T4 Thyroid Basics: The Test, The Medication, The Hormone

T4 Thyroid Basics: The Test, The Medication, The Hormone

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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 thyroid hormone molecular structure

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…


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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. 

High free T4 and low TSH in hashimoto's

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. 

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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. 


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! 

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

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

#3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4699302/

#4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673746/

#5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4650787/

#6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3092723/

#7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4980994/

#8. https://www.thieme-connect.de/DOI/DOI?10.1055/s-0043-125064

what you need to know about t4 thyroid hormone

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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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