Complete T4 and T3 Guide: What Do They Mean for your Thyroid?

Complete T4 and T3 Guide: What Do They Mean for your Thyroid?

T3 and T4 play a critical role in maintaining thyroid function. 

Perhaps more important is that they are often ignored by conventional physicians despite being easily available through routine blood testing. 

Learn the importance of T3 and T4 including how to interpret your lab values, how these hormones function in your body, and how to optimize treatment if you aren’t feeling well on thyroid medication

Thyroid Hormones Explained in Plain English

What is T3 & What does it do?

T3 is probably the single most important thyroid hormone. 


Because it is the most biologically active thyroid hormone and most of the functions we associate with your thyroid gland come from the interactions of T3 in your cells (1).

T3 is short for the term Triiodothyronine which is the name given to this form of thyroid hormone. 

But how does your body create T3?

T3 is created FROM the most abundant thyroid hormone in your body T4. 

Enzymes, known as deiodinases, remove an iodine moiety at a certain place on the T4 hormone which turns it into the most active thyroid hormone T3 (2).

pathways of thyroid metabolism

The process by which T4 is turned into T3 is known as thyroid conversion. 

And this conversion process is critical to understanding how your thyroid works. 

Thyroid conversion is the method that your body uses to tightly regulate thyroid function in the body. 

It is used to help create the EXACT amount of thyroid hormone needed at any given time and for all tissues. 

Think about it:

Your body has created a system in which it can convert, or not convert, T4 into the active T3 thyroid hormone based on the needs of the system (3).

So, if you are currently under a lot of stress, or if you need to produce more energy or heat, then your body can rev up the thyroid conversion process to produce more T3. 

On the opposite side of the spectrum, your body can also choose to slow down the conversion and put energy into more important bodily functions in instances where you may be critically ill. 

We see reduced conversion in people who are critically ill (4), those in the hospital, people taking certain medications, and so on (5).

More recently, we also see some patients with obesity and metabolic syndrome with lower-than-normal T3 levels due to a slowing of peripheral thyroid conversion (6).

deiodinase enzymes and where they are located

In this way, thyroid conversion is becoming a more important topic because it influences how Doctors should treat and evaluate certain patients

This regulation is one of the most important concepts to understand in thyroid physiology. 

Thyroid conversion occurs ALL the time and occurs in major tissues such as your liver, and your gastrointestinal tract (7) but it also occurs in each cell. 

So what does T3 actually do?

T3 the thyroid hormone floats around in your bloodstream until it hits the target cell. 

Once there it can activate cellular function in two important ways:

The first is through non-genomic changes in the cell. 

Non-genomic changes include directly impacting enzymes and transporters on the surface of the cell itself (8).

An example would be the way that T3 activates calcium channels in the heart which may cause palpitations when ingested. 

The second is through genomic changes in the nucleus of the cell. 

These changes directly impact the expression of DNA, so it is altering which DNA and therefore which proteins are created. 

The proteins created from genetic changes go on to increase mitochondrial function, increase ATP production efficiency, and so on (9).

Put into plain English:

T3 helps increase your metabolism, improves your gastrointestinal tract, helps regulate your weight, regulates your mood, and so on. 

Perhaps the most important changes that occur secondary to thyroid hormone occur as the result of T3 changes in the cell. 

What is T4 & What does it do?

The majority of thyroid hormone produced by your thyroid gland is actually in the T4 form (also known as Thyroxine). 

Your thyroid gland, if working properly, produces around 80% T4 and about 20% T3 (10).

This means that T4 is MUCH more abundant in your body when compared to T3. 

It’s also important to understand that T4 is NOT as biologically active as T3. 

This means that it doesn’t have the same cellular effects as T3 and it doesn’t activate your cells in the same way. 

So what is the purpose of T4?

T4 helps to serve as a reservoir for T3 conversion, so T4 levels are still critical to proper thyroid function. 

T4 also does have some cellular activity, but these are not felt to be as strong when compared to T3. 

T4 is important to understand because most of the medications that Doctors prescribe to treat hypothyroidism come in this form

The idea is that supplying the body with T4 (the less biologically active thyroid hormone) is safer when compared to using T3. 

T4 also has a longer half-life when compared to T3 (11) and comes with fewer side effects. 

What is T2?

T3 and T4 tend to get most of the attention when it comes to thyroid hormones, but there are also other forms of thyroid hormones. 

One such example is T2. 

