Range is 2.6-4.4) This was after he increased me from 10mcg of Cytomel to 20.
    So, here’s the question. I think my cytomel needs increased so he will probably lower my Nature Thyroid currently taking 45mcg in the am. Does it matter that my free t4 is also LOW? And would your supplement T3 conversion pill be safe to take if I can get him to increase my T3 dosage? Thanks

    • Hi Kelly,

      Our supplements are designed to be taken with or without thyroid medication 🙂 And it seems that some people can tolerate a low T4 but others need it to be optimal.

    • Hi Charlotte,

      Yep! The lab ranges are the same whether you have a thyroid or not because you still must convert T4 (in the form of thyroid medication such as levothyroxine or Synthroid) into the active T3 thyroid hormone.

  4. I find your website to be extremely informative. I have been on levothyroxine for many years, diagnosed by a rheumatologist due to leg pain. My primary md has been following my thyroid, but in the last few months, my TSH (which is all she screens) have been between 0.04 up to 185. My latest is 0.05. I have all the symptoms you list, except menstrual (I had my ovaries removed after breast cancer). I am so chronically tired, I can take 2 naps after sleeping 8 hours. I don’t know where you are located, but I would love to have a doctor like you. If you know of any that practice your kind of medicine related to the thyroid in the Mount Laurel area of NJ, I would appreciate a recommendation. Please help me.

  5. Hello. When you still have your thyroid and have been hypothyroid and no Hashimotos, can you ever ween off meds like Armour and Synthroid, doctor supervised, of course, to see where you do on your own? I skipped a couple days with no meds because my pharmacy was out of what I use and I felt so much better taking nothing, physically felt better.
    Thank you so much for any reply and expertise you could offer from your thoughts!

    • Hi Amy,

      You can certainly try if you’d like, but it wouldn’t be wise to do it without physician supervision. Some thyroid medications can stay in the system for 4 weeks so you may not feel poorly until after that time.

  6. Morning,
    I have watched your videos numerous times and read your posts. I can’t get enough.
    I have been switched to Armour, 90mg. My Free T3 went up to 5.8 but my Free T4 is still at 1.0. When I was on synthroid, my Free T3 was 1.4. Shouldn’t Free T4 be mid-range? I do feel a bit ‘hot flashy’, but my fingers and toes are freezing cold. I don’t know where to go next.

    • Hi Paige,

      Each person reacts differently to thyroid medications so just switching medications doesn’t mean that your lab results will optimize.

  7. Hi,
    I have been on synthroid for 8 years now. Relatively good numbers on and off however never fully felt great. For past year TSH is low end normal, free t4 is mid to high normal, free t3 us slightly below normal, reverse t3 is high.
    Tried supplementing with selenium, zinc and diet for past 5 months no change in number.
    Dr. willing to decrease synthroid and start t3 slow release at 2mcg.
    Worry/fear is hair loss (it is an ongoing issue with no other outside risk factors), weight loss (I am at a good weight) along with the jittery hyper symptoms. Pro is the potential to feel way better. Would the potential side effects be an issue given I’m not new to treatment and it being a low dose?

  8. Hi,

    I have had a partial thyroidectomy and have been on Armour 90mg for a few years now. My current PCP wants to lower it to 60mg since my TSH is .06. He does not check my Ft3 but tests my Tot T3 and FT4. Should I have my Ft3 checked before I lower my dose that much? I don’t feel bad, but he wants to change it d/t my TSH level.

  9. Hi Dr. Childs
    I am following your diet plan and take 50mg liothyronine per day I don’t think my body is utilizing the T3 as my weight will not shift. I have also been using Saxenda for nearly a year which initially seemed to help but as my weight is the same I think it has stopped working. Can you help??

    • Hi Liann,

      If you are referring to my weight loss guide then there are several other therapies listed in there to help you break through a plateau aside from Saxenda and liothyronine. Take a look at some of the other medications in that guide and the video/slides on breaking through a weight loss plateau.

  10. Can you help me select the supplements that I need? My TSH is in the low normal, T4 Free is 1.6 this lab’s normal is 0.8-1.8, my T3 Total is 79 this lab’s normal is 76-181 and my TPA is 1 this lab’s normal is <9. I am very fatigued which is not at all normal for me and have some hair loss.

  11. 2 questions:
    1) I didn’t read anything about the utility of total T3. I usually check TSH, ft3, ft4 but have stopped checking total T3 as I’m not finding it impacts any clinical decision making.

    2) Do you have any data on skipping the morning thyroid dose before testing? This is my training as well but I have not been able to find data to support it. While I do want to see my patient’s troughs, I am concerned about TSH suppression and overly elevated T3 as well, which we’d clearly miss with the skipped dose.

    Thoughts?

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