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Can Adding T3 Cause Weight Gain?

Why additional T3 can sometimes make hypothyroid symptoms worse (and yes, slow your metabolism!)…

This patient started gaining weight after adding T3 to their T-4-only medication strategy. Could the T3 be the culprit? The answer is “yes,” but it’s a loaded yes…

Video Transcript

Question for Dr. B’s videos or podcast? Send them via Instagram, or email info@rejuvagencenter.com!

Today’s question for our Thyroid Thursday is from a patient who recently asked me: could adding T3 to T4 medication cause them to gain weight and have a slower metabolism? 

In short, yes, it can. 

In this video, I’ll walk you through what’s going on. 


This person was on T4-only medication, but their T3 and free T3 levels were functionally low and they still had hypothyroid signs and symptoms.

So they went to an integrative practitioner who then said this person had a problem converting T4 to T3. Therefore, the solution was to give them T3 since they were a “non-con converter,” or they had a reduced ability to convert T4 to T3. 

So the doctor added some T3, and since then they’ve been tinkering with the dose; increasing T3 and decreasing T4.

Then, they got some follow-up labs. I don’t know what the actual values were, they just told me they had a TSH below 1.0, a free T4 and free T3 that the doctor considered to be “optimal,” and a reverse T3 that was around 8.0.

They were told that they had a higher reverse T3 on the T4-only medication and that the reverse T3 was blocking T3 from binding. (But that’s not necessarily the case. What reduces T3 from binding is really deiodinase 3, which deactivates T4 and T3 into less active forms of hormone.)

And so this doctor said, you’re now optimized because TSH is lower, they’re not as concerned about the TSH value, but because free T4 and free T3 were in this doctor’s optimal range, and reverse T3 was lower.

The person was told, “Hey, you’re optimized. We’ve got your thyroid panel where it needs to be, the meds are where they need to be…”

But the patient’s concerned because they still have hypothyroid signs and symptoms, plus they’re gaining weight and that’s the main concern. 


So what could be going on here? Well, there are a couple of things that could be going on. 

Part one is that we’re saturating the hypothalamus in the pituitary, and that can activate the sympathetic nervous system, reduce the parasympathetic nervous system, and it can create a little bit more stress and inflammation in the system. 

Part two – by dropping TSH, what happens here is now we’re getting less drive of the thyroid gland to produce T4 and the little bit of T3 that it typically generates. 

And if the person’s suppressing their TSH with added T3 and they’re not getting any help from their thyroid gland to produce T4, the dose of T4 that they’re on may actually be too low.

Remember, the half-life of T4 is like seven days, but the half-life of T3 is somewhere between 12 and 24 hours. 

So the person is probably not taking enough T3 to support their metabolism and they don’t have enough T4 to support their metabolism. So their metabolism is being downregulated.  On top of that, they probably still have the immune-inflammatory process that was actively deactivating the conversion of T4 to reverse T3. 

One of the big clues that this person may not have enough T4 in their system is this lower reverse T3. 

I know a lot of people in integrative functional medicine think reverse T3 has a blocking mechanism, but really, elevated reverse T3 is an indication that there’s either too much T4 medication, or there’s an inflammatory process going on that’s adaptively deactivating T4 to reverse T3 to slow down metabolism. 

And when that mechanism is in play, it’s not just T4 that’s getting deactivated, but T3 is also getting deactivated. 

So for this person, what’s likely happened is that they’ve got too little T4, the thyroid gland is being suppressed so it can’t help make T4, and they also probably don’t have sufficient T3 in the system, even though these free values look good or “optimal.”


So what do you do in this situation? 

I think one of the things to consider is that if you don’t feel well on your current dose, then it’s either 1) not the appropriate dose, or 2), there’s some type of inflammatory process going on at the cell level, regardless of what hormone you put in, that’s causing that hormone not to be converted.

The other challenge is that when people get put on a combination of T4 and T3 medication, they’re oftentimes not taking enough T3 or T4 medication to really support the physiology well enough on top of that immune inflammatory process. 

So to the person who asked the question, and I get this question asked a lot, could adding the T3 cause them to gain weight? And the answer is yes. 

You’re probably in a kind of gray zone where you’ve suppressed the thyroid gland, so it can help out. You’re not taking enough T4 and you’re probably not taking enough T3 either, given what you’re trying to do. 

Need 1-1 help with your thyroid? Schedule a free discovery call with me here.


What would I suggest for this person? 

Remember, I don’t prescribe, but if this person is trying to identify the root issues as to what’s going on with their physiology, then I really want them on the least amount of T4 possible. 

If they need some additional T3, especially if their thyroid gland is significantly atrophied or they’ve had thyroidectomy or they’ve had radiation treatment or somebody’s damaged or destroyed the thyroid gland, then maybe 5-10 micrograms of T3, tops. 

But pounding more into the system isn’t going to fix it. And just trying to manipulate blood values doesn’t always correlate with optimal thyroid physiology inside the cells. 

Overall, I don’t think this is a person who cannot convert T4 to T3. I think this person has a reduced demand to convert T4 to T3 due to some type of cell stress or inflammatory mechanism. 

What I would suggest is, depending on whoever their doctor is, identify an appropriate level of T4, and if they need it, a little bit of T3. But to get busy trying to find out what’s creating that cell stress response that’s preventing them from converting T4 to T3 at the cell and tissue level. 

Remember, the thyroid gland only makes about 5-10 micrograms of T3 per day. The rest of the 20-25 micrograms of T3 are made in the peripheral tissues, not at the gland. 


So what drives the cells and tissues to convert T4 to T3? 

When cells are in low-stress homeostatic regulation, they want to bring a lot of T4 in, convert it to T3, and have a higher level of T3 in the cell to turn on the manufacturing process, to burn calories, to make energy, to make hair and skin and hormones and peptides and all this fantastic stuff. 

But when cells are perceiving danger, then there’s an active downregulation of T4 to T3 in a lot of the tissues, and a ramping up of the cell defense response. 

Bottom line: you can’t force more thyroid hormone into a system that doesn’t want it, and expect to have good results!

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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.