T2, also known as 3,5-Diiodo-L-thyronine (or T2 for short) is not as powerful or potent as T3 and T4, but it does have some interesting effects.

The main difference between T2 and T3 is that T2 is MUCH less potent than T3 at stimulating genetic expression (12).

But that doesn’t mean that it can’t. 

Studies have shown that T2 can still stimulate genetic changes in the liver and it does possess thyromimetic effects. 

Thyromimetic is a fancy way of saying that it can stimulate thyroid function in the cells. 

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T2 also shows that it can compete (although weakly) for binding on the T3 receptors in your cells (13).

One important point worth mentioning about T2 is that it is available as an over-the-counter supplement in the US. 

T3 and T4 both require a prescription from a physician, but T2 can be taken over the counter. 

We do not understand exactly how T2 functions or the importance of this thyroid hormone but we know that it may be beneficial in certain patients. 

Using T2 can help increase energy metabolism in the mitochondria (14) and may be useful as a supplement to existing T4 & T3 dosing. 

Adding T2 may be helpful in treating obesity for certain patients. 

What is Reverse T3?

Reverse T3, known as RT3 or 3,3′,5 triiodothyronine, is an anti-thyroid metabolite

Reverse T3 is created from the conversion of T4. 

But wait, didn’t we already mention that T3 is created from T4? How can reverse T3 be created from T4 as well?

Both statements are actually true. 

You can think about thyroid conversion as a fork in the road. 

Your body has the ability to turn T4 into reverse T3 or T3 itself and it must choose one or the other. 

If it chooses reverse T3 over T3 then that is one less molecule of thyroid hormone that can be activated. 

Beyond “stealing” from your potential T3 supply, reverse T3 can also block thyroid action at the cellular level. 

Reverse T3 has been shown to compete for cellular binding with T3 (15).

So not only does converting T4 into reverse T3 lower your circulating T3 levels but it also makes it more difficult for what little T3 is left to bind to and activate your cells. 

In this way, you can think of reverse T3 as an anti-thyroid metabolite. 

Your body will preferentially turn T4 into reverse T3 in certain and specific situations. 

Inflammatory states (16), obesity, metabolic abnormalities, exposure to LPS, exposure to EDCs, illness, medications, and so on may all trigger the conversion of T4 into reverse T3. 

Reverse T3, like the other thyroid hormones, can be assessed in the serum through simple lab tests. 

Ordering reverse T3 can help give you an idea of what is happening in your body and how well the thyroid conversion process is taking place. 

TSH vs Thyroid Hormones

It may be confusing to hear about thyroid hormones such as T4 and T3 when you are so used to hearing about TSH. 

TSH, while important, is not necessarily more important than your thyroid hormones

TSH, known as thyroid stimulating hormone, is a hormone secreted by your pituitary gland. 

TSH then acts to stimulate the RELEASE of thyroid hormones from the gland

TSH doesn’t necessarily say how much T4 and how much T3 to produce, however. 

It just “stimulates” the gland to produce thyroid hormone in general. 

The actual activation and conversion of T4 into T3 (which is more important as it relates to thyroid activity) is controlled by the liver, gut, and individual cells. 

TSH is important, however, in helping to diagnose gross abnormalities in your thyroid gland and in helping to determine how “responsive” the gland is to TSH. 

If you have high TSH in your bloodstream then that is often a sign that you have LOW thyroid levels in the body or a condition known as hypothyroidism. 

High TSH is an indication that your thyroid gland is not responding to the TSH. 

Low TSH, on the other hand, may be an indication that you have TOO much thyroid hormone in the body (known as hyperthyroidism). 

TSH is an important factor in thyroid management, but it should not overshadow the evaluation of free thyroid hormones such as T3 and T4. 

In addition, there are situations in which the TSH becomes less helpful as a tool for evaluating thyroid function. 

You can read more about this concept here

Testing for T3 & T4 + Optimal Ranges for Both

Because of the importance of thyroid hormones in thyroid physiology, it’s important to evaluate and monitor their levels in your body. 

You can do this through simple blood tests for both hormones. 

These hormones should be evaluated in addition to the standard TSH (Thyroid stimulating hormone) which is often ordered to evaluate thyroid function.

TSH gives you an idea of how the pituitary is functioning and how responsive your thyroid gland is, but T3 and T4 give you much more actionable information on how thyroid hormone is being utilized in your peripheral tissues. 

Always use these blood tests in conjunction with TSH. 

