Triiodothyronine (T3) Hormone Guide: Why It’s So Important

Triiodothyronine (T3) Hormone Guide: Why It’s So Important

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Learn everything you need to know about the thyroid hormone Triiodothyronine including how it functions in your body, how it is produced, the symptoms associated with deficiency, and how to improve (both naturally and through medication) and optimize your levels in this post

What is Triiodothyronine?

Triiodothyronine is the complete name given to the most powerful thyroid hormone in your body. 

It is often abbreviated as T3 for short, but whether you refer to it as triiodothyronine or as T3 it is the same molecule. 

T3 is the most powerful form of thyroid hormone that your body creates and produces and it should be differentiated from the less biologically active thyroid hormones. 

The most common thyroid hormone that your thyroid produces is known as Thyroxine

Triiodothyronine is actually a byproduct of Thyroxine when an iodine moiety is cleaved off of it during the thyroid conversion process by special enzymes (1).

Basically, your body produces massive amounts of thyroxine and it then activates that thyroxine into triiodothyronine in specific tissues based on the needs of the body

This is the mechanism, that your body uses to regulate how much thyroid hormone is being produced at any given time. 

The reason that your body has to regulate Triiodothyronine in this way is that it is the most powerful thyroid hormone. 

All of the benefits and effects of thyroid hormone come from Triiodothyronine. 

Benefits such as the regulation of your metabolism (2), the regulation of your body weight, the regulation of heat and energy production, the regulation of cholesterol synthesis (3), and heart rate ALL come from Triiodothyronine and its impact on your cells. 

If we understand how your body creates T3 then we can reverse engineer the process and use that information to help us improve thyroid function in the entire body. 


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How does your Body Get it & What does it do? (Basic Physiology & Function)

So we know that triiodothyronine is important, but how does your body get it and where does it come from?

The majority of the triiodothyronine in your body is actually created from Thyroxine through deiodinase enzymes in specific tissues such as the liver and your intestines. 

Your thyroid does produce some T3 directly as well from the thyroid gland. 

Around 80% of all thyroid hormone created from your thyroid gland (4) comes out as thyroxine which leaves around 20% or so which is produced directly as triiodothyronine. 

Your body then takes that 80% of thyroxine which is floating around in the bloodstream and alters the chemical structure of thyroxine to produce triiodothyronine. 

T3 then circulates in the bloodstream and enters target cells where it sits on receptors on the outside of the cell as well as the inside. 

Thyroid hormone is special in that it interfaces directly with the nucleus of your cells and alters DNA production directly (5).

So in a way T3 is directly altering the production of proteins and enzymes in your cells. 

These proteins then go on to improve enzyme function in specific organelles in the cell. 

The net result to you is that you have more energy, your body produces more energy, your metabolism is increased and your mood maintains controlled and regulated. 

Triiodothyronine has also been shown to directly activate pumps and channels on the outer membrane of the cell (so-called nongenomic effects (6)). 

This is the mechanism by which your thyroid uses to help manage heart rate and blood pressure. 

The bottom line?

Triiodothyronine regulates MAJOR systems in the body and it is both created and produced. 

Triiodothyronine Deficiency & Symptoms

Is it possible to have too little T3 in your body?

The answer is a definite yes, and it is well-known in the medical community.

There are two major ways that you can develop low T3:

The first is through a reduction in Thyroxine (T4)

If your body, for any reason, is unable to produce Thyroxine then you will automatically have an issue with triiodothyronine. 


Because the main way that your body produces T3 is by altering Thyroxine (T4) through peripheral conversion. 

Through this mechanism, your body may develop low T3 through any condition that damages your thyroid gland or reduces thyroid hormone production. 

The most common condition that fits into this category is known as Hypothyroidism, but other conditions such as Hashimoto’s thyroiditis can also cause it. 

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Other medical conditions that result in damage to the thyroid gland (such as radioactive iodine ablation, trauma, or inflammation) or surgical removal of the thyroid gland (known as thyroidectomy) will also cause the same issue. 

