This is video #3 in my thyroid beginner series and today it’s all about T4. 

T4 is essential in understanding your thyroid because it’s actually 3 different things in one. 

T4 is a prescription medication, a hormone that your body produces naturally, and a test that you can order. 

Learn the differences, what they mean and how they can help you manage your thyroid: 

What is T4?

T4 is the most abundant thyroid hormone that your body produces naturally (assuming it’s working properly). 

It’s also known as Thyroxine which is the “official” or scientific name. 

If you hear someone refer to thyroxine just realize that they are referring to T4 and vice versa. 

So what’s the big deal with T4?

T4 is important in understanding your thyroid because it’s involved in the thyroid feedback system. 

T4 is produced by the thyroid gland after the gland is stimulated by TSH from the pituitary. 

T4 then circulates through the body where it is converted (on demand) by your cells into the active T3 thyroid hormone. 

If you have low circulating T4 then you will have low circulating T3 and this will cause the symptoms of hypothyroidism!

Let’s dive into the various definitions of T4 and how understanding what they mean can help you as a patient…

T4 the Hormone

As I mentioned previously, T4 is the most abundant thyroid hormone in your body. 

Your thyroid produces two major thyroid hormones: T4 and T3. 

About 80% of the thyroid hormone that your thyroid produces is T4 or Thyroxine. (1)

This leaves around 20% left as T3 (the primary active thyroid hormone). (2)

But why does your body produces so much T4 relative to T3?

The reason is most likely related to the control of the thyroid system. 

For instance:

T4, by itself, is not an active thyroid hormone. (3)

It must be activated, by enzymes, into the active T3 thyroid hormone. (4)

So your thyroid spits out some amount of constant T3 (directly from the thyroid gland) and then converts the rest of T3 that it needs from the abundance of T4 in the bloodstream. 

This way it can control how much thyroid hormone your cells need based on the demands that YOU put on your body. 

I think it’s most helpful to consider the analogy of a dam. 

In this analogy the dam is thyroid conversion (T4 to T3 conversion), T4 is the water behind the dam, and T3 is the water that is allowed through the dam. 

By setting up this system your body can titrate and carefully control thyroid function. 

And this makes sense, considering how important your thyroid is to your entire body! 

T4 the Test

But what about T4 as a test?

It should come as no surprise that we can test the amount of T4 that your body produces naturally (or the amount that you take by medication) through the blood. 

Doctors can do this by ordering what is known as a “free T4” test. 

This test is incredibly important in understanding how your thyroid is functioning because it is THE primary hormone that your gland produces. 

In many cases, I believe that the T4 (and T3) hold more value than the TSH when evaluating thyroid function. 

You can read more about how and why TSH can fall short as a predictor of thyroid function in thyroid beginner series #2 here. 

But what does T4 tell you when you measure it?

Several very important things:

#1. It gives you an idea as to how your thyroid gland is functioning. 

Your thyroid gland is stimulated by the pro-hormone TSH. 

This hormone tells your thyroid gland to produce both T4 and T3. 

If you test for T4 and find that it is low then it can tell you how responsive your thyroid gland is to TSH. 

The normal hypothyroid lab pattern is usually a high TSH accompanied by a low free T4. 

And this makes sense if you think about it:

If your thyroid can’t produce T4 then it responds by increasing the amount of TSH to try and overstimulate the gland to produce more hormone. 

But if your thyroid can’t produce thyroid hormone because it is damaged then the TSH will remain raised and your T4 will remain low. 

This is the “standard” pattern that most patients who have hypothyroidism or Hashimoto’s present with. 

#2. It gives you an idea as to how well your body is converting T4 into T3. 

We’ve already discussed how important T3 is for thyroid function in your body. 

And testing for Free T4 can help you identify issues with this conversion process. 

How does it help?

You can test both Free T4 and Free T3 and look at the ratio between the two of these hormones. 

Your T4 should be in a “healthy” range (usually the top 50% of the reference range) and your T3 should be in about the same range. 

If you have issues with T4 to T3 conversion you may see your T4 increase while your T3 decrease. 

In cases where your body isn’t able to produce thyroid hormone, you may see both the T4 and the T3 low. 

But both situations may be a clue that your body isn’t converting as well as it should. 

#3. It gives you an idea if you are absorbing or utilizing thyroid medication that you are taking by mouth. 

T4 can, and should, be tested when you are taking thyroid medication by mouth. 

It goes without saying, but in order for your body to actually “use” thyroid hormone that you take by mouth, it must be absorbed by your body. 

There are several factors, including the time of day that you take thyroid medication, (5) that can impact the rate at which you absorb thyroid medication. 

You can easily determine if you are absorbing thyroid medication by checking Free T4 before you start medication and 8 weeks afterward. 

You should see your Free T4 increase as you take thyroid medication and you should see your TSH drop. 

If these things don’t happen then you may need to consider that you are not adequately absorbing your medication. 

Other factors such as supplements, calcium, (6) and food intake can all impact how well you absorb thyroid medication (and this is why it’s generally recommended that you take thyroid hormone on an empty stomach). 

You can learn other tips and tricks to help maximize thyroid hormone absorption here. 

T4 the Medication

T4 is also the most commonly used thyroid medication. 

Prescription thyroid medications, especially those used by conventional doctors, contain T4 thyroid hormone. 

Medications that fit into this list include Synthroid, levothyroxine, Tirosint, and Levoxyl. 

These medications contain ONLY T4 or Thyroxine and should be compared to other medications which contain either combinations of T4 and T3 or just T3 alone. 

T4 has become the most commonly used thyroid medication over the last 30 years or so (7) because it is felt to be the most “consistent” thyroid medication and because it has the longest half-life (which means it stays in your bloodstream the longest). 

But it should always be remembered that your body naturally produces both T4 and T3 and that replacing only T4 may be part of the reason that so many patients remain symptomatic despite taking thyroid medication. 

Doctors only use T4 because they assume that your body will have no issue in converting T4 into T3. 

But this logic doesn’t take into account that each person converts T4 into T3 at a different rate and that we aren’t all equal in that regard. 

The problem with T4-only medications is that newer studies have shown that many people who use T4 have a lower than normal T3 (8) (the active thyroid hormone) even though they have a normal TSH. 

This approach to thyroid hormone replacement may be part of the reason that so many patients remain symptomatic despite having a “normal TSH”. 

It should also be noted that you can safely use T3/T4 thyroid medications without negative side effects provided they are used appropriately. 

Conclusion

That concludes our discussion on T4 basics!

Just remember that while T4 is an important thyroid hormone for your body, it is not as powerful as T3 thyroid hormone. 

But it still has value in terms of testing and using T4-only thyroid medication. 

Some people are able to use T4 thyroid medication and feel great while others may need some T3 to feel optimal. 

If you have thyroid problems make sure that you can differentiate between T4 the test, T4 the hormone, and T4 the medication. 

Now I want to hear from you:

Have you had your T4 tested? Was it low or normal?

Are you taking T4 medication? Is it working for you?

Why or why not?

Leave your comments below! 

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: 

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. 

Why?

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). 

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

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. 

NDT

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. 

Conclusion

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’re having trouble, don’t hesitate to try T3 support supplements, which can help your body naturally create more T3.

Between the two, you will be able to optimize your T3 status.

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!