You can find the list of thyroid hormone testing below:

  • Free T3 – Free T3 is the number of FREE and ACTIVE thyroid hormone. Remember that T3 is the most potent and biologically active thyroid hormone. For optimal results, this value should be in the top 50% of the reference range (top 20% for weight loss). 
  • Free T4 – Free T4 is the number of FREE and ACTIVE thyroid hormone but is NOT as biologically active as Free T3. For optimal results, this value should be in the top 50% of the reference range
  • Reverse T3 – Reverse T3 is the anti-thyroid metabolite that competes for binding with T3. For optimal results you want this value to be less than 15 ng/dL

When using thyroid medication (such as Levothyroxine or Synthroid) you want to optimize your Free T4 level to be in the top 20% of the reference range and your free T3 to be in at least the top 50% of the reference range. 

If weight loss is your goal then aim for even higher free T3 levels (top 20% of the reference range). 

In order to achieve these levels, you may need to use a combination of T4 + T3 thyroid medications. 

This may require adding Cytomel or liothyronine to your existing dose of T4 or by switching to NDT medications such as Armour Thyroid

Final Thoughts

Understanding how T3 and T4 work in your body will help you to understand how well your body is functioning. 

Remember that your thyroid gland controls very important functions in your body ranging from your metabolism to your mood and everything in between. 

These functions are TIGHTLY controlled and regulated by your thyroid gland, thyroid hormones, and thyroid conversion.

Small changes, or dysfunction, to any of these systems, may result in quality of life issues that will manifest as weight gain, depression, fatigue, hair loss, and so on

If you suffer from hypothyroidism or another known thyroid disease, please make sure that you evaluate your thyroid hormones in addition to TSH. 

Ordering T3 and T4 will help shed insight into how well your body is utilizing thyroid hormones and how well you are converting thyroid hormones. 

If you find that you are not optimal, even if your TSH is normal, then you may need to have your medication adjusted to include different medications or a different combination of T4 and T3. 

Now I want to hear from you:

Have you had your T3 and T4 tested?

Are you suffering from high reverse T3 or other thyroid issues?

Are you taking thyroid medication?

Leave your comments below! 

















how to get optimal t3 and t4 levels

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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 80,000+ people have used them over the last 7 years. You can read more about my own personal health journey and why I am so passionate about what I do.

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32 thoughts on “Complete T4 and T3 Guide: What Do They Mean for your Thyroid?”

  1. Hi Dr. Westin,

    I am a Chinese Medicine practitioner treating a patient base whose issues run the whole gamete. I just recently came across your page and I am finding it an incredible resource to be able to support my patients as many of them concurrently navigate more allopathic care. Thanks for your work- especially your thoughtful but easy to read writing style.

  2. I am a 48 y.o. Woman who has been on hormone replacement for 30 years. I have mostly been prescribed Levothyroxine for all of those 30 years, I lost my thyroid to papillary thyroid cancer at age 18, I have been very successful in maintaining my health and having two children but the weight loss has always been very hard, as well as the mood leveling.
    My question is, What should I ask my doctor to look for? Should I be seeing an Endocrinologist or an Internal Medicine Doctor and what test and results should I need to be aware of?
    This has bothered me for years but I just “go with the program”.
    I don’t expect to hear from you but thought it was worth a shot.

    Thank you!

  3. Hi Dr. Westin

    Every morning I take 100 mcg levothyroxine as two 50 mcg tablets to avoid the additives in other dosages. Also at the same time, I take 12.5 mcg liothyronine. Although my TSH is very very low like .02, my free T3 and T4 are in the optimal range and I was finally able to lose 10 lbs. Only 7 to go! 🙂

    • Hi Rachel,

      Awesome! Thanks for sharing 🙂 It can be tough to find optimal dosing for each person, but once you do the results are very much worth the struggle.

      Keep us updated on your progress.

  4. I’m a 67 yr. Old women who has suffered badly with genetic diseases, and Hashimoto’s thyroid disease. The Hashimoto’s disease has destroyed and disintegrated my thyroid gland, I had an ultrasound done on my neck, there’s only a piece of thyroid tissue left inside my throat with the 4 thyroid cyst’s floating around my throat. I was diagnosed in my late 20’s with a dead thyroid, I was on Synthroid up until my 60’s, then a naturopathic doctor tried me on natural thyroid meds. They kept playing with the dosage’s going up and down for 5 yrs. My T-3, T-4, my TPOA antibodies were and still is very high, my TSH goes very high, or low. Today I’m on 100 mg. Of only Synthroid, my dr. From the thyroid clinic only goes by TSH test results. She said that they don’t evaluate or give you meds for the other test results. What do you advise??

