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Let’s Talk Thyroid Lab Interpretations: Signs of Poor T4 to T3 Conversion

Where to look if lab values appear to be “normal”…

This patient was experiencing hypothyroid symptoms with a slightly high TSH. However, her other numbers are in a “normal” reference range. Here’s what we looked at to uncover why she’s still having symptoms.

Video Transcript

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Today we’re doing another lab review or lab interpretation. 

Now for this person, I don’t have a lot of information, just what is here on the board. I don’t know a lot about their history. This is just somebody who sent me some of their labs and had some questions, but I thought this would be a good teaching moment for a Thyroid Thursday.

Here are their lab values:


So this is a person whose TSH is 4.17. They’re on 100 micrograms of Levothyroxine. 

Total T4 is 131 nmol/L or 10.18 ug/dL. That is well within the optimal reference range for total T4.

Free T4 is 20 pmol/L and 1.55 ng/dL. Again, at the higher end of what we might consider an optimal range, but still it’s at the higher end of the lab reference range. Typically in the US that reference range is about 1.7, and we typically look at 1.5 maybe being the top end of the optimal range.

Free T3 is 4.43 pmol/L, 2.88 1.55 pg/mL, and that would be below the optimal range. And we kind of look at an optimal range, maybe somewhere 3-4 range. The lab low here would be 2.0, but let’s say this is, maybe this is a person who feels awesome, maybe it’s a person who feels hypothyroid, but we’ll kind of go through both scenarios.


One of the things that I typically look at, especially when all these labs might be considered “normal” by their prescribing doctor, is the free T3 to free T4 ratio. How well is the person converting the T4 that they are getting from their medication?

Now remember, the thyroid gland typically makes about 80-100 micrograms per day, based on somebody’s size.

This person is taking 100 micrograms of Levothyroxine, so they’re taking a day’s worth of T4 on a regular ongoing basis.


There’s plenty of T4 in the system, but what we need to see is how much of that T4 is actually converting to T3 at the cell level. That’s what we want to see. 

So we look at this free T3 To free T4 ratio – and for this person, it’s 0.22.

If you’ve listened to some of these other videos, I like a range 0.31-0.34, and that’s based on the scientific literature.

And so this is a person that – despite plenty of T4 and what maybe her medical physician might think is plenty of T3 – they’re not doing a great job converting that T4 to T3.

This is likely a person who is still having hypothyroid symptoms and the reason why her TSH is still at 4.17 is because with less T4 to T3 conversion in the peripheral cells away from the thyroid gland, the pituitary gland is sensing this lower T3 state and that is keeping the TSH level higher. 

So to try and get the thyroid gland to support more T3 production for the under-conversion, it’s having to do it peripherally. 

This is a person whose doctor probably did the appropriate thing. They gave them T4 medication because they were diagnosed with hypothyroidism. 

Now the question is, could this person still be feeling hypothyroid? And I would say, yeah, the person might still be feeling hypothyroid, and if they are, the question is should their physician give them more T4?

I don’t think so. There’s plenty of total T4, there’s plenty of free T4. More T4 into this system isn’t going to fix it. 

So I don’t think more T4 is the right solution because there’s already enough T4 and free T4 available. If the cells and tissues wanted to convert the T4 to T3, they would be doing it. 

Remember, the thyroid gland only makes about 5-10 micrograms per day. The rest of that T3, the other 20-25 micrograms of T3 per day, comes from peripheral tissues converting T4 to T3, using that T3 temporarily, and then pumping it back out into the bloodstream. Then that becomes the circulating T3. 

So what do we do? Well, I don’t think the solution here is to provide more T4 because they’re already not converting the T4 to T3.

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So is the solution giving this person T3? Because somebody might say, well, especially if they still have hypothyroid symptoms, we’ll just give them T3 and that might raise the T3. 

And yes, it might, if you’re measuring the free T3 levels, right after you gave that person the T3 medication, you’ll see that rise in T3. 

But remember that the half-life of the T3 is about somewhere between 12-24 hours. So over the next 24 hours from the time of that T3 dose, you’re going to have a falling level of T3 in the system because that T3 is being metabolized out. 

So the person’s still going to have some of those symptoms. So I don’t think that’s the solution either because there’s enough T4 to convert to T3. 

I doubt this is a person who cannot convert T4 to T3 because we already see that their free T3 is 2.88. 


So what should we do with this person?  Well, it really does depend on the goal. 

If the goal is to suppress TSH, then more T4 might be appropriate, but the person’s probably not going to feel better. They’re probably going to have mixed symptoms of hypo.

If you’re just trying to manage some of their symptoms temporarily, then some T3 might manage their symptoms for a short period of time. And if you’re trying to manage this free T3 level and raise it, then yes, you can provide them with T3 medication. 

But the real question is, does it fix what’s happening at the cell and tissue level, or is it just managing numbers and short-term symptoms?


But what often happens with people who don’t convert T4 to T3 well is that they don’t feel well on higher and higher doses of T4, especially because it starts to suppress their free T3 levels

And then they often switch to T3. It feels good for a while, then they have to go up and up and up, and now they wind up on higher and higher doses of thyroid hormone. 


So what do you do in this situation? What I typically recommend in a situation like this is based on the patient’s goal. 

Is the goal of the person to feel better and function better? If the answer is yes, then my suggestion would be we start figuring out why the body is under-converting that T4 to T3.

To understand why that might be, we’d need to look at their health history, their health timeline, and the rest of their labs. 

Are there inflammatory mechanisms going on that are reducing the T4 to T3 conversion? If there is, then the solution is to identify and address what’s triggering and driving that inflammatory process.


Because if you put more T4 into a stressed system, what’s going to happen? You’re going to deactivate it to reverse T3!

(…which this doctor can’t see if that’s happening because they’re not running reverse T3.)

What if we just put more T3 into the system? Well, the inflammatory mechanisms are still going to cause deactivation of T4, but they’ll also create deactivation of the T3 you just gave. 

So some will get to the tissue, but some’s going to be deactivated anyway, and so that’s why you get the initial boost, the initial high, and then they’re going to need more in time.

Ultimately, if this person wants to feel and function better, if they want to take less thyroid medication and have it work better, if they ever want to have the possibility of their thyroid gland actually doing its job making hormones and helping regulate metabolism naturally…

You don’t want to keep just loading, loading, loading this person with thyroid medication….

You want to identify what’s creating the excessive cell stress that’s causing the body to reduce the conversion of T4 to T3. It’s an adaptive response that drives thyroiditis and damage to the gland. 

That’s going to give you the long-term solution, not just pounding this person with more thyroid hormone. 


So again, what we have to do is determine what is the patient’s goal. 

If a person said to me, “Hey, I want to feel and function better,” but they don’t want to make diet and lifestyle changes, they don’t want to look at making any changes to their habits, their behaviors, they don’t want to work on themselves or their environment… then managing their labs with medication and their symptoms with medication is probably the best option. 

But if they really do want to address what’s driving that excessive cell stress response, if they want to address what’s causing the under-conversion of T4 to T3, the thyroiditis, then the best solution is a functional medicine approach, less medication, and finding the root cause.

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Hi, I’m Dr. Eric Balcavage, owner and founder of Rejuvagen. If you’re struggling with health issues or have questions, let’s chat. You can schedule a 15-minute call with me to get started.