T3 Thyroid Hormone: What Your Doctor Isn’t Telling You

T3 Thyroid Hormone: What Your Doctor Isn’t Telling You

This is lesson #4 in my thyroid beginner series and today it’s all about the hormone T3.

The last lesson we discussed the value of T4, how it is produced, what it does, and how it influences your body. 

We’re going to do the same thing in this lesson but discuss the other (more active) thyroid hormone: 


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What is T3?

T3 is the most powerful thyroid hormone that your body produces. 

If you were paying attention yesterday then you know that your thyroid gland, under the influence of TSH, produces T3 thyroid hormone directly. 

In fact, about 20% of the total amount of hormone that your thyroid produces is T3 (1).

This hormone influences almost every cell in your body, through nuclear receptors, and can be tested through routine blood work. 

Your doctor can also prescribe T3 as a medication directly. 

Later in this video and post, we will talk about why many people may actually need T3 medication to feel optimal. 

But let’s focus on T3 as a hormone first. 

T3 as a Hormone

T3 is often referred to as triiodothyronine, especially on lab work or in scientific studies. 

But don’t let this confuse you, because triiodothyronine is another name for T3 and both are referencing the same active thyroid hormone. 

You know that T3 is the most active thyroid hormone in your body, but how does your body get the hormone?

Through 2 main ways:

#1. Your thyroid gland produces it directly (20% or so)

#2. And through the conversion of T4 into T3 (2).

T3 then circulates through your body where it impacts nearly every cell, either on the surface of the cell or directly in the nucleus (3), to cause all of the positive benefits of thyroid hormone. 

T3 is responsible for helping your hair to grow, providing you with energy, helping you lose weight, lifting your mood, increasing your heart rate, managing your cholesterol, and so on. 

All of these benefits come from the effects of T3 on the cell. 

How do you end up with low T3?

Well, if we go back to the two main ways that we know your body produces T3 we can reverse engineer what can cause low T3. 

#1. Your thyroid gland isn’t producing ENOUGH of it.

Conditions such as hypothyroidism, Hashimoto’s thyroiditis, inflammation of the thyroid gland and obesity can all result in reduced production of thyroid hormone directly from your thyroid gland. 

A reduction in either T4 or T3 will lead to low T3 in your body. 


Because of reason #2:

#2. And/or you aren’t able to convert T4 into T3 adequately. 

The thyroid conversion is the process by which your body takes circulating T4 and turns it into T3, through certain enzymes (4), when it needs to. 

If you have a low supply or reservoir of T4 then obviously the amount of T4 that your body can draw upon to create T3 is limited and this may lead to low T3 in your serum (that you can test with lab work). 

If you can identify which issue is causing low T3 in your body then you can help direct your therapy and treatments. 

But just realize that regardless of the reason, if you have low T3 you will likely experience the symptoms of hypothyroidism. 

T3 as a Lab Test

T3 can be easily tested in the blood through two main tests:

#1. Free T3 (amount of unbound active thyroid hormone)

#2. And Total T3 (total amount of T3 in the serum)

Testing for T3 gives you an idea of how much function your thyroid gland has on your body because you are testing for the active thyroid hormone (compare this to T4 which is less biologically active (5)).

T3 is therefore probably the single most important thyroid test that exists (superior to even TSH) (6).

With this in mind, you will want to put a priority on your T3 and ensure that it is in the “optimal” range. 

You can use the example below to illustrate the point: 


In this example, the free T3 is measured at 2.3 with the reference range of 1.7 to 3.7. 

So, from a technical standpoint, the lab is measured in the “normal” range and this is probably what most physicians would tell you. 

But what they fail to realize is that most of your hormones operate on a spectrum ranging from zero function to optimal function and everything in between. 

You can still “function” with a sub-optimal T3 but you may pay the price of fatigue, hair loss, constipation, and weight gain as a result. 

It’s in your best interest, then, to ensure that it is “optimal” for your age. 

You do not want to compare your personal T3 to that of someone in their 80’s or 90’s, instead, you want to compare it to a healthy person that is age-matched to you (currently labs are not set up to give this information) (7).

The “optimal” value will vary from person to person but a safe assumption is that you want your free T3 value in the upper 50% of the reference range that you are given (reference ranges change based on the lab). 

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In a perfect world, you would have tested your free T3 when you were “healthy” and have that as a comparison, but that’s incredibly rare. 