  5. Hi Dr. Westin,
    I’ve been on Armour for almost a year and for a while I felt good and had energy but did not lose weight and I actually gained 10 pounds since starting although my TSH was almost at 0. And my t4 and t3 were on the higher side of the range. It seems that when I exercise and diet my energy plummets and when I start to eat normally I gain very rapid weight at one point I’ve gained 4 pounds in two weeks and I wasn’t eating that much. I’ve read that some people actually gain weight from Armour. Any information would help. Thank you.

    • Hi Oksana,

      Yes, it’s certainly possible to gain weight on both T4 and T3 medications. This is not considered a common side effect but it certainly can happen.

      Usually, the key to understanding how this happens is by evaluating your reverse T3 level. You can learn more about that here:

  6. Hi Dr. Westin
    I have been on T4 replacement for about 10 years now with no luck losing any weight. My skin is very dry and itchy my moods are all over the place my whole body just aches my hair is thinning. I went to a new doctor who looked more thoroughly into my blood and also prescribed T3 replacement. Was on both T3& T4 for about 6mths and ended up in the hospital for 4 days with afib. The doctor in emergency went crazy that I was taking both T4& T3 replacement and took me off T3 and put me on blood thinners. On my check up appt, they increased my T4 replacement to 200mg per day as my blood results were all over the place. I feel I’m Back to square one again. The best I have felt in years was when I was on the T3. What do I do.???

  7. I have run the gammut of trying different meds and I am big time allergic to most fillers it seems. I have been taking Tirosint and am not adding Cytomel. Can you tell me the proper ratios to take the two (and from there I can figure out my optimal titration) and also do know where I can get a “pure” T3 with no dillution (usually microcrystalline cellulose that I do terrible with) and a good filler? My doc here is gret in leeting me try what I need to, but not quite as informed on the thyroid meds/stuff. Thanks in advance!

  8. My FT4 is always on the low end. The highest I’ve seen it was a .89ng/dl in 2016. I finally got my RT3 checked in 10/2016 and it was at 15 ng/dl. At the same time, my FT3 was 3.68pg/ml, FT4 was .80 ng/dl, and TSH was 3.64 ulU/ml. in 2016 I had an ultrasound of my thyroid and had one nodule and in 2017 I had another ultrasound and it showed another nodule and a cyst. And in the last year, I started having what feels like swollen lymph nodes, pain/pressure in thyroid area of the neck. ENT thinks I have silent reflux and has me on meds for that. I’m wondering if I should be trying to pursue looking at a possible thyroid issue? I do have a long list of symptoms that if put together should indicate a thyroid issue/autoimmune issue but conventional Dr’s can’t seem to figure out what it is.

  9. Hi, Dr. Westin,
    My T4 is normal low range 90, and rah normal. I took my naturethroid pill the morning of the test (forgot) and t3 was high.

    I was recently increased from 90 to 130 Naturethroid and there was no increase in Free T4.

    Still, have Hashimoto symptoms.

    Any suggestions are greatly appreciated!

  10. Hello.

    I have recently had labs drawn and the reference range is higher than any other lab I’ve had drawn. Has there been a change in reference ranges in the last 3 months? Also, my endo has never tested reverse t3. How important is this test and should I be requesting full panels each time?

    Thank you, Dr. Childs.

  11. Hello, I have been post-surgery on the right side, nodule for a year and a half. I’m on armour thyroid. I’m 60 yrs old and have a problem losing weight all my life. Had gastro bypass 15 yrs ago, kept off most of the weight for 14 yrs, then menopause gained 20 lbs in 6 months found out my nodule was growing and had it removed. Recently got my TSH done with t4 free and t3 free. My results are tsh 0.848 t4 0.67 and t3 2.43 they said it was perfect. Started a diet with no carbs keto down 6 lbs in 1.5 weeks. Are these normal readings and what is the ranges suppose to be, everytime I look up I get all different numbers.
    Thank You

  12. Hi Dr. Childs, when is the best time to test for thyroid hormones? I have read conflicting information. I am currently on a T4 (tirosint) and T3 (Cytomel) dosage. I have read to not take it the morning of tests which could mean 24 hours after you take your last medication. I have heard 4 hours after, then I have heard 12 – 18 hours after last medication.