Because low triiodothyronine may result from these conditions it’s always important to test for both T4 and T3 (more on that below). 

The second reason you may develop a deficiency in triiodothyronine is due to a reduction in Thyroxine conversion

Any condition which results in reduced Thyroxine conversion to Triiodothyronine will automatically result in low T3. 

This condition is often referred to as Low T3 syndrome and it has many different causes. 

Perhaps the most common cause of reduced thyroxine conversion is secondary to dieting or calorie restriction. 

Calorie restriction, if prolonged, can blunt thyroxine conversion and lead to reduced circulating triiodothyronine (7).

Put simply: dieting = reduced triiodothyronine = reduced metabolism = weight gain

This mechanism helps explain why so many patients regain weight after diets and have issues in maintaining weight loss long-term. 

But other problems can cause reduced thyroxine conversion as well. 

Documented conditions such as inflammation, gut dysfunction, liver problems/damage (8), nutrient deficiencies (9), illness, and the use of certain medications (10) can also blunt thyroxine conversion. 

Patients who suffer from these issues often present with symptoms that mimic hypothyroidism. 

The severity of their symptoms depends on how great the reduction in circulating triiodothyronine is in their body. 

Some conditions may result in only minor reductions of 5-10% whereas other conditions may be more serious with a 25-40% reduction in triiodothyronine. 

Symptoms that often accompany triiodothyronine deficiency include: 

  • Weight gain (10-50 pounds depending on severity)
  • Fatigue or low energy
  • Low body temperature
  • Low resting heart rate
  • Inability to lose weight despite diet and exercise
  • Elevated cholesterol 
  • Cold intolerance
  • Hair loss

If you are experiencing these symptoms you can confirm a deficiency by checking serum lab tests.

How to Test for Triiodothyronine (Total Triiodothyronine vs Free Triiodothyronine vs Reverse Triiodothyronine)

Triiodothyronine can easily be assessed with simple and routine blood tests. 

You can ask your doctor to order these tests and they should be covered by insurance, especially if you have a known or suspected thyroid issue (don’t let your doctor tell you they won’t be covered!). 

There are 3 ways to assess triiodothyronine in your blood: 

  • Free Triiodothyronine (Free T3) – This test gives you an idea of the amount of free and active T3 in your bloodstream. You want this number to be in the top 50% of the reference range provided. Your free T3 may be the most important measurement of thyroid activity in your body. High levels, however, may cause the symptoms of hyperthyroidism. 
  • Total Triiodothyronine (Total T3)- Total T3 can help determine the amount of T3 in your serum from all sources. Total T3 can help you understand how much T3 is available, even though not all of it may be active. Like free T3, you’ll want this number in the top 50% of the reference range. 

These lab tests are often considered “advanced” and supplemental, but they should take priority and always be ordered with other thyroid lab tests. 

Knowing and understanding the value of triiodothyronine in your body gives you valuable insight in regard to thyroid gland activity and peripheral thyroid conversion. 

These cannot be reliably assessed with just the TSH and Free T4. 

In addition to the tests listed above, you might also find additional value in ordering these tests: 

  • Reverse Triiodothyronine (Reverse T3) – Reverse T3 is an anti-thyroid metabolite that can be produced from Thyroxine through a similar conversion process as T3 is created. Reverse T3 directly competes with T3 and high levels may cause hypothyroid symptoms. 
  • Free Thyroxine (T4) – Thyroxine can help you determine how active your thyroid gland is and give you an idea of how much thyroid reservoir exists from which your body can produce T3. 
  • Thyroid-stimulating hormone (TSH) – TSH is another test that helps determine how responsive your thyroid gland is to stimulation. High TSH generally indicates a slow thyroid while a low TSH generally indicates a hyperfunctioning thyroid. TSH is a very useful test but it does have some limitations which you can find out more about here

Understanding these tests, their optimal ranges, and how they interact with your body is very important. 

If you find that you have sub-optimal T3 levels AND you are experiencing symptoms associated with deficiency, then your next step should be to consider increasing your T3 through supplements or bio-identical medication. 