So, without this knowledge, you can assume that a healthy range is probably near the top of the range. 

In our example that would mean that the “optimal range” would be anything greater than 2.7 putting this patient at a slightly sub-optimal level. 

Another important point to mention is that your T3 should always increase if you are taking any type of thyroid medication. 

If your T3 doesn’t increase with medication then you need to take a look at conversion, absorption, or look into using a different type of thyroid medication

T3 as a Medication

T3 can also be used as a medication either by itself or in combination with T4 (and other thyroid medications). 

We’ll get into the various formulations of T3 below.

T3, because it is so powerful, tends to have more side effects when compared to T4. 

This doesn’t necessarily mean that you should avoid T3 medication but it does mean that it’s more difficult to dose correctly and this may be part of the reason that Doctors shy away from using it. 

T3 has a direct influence on your heart cells which can cause your blood pressure and heart rate to increase immediately after taking the medication. 

You may feel this sensation as heart palpitations and/or anxiety. 

These types of symptoms usually go away over time or as you adjust your dose and aren’t necessarily a reason to stop using the medication. 

It’s also important to realize that T3, like thyroid medication, is probably the most effective medication for helping with weight loss (8).

If you are struggling with weight loss, and you have thyroid disease, you need to take a close look at your Free and Total T3 levels (see examples above). 

Liothyronine & Cytomel

Liothyronine and Cytomel would be considered “immediate release” versions of T3 because they are usually rapidly absorbed into the body after ingestion. 

And because T3 has a short half-life, it’s not circulating around in your blood for a significant amount of time (9).

This may be an issue for some people who then opt to take more frequent, but smaller, doses throughout the day. 

This problem is largely solved with SR T3. 


SR T3 is a compounded medication in which the active T3 hormone is bound to a “sticky” material which delays or slows down the absorption of T3 in your intestinal tract. 

This allows for a slow but steady stream of T3 into your body throughout the day. 

SR T3 is often preferred if you experience symptoms such as heart palpitations, anxiety, or headaches when using T3. 

One potential problem with SR T3 is that it may dramatically reduce the absorption of the dose that you take. 

For instance:

If you take 50mcg of SR T3, your body may only absorb some fraction of the total (such as 25mcg or 30mcg of the original 50mcg). 

This typically doesn’t happen with the IR T3s. 


T3 is also found in combination with T4 in medications such as Natural Desiccated Thyroid. 

Natural Desiccated Thyroid (or NDT for short) is dosed differently than other thyroid medications and is referred to as “grains”. 

1 grain of NDT contains around 38mcg of T4 and about 9mcg of T3. 

It’s better to use grains as a unit of measuring the potency of NDT because each formulation has a different set of milligrams which equals 1 grain. 

For instance:

65mg of Nature-Throid = 1 grain while 60mg of Armour Thyroid = 1 grain. 

Don’t let this confuse you though, just realize that NDT contains both T4 and T3. 

Because your thyroid gland (when it is healthy) produces around 80% T4 and around 20% T3 it makes sense to supplement your body with similar ratios. 

This may be why patients who start taking T3 medication often feel much better compared to when they were on T4 medications by themselves. 


T3 fits into the categories of tests, medication, and hormones all in one. 

Understanding what T3 does and how it works is critical to understanding thyroid function in the body because it is considered to do all of the “heavy lifting”. 

Getting your physician to test for T3 and to prescribe T3 medication can be difficult, but certainly not impossible

If you are struggling to get help you can use this as a resource to help you find a knowledgeable physician

But now I want to hear from you:

Have you had your T3 tested? Were your levels optimal?

Are you currently taking T3 medication? How is it working for you?

Are you struggling to find a doctor willing to work with you?

Leave your comments below! 

#1. https://www.ncbi.nlm.nih.gov/pubmed/12915350

#2. ncbi.nlm.nih.gov/pubmed/6479377

#3. ncbi.nlm.nih.gov/pmc/articles/PMC329808/

#4. ncbi.nlm.nih.gov/pmc/articles/PMC3673746/

#5. ncbi.nlm.nih.gov/pmc/articles/PMC4699302/

#6. ncbi.nlm.nih.gov/pubmed/27700539

#7. ncbi.nlm.nih.gov/pubmed/27440910

#8. ncbi.nlm.nih.gov/pmc/articles/PMC3205882/

#9. ncbi.nlm.nih.gov/pmc/articles/PMC5167556/

T3 basics: what your doctor isn't telling you

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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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27 thoughts on “T3 Thyroid Hormone: What Your Doctor Isn’t Telling You”