  13. Hi, Dr. Child’s I was recently put on Cytomel along with Synthroid since it was discovered I have low free t3 as well. I have hypothyroidism caused by subacute thyroiditis. I noticed that if I do my blood work after taking medication my free t4 is in the upper range and my free t3 as well but my TSH is very suppressed (.03). I’m on 15 mcg Cytomel and 88 mcg Synthroid a day. If I do my blood work before my medication my free t3 is below range and my free t4 is low but in range and my TSH is the same. Which results are more accurate? Is it safe to add more Cytomel without increasing Synthroid? Your feedback is much appreciated. Thanks

  14. Hi Dr. Westin,

    I am a 49 year old woman that is in good shape, active and works out 3-5 days a week. Up until now my thyroid has always been good and now I got a reading of my T3 at 7.4 . Over a month ago I started taking zinc every day and had my blood tested and it came back high. Then I re-took the blood test checking my thyroid after I got off the zinc for three weeks and it is still reading 7.4.

    I am on bio-identical hormones and have been for 3-4 years now. I take estradiol/testosterone under the tongue and progesterone lab created.

    I am going through some stress in the last month as well.

    I would like an expert opinion as to what I should be doing or consider or what I can do to correct my T3.

    Very concerned,

    Sandy Bobeldyke

  15. Hi Dr. Westin.

    I asked my doctor to do a rT3 test, but she refused and said it’s done differently than the other thyroid tests. Is that correct or is it done the same way?

    Thanks in advance,

    • Hi Kolbrun,

      I’m not sure I understand the question. Reverse T3 is just a standard lab test like any other thyroid lab test but the assay is different (but this is true of all lab tests).

  16. Yes — for quite a few years I have been having my Free T4 and Free T3 tested. They still test my TSH even though I have pituitary damage from a severe hemorrhage during childbirth. My TSH usually comes back at <0.01.
    I haven't really been able to get some good help with my situation. I have taken some Timed release T3 meds and thought that they were working pretty good for me, but at one lab test the Free T4 and the Free T3 went over the range — Not sure what happened there!
    My functional medical doctor refused to renew my prescription for my Timed release T3 med so I have been off that med for several months.
    Now my Free T4 levels have gone up (I'm taking synthroid — 175mcg) to the top of the range (I started taking them early in the morning before getting up and when I take my Cortef) so maybe they are absorbing better. And now with no Timed release T3 medication those levels are at the very bottom of the range — about 5% or less from the bottom of the range and every subsequent lab report has the Free T3 LOWER still!
    My family doctor and my functional medical doctor have refused to order me ANY T3 medication because I am in my 70's and they are "paranoid" that I'll have afib! Their story goes that no one over 60 gets any T3 med — to prevent that happening!
    AND they try to "scare" me by saying that IF that happened I would have to take "blood thinners" for the rest of my life! AND I have ALWAYS had rather THIN blood — nose bleeds that didn't stop, veins in my foot that burst open and were VERY hard to stop the bleeding, etc!!
    The endos here are pretty useless — almost all wanted me to lower my Thyroid meds in order to "make" the TSH go up!!
    I want to ask my functional medical doctor for Armour Thyroid, but don't know if he'll agree! I have been having quite a lot of shoulder pain and I usually don't feel pain easily! My substance P levels are WAY down!!
    ANY suggestions are welcome!

  17. Hi dr I have try to call your office and nobody answer the phone I REALLY LIKE TO MAKE AN APPOINTMENT I live in Phoenix Az and is hard to reach out to you I have thyroid problems and I been taking the same medication for years I think I need to change but I think I need another doctors opinion thank you

  18. Hi please accept my apologies in advance if I have already posted this, but I cannot seem to find my question so I am reposting. My name is Cindy I’m a 58 year old female, who has been suffering for over 6 years from severe overheating issues. No blankets no heat no hot showers. I can’t walk even a hundred feet without getting so overheated, I have to run into my place and sit underneath a fan. I have had my TSH, T3 and T4 levels checked, and I was told they are normal. Over the last few weeks I started losing my hair, and have lost almost 50% of my hair in patches. The inability to regulate my body temperature or cool down, is so severe and debilitating it is interfering with my daily life. What tests can I ask my doctor to do next? She threw her hands up and said I was the worst case you have seen. I can’t leave it at that I need to know what other tests can be done. Can you tell me what other tests can I ask my doctor to perform? Desperate in seeking an answer.

    • Hi Cindy,

      Have you checked to see if this is menopause related? It could be your thyroid but it could also be related to your sex hormones as well.


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