rT3 dominance...real or not...?: Sometimes, it... - Thyroid UK

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rT3 dominance...real or not...?

Sometimes, it feels like rT3 dominance is like the thyroid hormones T1 and T2...that is, real or myth? Is it metabolically active on its own or is it simply an inactive byproduct of T4 and T3?

Many articles online claim rT3 dominance (rT3 is often referred to as the "hibernation hormone") can cause hypothyroidism although you take high doses of NDT or T4, as too much T4 is converted to rT3 which then blocks the activity of free T3. The solution proposed is to go on T3 only, or cut back on NDT/T4 and take no more than 50 mcg/1.5 grains daily, and then add as much T3 as you need to clear your system of excess rT3. This is said to take 3-6 months.

But is rT3 dominance a real thing, or just a myth...? I just found the following article that pretty much says it is:


Any ideas or personal experiences...?

15 Replies

PS. I also find this article very interesting, especially what it says about rt3 being the real reason many regain weight after weight loss:


It makes perfect sense to me...

haggisplant profile image
haggisplant in reply to

I’m not sure but I’m wondering why I’m experiencing hypothyroid symptoms of freezing feet and tingling (but no other symptoms) despite being slightly over replaced and thinking it might be an answer.

in reply to haggisplant

What are your symptoms of being over replaced?

haggisplant profile image
haggisplant in reply to

Sleep issues - super woman on a few hours sleep. Very hot. Slightly loose bowels. Itching. It’s only slightly out of range. But I recognise the symptoms. But couldn’t understand the freezing feet! I have crap circulation though. I’m going to drop by 12.5 and see if it helps.

The thing is that a healthy body is very good at maintaining homeostasis so it would make sense to me that there’s mechanisms to avoid going to far one way or the other (if your thyroid system works well.)

I’d recently read into the role of the enzymes involved in t4 to t3 conversion and there are similar feed back loops there to maintain the right level of t3. Obviously all people are different though. I haven’t had time to read your links unfortunately.

There is no evidence at all that rT3 blocks T3. In fact the blog actually says that rT3 does not affect T3 receptors and there are papers to prove this. In nonthyroidal illness, rT3 is often elevated and this may have some indirect effects, as FT3 is simultaneously lowered. But rT3 is cleared very quickly from the body in a few hours, whereas T3 has a halflife of one day and T4 8 days. The idea that rT3 can cause weight gain directly is I think an unproven and unlikely idea. In therap. Taking too much T4 can raise rT3, which is used as an inactive drain to get rid of the excess T4 quickly. So we don't really know if it is the elevated rT3 that is having an action or whether changes in the other hormones are more important - which I suspect is the primary situation.

Dawid86 profile image
Dawid86 in reply to diogenes

BEFORE TO START ANY THYROID, last year, i had:

Ft3 3.9 1.5-4. 1

Ft4 0.96 0.8-1.76

Rt3 0.5 0.10-0.24 **

Feeling severly hypo!!

Taking t3 at replacement dose, now 80mcg has improved a lot of hypo symptoms. Also has made higher my total and free testosterone at optimum level.

Rt3 maybe does not block t3 receptor, rt3 has his receptor, but for sure it is a brake for metabolism and thyroid effect!!

I saw a lot of people who was good only after clearing their high rt3 even if they had good ft3...!

Rt3 adrenals group, t3 group on fb, adrenal thyroid care on fb based on sttm, and Paul Robinson confirmed that.

Also yoy have to study biochemistry to better understand the role of rt3.

Too much rt3 is hurting your thyroid fuction and not only because you could have a decrease t4=t3 conversion.

rT3 doesn't block T3 as it has its own receptors. It is just made in preference to T3 to keep hormone levels down when they are too high (either T4 or T3) for you. Lots of things (mostly illnesses and low calories) cause rT3 to rise, so knowing that it is high (or not) isn't really helpful. If you have no thyroid, taking T3 only would stop rt3 production as there'd be nothing to make it from, but if you have a thyroid and take too much T3, you'd still make rT3 from the t4 your thyroid can still produce.

in reply to Angel_of_the_North

I have been wondering about this myself as I've read lots of articles claiming that you can "flush out" excessive rT3 levels by going off or decreasing T4/NDT and adding pure T3. The reason for that, they say, is that rT3 can only come from T4, not T3 so less T4 = less rT3. But if what you say is true (and it makes perfect sense to me), excessive T3 levels can also lead to rT3 dominance...?

Angel_of_the_North profile image
Angel_of_the_North in reply to

We've seen people here who have tried that and still have high rT3 even though they are on T3 only and have low FT4 - and I can only assume that it is because they still have thyroids and pretty much all the T4 their thyroids produce is now being converted to rT3. If FT4 is too high and FT3 is low then reducing levo and adding some T3 may be what you need, but taking too much T3 only is not going to "cure" you. If you don't have a thyroid, that would stop your production of rT3, but it wouldn't necessarily be a good thing.

in reply to Angel_of_the_North

Makes a lot of sense to me!

But there also are a lot of people who was able to switch off their thryroid allow the flushing out of t3! To totally suppress your thyroid, you must have a tsh suppressed 24 /h, some people to do that need 3 big dose of t3, or 4 dose.

My rt3 now is 2 times above range, taking t3 only at 80mcg before was 6 times above, and before while taking 50mcg t3 was 10times above and felt horrible!


If you read the previous post it explains how the rt3 is linked to the enzyme conversion system as a feed back loop to up or lower conversion rates.

in reply to haggisplant


helvella profile image

Just to address one specific point, T2 and T1 are absolutely real hormones.

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