  1. 60 y/o. Have high RT3 and high Leptin (diagnosed Leptin resistant). Urate also above normal range. From what I read these 3 are linked. My TSH is usually suppressed less than 0.2 mU/L simetimes less. FT3 and FT4 usually within normal range (upper 75% range). Because RT3 so high GP reduced T4 meds and supplemented a small amount of S/R T3 (10mcg). (I developed heart palpitations when raised to 15mcg so dropped back – still get heart palpitations with a drink of alcohol). Hoping this settles down. I have had mixed results with testing as I have been self-medicating with more T4 as depression and anxiety set in when GP reduced. However I have stuck to her instructions for 3 months and have just been re-tested hoping TSH within normal range and RT3 has reduced. Results not back yet. Keen to know. Will begin work on leptin resistance by cutting out fructose, yo yo dieting and include some high intensity training. It’s a journey.

    • It took me about 15 years ( on T4 only) to get someone to even listen to what I said about still feeling horrible on this alone….I was ridiculed, called a liar, kept on starvation diets for all this time and of course, all that happened was that I became fatter and fatter…..which of course, led to more criticism. They said I didn’t eat what I said I ate….I never lied to them, EVER! I was desperate for someone to help me! I put on so much weight that I became scared and began to research for myself…..I asked time after time for a T3 test and was refused every time. I had almost given up, feeling so I’ll, I thought that was the end for me. I could not function in my daily life….showering, dressing, toileting…simple tasks left me sobbing with exhaustion. All I kept getting from numerous G.P.s and even endocrinologists was that I needed to stop lying to them and to myself about my diet. I followed every instruction they gave me, to the letter! I was on less than a thousand calories a day for over ten years….650 when I saw my last endocrinologist, who told me, and I quote,” take it down to 500 calories a day and don’t eat peas or pumpkin! “That was it ! That’s when I started to educate myself. I have since found a Functional Therapist who actually LISTENED to my pleas to test for T3….he did, and now I’m on T4 and T3 and my life has changed completely….I no longer struggle with little things and can do more normal, everyday things that I hadn’t been capable of for years. I lost 26 kg in three months eating normal, balanced meals…..my weight has plateaued, but it’s given the loose skin time to retract and I’m exercising agin, lightly, for the first time in 15 years. I’m 68 years old, riddled with Osteo Arthritis, am now 125 kg( down from 150.7 kg) ….and I feel amazing! I wish someone had listened to me. My health just got worse and worse….it was frightening. Just a couple of weeks ago, I asked a G.P. if she would do a T3 test and she hesitated for so long I asked her what was wrong….she told me she would test me, but it would be very costly….she also let slip that they did not do the test routinely because the Government didn’t subsidise the clinic for that test! I was stunned! All those miserable, sad, difficult years, being told I was a liar and that I needed to see a psychologist or a psychiatrist! If only I had had more gumption or even the energy to argue the point, I need not have suffered for so long. The treatment for Hypothyroidism needs to be upgraded in Australia! G.P.s need to be taught how to treat it properly….I was so bad I would go to bed hoping I wouldn’t wake up in the morning….there is a lot to answer for…..I can’t be the only woman who has gone through this hell without help. Sorry to be so long winded….at least I’m alive enough now to care. Thanks for listening, so to speak…xx

      • Hi Vivienne,

        I think it will be updated eventually but the bad news is that it will probably take another 5-10 years (potentially longer). Eventually, though, you will see the therapies that I have listed here on my site become the standard, there’s just no other way around it.

  2. Recent U/S conclusion nonfunction partial thyroid. partial thyroidectomy 1979 at 29years.
    MD states with age and Hashimotos; Levothyroxine 50mcg and at my request 30mg Armor. (increase risk of cardiac issues with higher doses of medications) No hx. of cardiac problems. Symptoms; lack of energy, dry hair, cracking nails and weight gain even though no change in diet. Aprox 1600 cal high fiber-vegetables and dairy, fish and chicken. Open to suggestions.

  3. I have never had my T3 tested, just my T4, TSH and antibodies. I will use this as a resource and ask my doctor at my next visit. It’s worth trying to get it checked. I’ve had thyroid disease almost ten years and I don’t recall it ever being tested. It’ll be interesting to hear what she says when I bring it up.