Using T3 Medication (Side Effects)

If you suspect a deficiency in T3 there are ways to supplement with this hormone. 

Medications that contain triiodothyronine include Cytomel, Liothyronine, and formulations of natural desiccated thyroid hormone.

Most patients who suffer from thyroid issues, including low T3, often supplement with Thyroxine as opposed to Triiodothyronine. 

The reason for this is that Thyroxine is considered to be more stable in your bloodstream and more consistent because it has a longer half-life (11).

This means that taking Thyroxine medication (such as levothyroxine or Synthroid) will last longer in your body when compared to Triiodothyronine. 

But just because most physicians prefer not to use it doesn’t mean that it isn’t tremendously beneficial for some people. 


Triiodothyronine is the most active and potent thyroid hormone in your body. 

Compared to Thyroxine medication it is about 3-4x more powerful at altering the TSH (12).

Because of this patients may experience side effects when using T3 medication, more so when compared to Thyroxine. 

Side effects of taking Triiodothyronine medications include:

  • Heart palpitations
  • Jittery sensation
  • Increased anxiety
  • Rise in heart rate
  • Increased body temperature
  • Increased hair loss
  • Shortness of breath

These side effects are usually seen when the dose that you are taking is too high and may indicate that you are getting too much hormone. 

Triiodothyronine can safely be used as part of a hypothyroidism medication regimen (often used in conjunction with Levothyroxine) as long as it is dosed correctly and monitored correctly. 

Some patients do abuse T3 specifically for the use of weight loss, but this should be avoided!

Using T3, in supraphysiologic doses (higher than normal doses), may cause weight loss but it does put extra strain on the heart

Long-term use in high doses may cause cardiac enlargement and other heart issues such as atrial fibrillation (13).

Triiodothyronine can be considered in patients who are taking Thyroxine medication but not experiencing symptomatic improvement. 

As long as it is dosed in accordance with the TSH and Free T3/Total T3 levels it will not cause harm

Supplementing to Boost Triiodothyronine

Taking bio-identical triiodothyronine can be difficult because it requires a prescription from a physician. 

Another strategy to increase T3 levels is through supplementation with certain vitamins and nutrients. 

It is well known that up to 13 nutrients are required for proper thyroid hormone synthesis and conversion

Optimal levels of all of these micro-nutrients are required for proper thyroid function. 

In addition, many patients may actually have small but noticeable deficiencies in many of these nutrients and trace minerals. 

This opens up the opportunity to use certain supplements to help promote optimal T3 levels in the body. 

In addition to these nutrients, there are also certain botanical supplements, such as Guggulsterone, which can help improve T3 levels as well. 

These nutrients work by improving the rate at which your body converts T4 into T3 (through the conversion process) and by helping your body produce optimal Thyroxine and Triiodothyronine from the thyroid gland by providing relevant precursors. 

You can use a supplement such as this one, which contains all relevant nutrients, or you can supplement with each nutrient individually. 

Here is a list of supplements that have been shown to help improve thyroid hormone function: 

You can find other ways to naturally increase triiodothyronine in this guide as well

If possible, your first approach to managing your T3 levels should be to use natural approaches first prior to using bio-identical hormone therapy. 

Natural approaches are often quite effective and reduce the risk of developing negative side effects. 


Triiodothyronine is the single most important and biologically active thyroid hormone in your body. 

It regulates the function of major processes in your body such as your metabolism and also helps to regulate other hormones.

Preserving the amount of activity of T3 in your body should be a top priority, especially if you are experiencing quality-of-life symptoms such as weight gain or fatigue. 

If you suspect that you have a triiodothyronine deficiency in your body then make sure that you get proper testing with a TSH, Free T3, and Total T3

Fixing your problem then becomes a matter of supplementing with bio-identical hormone therapy or by taking the natural approach. 

Now I want to hear from you:

Are you experiencing low T3?

Do you have the symptoms of T3 deficiency?

Have natural approaches worked for you? Why or why not?

Leave your comments below! 
















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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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