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Thyroid UK
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Excess T4 conversion to Rt3 - question?

Hey guys.

Have been on T3 only for nearly 15 yrs as didn't show any real conversion to T3 in tests but have just added a little T4 back in due to ongoing issues and worsening symptoms in the last couple of years since thyroid died and stopped producing it's own tiny amount of T4. Seems to have resolved the issue.

Anyway, I know there are problems with excess T4 inhibiting T4 to T3 conversion in those that still rely on T4 conversion to get their T3 etc but would any of these problems still apply to those not taking T4 for conversion use - just for it's possible other direct biological actions? Would it matter how much Rt3 is about if taking enough T3 for use elsewhere anyway?

I'm sure I read somewhere that excess Rt3 can actually physically block T3 receptors but I can't seem to remember where I read it or if it was just hypothetical or actually based on facts. Does anyone have any research paper or links to anything more solid regarding this? If this is the case, I'm assuming I should be really careful with what dose of T4 I take to ensure just the right amount - historically before these issues, my T4 levels were around 3-5 -normally 4 or under. If I still can't convert T4, any excess might just end up as Rt3???

Not hugely worried at this point just wondering if I need to monitor my T4 levels a bit more regularly to begin with? There is also the possibility that I can convert now as used to have chronic inflammation and other issues which I'm sure I also read somewhere that this can prevent conversion (again not sure where) in which case might not be so much of a problem anymore as resolved but I'll go initially with what has happened historically lol!

Thanks :-)

Ps - sorry just being lazy to see if anyone has anything to hand rather than spending hours searching lol :-D

9 Replies

T3 only with a nonexistent thyroid shouldn't produce too much rT3. You'd be hugely hyperthyroid (overdosed) to achieve that. The idea that rT3 blocks T3 receptors is now fairly well disproved. All it seems to be is a drain of inactive pseudohormone without activity if T4 is given in too high an amount. Overtreating on T4 produces rT3 in treatment as the body sees it as a kind of toxin to get rid of.


Thanks diogenes , glad to hear mostly disproved so saved me a lot of time hunting lol :-)

I've only added 25mcg T4 for now and am assuming it's best to just try and keep to my previous levels of around 4/5 (7-20) if possible?

Many thanks again :-)


I'd like to ask more about this but don't want to muddy up this thread so I'll post a separate question.


In the meantime this might be of interest:-


Go to the date March 24, 1999 on the following:-


1 like

THanks shaws. A lot of good stuff there in regards to Rt3 that's easy to follow, I'm not so concerned about excess Rt3 now. I do disagree with the not needing T4 part though - I've just had too many problems with my brain function/memory to the extent I thought I might be suffering dementia plus unable to stabilise thyroid levels and even hair growth stopped since I stopped producing it myself. These are problems I have never experienced in all my decades of autoimmunity, thyroid and multiple disorders. This is all slowly but surely getting better now since added in T4 so am now 100% convinced this was the problem. I was worried that it was a permanent issue and maybe lost forever but they all seem to be coming back little by little so maybe a communication or activity issue.

Maybe it's just me and something to do with the reason I can't convert T4 in the first place but something was seriously missing in the whole process - maybe the Rt3 pathway - obviously without T4, I therefore could not make Rt3, T2 is even supposedly made mostly or faster from Rt3 and is also the preferred substrate of D1 I think. Don't quote me on it as I couldn't understand any of this stuff a month ago due to amount my brain had degraded so literally only just starting to read into it properly the last week and be able to make sense of it so a way to go before I can any reason why with any conviction.

Put it this way, I wouldn't have been able to understand any of this stuff up until a a couple of weeks ago but I'm slowly able to start reading up on this stuff again and understand it plus lots new hair growth. These problems wouldn't have been so noticeable if I hadn't figured out the root causes of my health issues before this happened as I was just getting worse and worse and would have put it down to this but where I had a period of relatively great health, it was more prominent :-)


Thousands and thousands of people do o.k. on T4. It then converts to T3. If we don't have T3, and our brain contains the most of the billions of receptor cells in our body nothing in our body works efficiently.

We get lots of people say they have 'brain fog' or we feel it's not working properly as we doesn't seem to be able to work out things like it used to.

Also get your B12 checked as well as all of the vitamins/minerals. B12 has to be around 1,000 in order to help prevent dementia and if it is low, we can also get nerve damage.

When we begin to feel an improvement in or health it's a great feeling as at times we don't think we'll ever improve.

I'm glad you're feeling much better.


Yes, I'm definitely still working on it lol :-) I had tried to up my B12 levels but kept reacting so badly to anything I was taking during this period. I have started them again over the last week and am not experiencing the same issues so hopefully okay now :-)

Yes, I've known brain fog all my life and even had a diagnosed sleep disorder and have had many issues over the years. All this cleared up completely for quite some time after gluten free. I've also always been deficient up until that point on everything and have higher levels now than I've had previously. None of these issues had ever stopped my brain from actually thinking or not understanding (apart from when I was asleep lol). This was not similar to any brain fog I have ever experienced before and have not been sleepy in the slightest - in fact wide awake. I've had memory issues before when not medicated etc but these were only temporary memory issues involved in storing short term memories or taking info in and remembering it. It has never taken away huge chunks/years of my past/identity away like it did this time. I've been slow at doing things before but not completely unable to do them.

My T3 levels were fine, the only difference this time was the lack of T4. Even when my T3 levels were well over range, my tsh was still raised indicating it wasn't getting something it wanted.

Anyway, because these issues have all started reversing since taking T4 and having not made any other changes within that time, I can only assume that this must be the cause unless something else crops up or goes backwards.

Thanks for your advice :-D


Check your Iron level . Dosing with Iron supplements mat help with RT3 .


THanks, I've been on iron for many years and still prescribed. This has improved since going gluten free as now only low in range for things like haemoglobin and ferritin rather than under - I've struggled to take anything since stopped producing T4 but this seems to be going the other way now so hopefully sorted and am taking proper doses again :-)


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