  4. I have read elsewhere that too much reverse T3 can cause lower than normal metabolism, make it difficult to lose weight, experience fatigue and chronic pain. Can you address this?

    • Hi, yes too much reverse T3 can do those things which is why, even when you have normal thyroid lab results (TSH; FT4; FT3) you can still feel awful, put on weight etc. As my GP explained it to me the hormone Reverse T3 is normal to have in your body but when there is too much of it then it blocks the hormone T3 from entering the cells. In simple terms she said: “It lands upside down on the cell and blocks access of T3”. Well, that’s in layman’s terms in a nutshell. I am sure Dr Childs has a more scientific explanation, but basically it will say the same.

    • Hi Teddee,

      Yes, it can certainly cause those symptoms as it blocks the cellular effects of T3, leading to low cellular T3, and low T3 is associated with all of the symptoms you addressed.

  5. Hi,
    I have been on 100mcg of levothyroxine for 25 years but have been feeling the cold for many years as well. I have been feeling extremely tired now for months and have had my Thyroid checked again (TSH and T4 are within normal ranges) but Free T3 is low below the normal range. My cholesterol is high even though I am thin (that is because I keep a very strict diet 1000 cal a day anything more and I gain weight). I am 55 years old. I am now trying to get T3 prescribed but that is difficult in the UK. not possible on NHS as the drug is too expensive. I may need to order online initially. I am very confused as to whether I should start with SR or IR? There doesn’t seem to be a straight answer on your site. Also, would one start with 10mg and add 5mg and assess every 10 days or start on 25mg and go up or down from there?

    • Hi Elise,

      The decision to use IR vs SR T3 really depends on the individual, so on my site, I can only give basic guidelines that some people may consider using when using these medications. If you aren’t sure where to start then your next best step would be to seek out a knowledgeable physician to help guide you.

  6. Hi, I’m trying to understand my recent blood work thyroid results.
    TSH: 3.68
    Total T3: 79 (I requested Free T3 but they gave me total instead)
    Free T4: 1.1

    I’m having a hard time finding info about the Total T3 so I can better understand the big picture of what these results are telling me about my thyroid. I have strong family history of Hypo.
    Thank you!!

  7. Hi Dr. Childs,

    I have Hashimoto’s with normal TSH. Do you know any studies that show benefits of taking T3 in this case (with normal TSH)? I also wonder if taking synthetic T3 suppresses your own production of T3.


    • Hi Luna,

      I don’t have any studies off the top of my head but you can check the bottom of my blog posts for references. And, yes, taking T3 will suppress your own production of T3 and T4.

    • Hi Becky,

      That’s somewhat of a controversial topic. Some people believe that you still need T4 and others don’t. I happen to think that taking some T4 is still a good idea but there are some people on T3 only who do quite well.

  8. I been on levothroxine 75mg and still have the symptoms. I even tell two doctors about my problems and still don’t get answers what I want. I even did two blood test that my TSH 1.48 and other is 1.42. T3 91 mg, Free T3 2.6, and Free T4 1 ng. It like the only test they go on is TSH is that normal. I don’t feel normal mind energy and weight is still the same . Also, if I could add T3 medication to levothroxine to thyroid medication or switch medication to natural thyroid medication.

  9. I have had a thyroidectomy about 5 or 6 years ago and I have only been on a Levothroxine medicine…no t3 was ever prescribed to me..at the time I was around 233 lbs now I am well over 279 lbs…is it possible to be put on T3 at this late stage of the game and loose weight…?? I have not talked to my doctor about it yet but have been wondering..if it is possible to reverse the weight gain…

  10. I have re-read this blog after at least a year, maybe more. I think I my interpretation of your charts was wrong and I incorrectly calculated how much T3 I needed. What I discovered is my armour thyroid dose was too low and the addition of the T3 opened a door to a new life! My TSH came back as zero on last lab but I was taking steroids and pain medications so that might impact the test. I now think I should lower my T3 dose. I feel great on the current dose but my Dr. seems to feel the TSH needs to be above 0. I still wish I could get rid of the belly fat.

  11. My doctor increased my Armour thyroid because my tsh was high, low t4 and normal t3. I just had my blood work done after 6 weeks on the new dose and my tsh has gone down to 0.02. The doctor advised me to continue on the higher dose because of the t3 in Armour causing the low tsh. I have side effects of nervousness, weakness, sweating, etc. Is taking the higher dose the right decision?


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