Thyroid UK
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Thyroidal T3 stays in blood only?

Since I have always felt that taking exogenous T3 does not alleviate any of my symptoms and it cannot be compared with the way FT3 coming from FT4 conversion acts upon me, I have googled around for answers and found this piece of information at

Local conversion (at the cellular level)

D2, another deiodinase enzyme, converts T4 to T3 within the cell. The level of available T4 affects the conversion rate. When T4 levels are high, there is less T4 to T3 conversion at the cellular level. This is why continuing to raise the T4 dose doesn’t seem to be effective after a while, and why adding some T3 instead may be more beneficial. T3 decreases D2 gene transcription, while it increases D1 transcription. However, T3 from D2 has two to three times the effect on gene transcription than T3 from D1. This means T3 converted from T4 at the cellular level has more impact than circulating T3. Someone taking only T3 only adds to circulating T3. There would be little T3 from cellular conversion, because there is negligible T4 for the D2 enzyme to convert.

Then another question popped up: does thyroidal T3 stay in blood only? By thyroidal T3 I mean the tiny amount of T3 produced by the thyroid ITSELF, and not coming from FT4 conversion; a healthy thyroid produces 85-100 mcg T4 and 6 mcg T3. I am talking about healthy people here, actually it does not matter.

That would mean that about 25% of the 85-100% T4 turn into FT3 inside the body, so at a cellular level a healthy person would have like +/- 25 mcg T3.

The rest 6 mcg T3 come directly from the thyroid.

Do these 6 mcg stay only in blood? For any other purposes? Maybe some organs which cannot convert FT4 into FT3 into their tissues will use that T3 from blood?

I haven't been able to find anything on this question of mine?

Thank you!

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I think the way you have worked out the above would be fine if you were talking about NDT (natural Dessicated Thyroid Hormones), the very original replacement which is made from pigs/cows glands and contains all of the hormones a healthy gland would produce.

I don't think levothyroxine a synthetic T4 suits many. It doesn't suit me whatsoever, even when some T3 was added to it. I had many visits to the A&E and by ambulance due to severe palps and all resolved when I took T3 only. I have felt normal health for ages, all symptoms have disappeared. I cannot take synthetic levo but because synthetic T3 doesn't need to be converted it works for me.

The purpose of T3 is to enter into our receptor cells and its work then continues between 1 to 3 days and it sends out 'waves'. I am not medically qualified but it seems simple to me. T4 I can live without, T3 I cannot.


Thank you, Shaws, although I think you are an outlier.

The majority of people can do well on T4, they can convert T4, it is just their doses that are not titrated the right way and thus comes the delusion that they are bad convertors, which is not true.

I was a bad convertor when on 50 T4, an extremely bad one, but I convert hell well when I am on a lower dose T4, because there are some conditions that have to be met when on T4 in order for conversion to take place and they were not met for me when I was on 50 T4 and higher.

So, sometimes less is better.

And T3 does not touch me a hint. It just lowers my FT4 and makes me more and more hypo and I have to pour more and more T4 to compensate for the lowering of FT4. And 70-80% are like me. The others are just outliers, who can do well on T3 only.


Yes, I've just started looking into this side of things myself although still looking at mo. I was happily on T3 for nearly fifteen yrs as my body just doesn't seem to like T4 and really can't seem to convert. However, when my thyroid finally gave up the ghost a couple of years back and I stopped producing a little T4 of my own, I've had increasing symptoms that I've never had before especially regarding my brain and was unable to stabilise my thyroid levels during this period and had hyper issues even though blood looked hypo. I've added some T4 back in and symptoms finally seem to be dissipating even though my body still does not like levo but it's better than the alternative lol!


There is deff a difference between people that have an albeit poor functioning but still there thyroid and those that have no thyroid due to TT or RAI.

I'm 2 1/2 yrs post TT myself so unfortunately have first hand experience of this. Currently trying to get a t3 trial but its proving to be a very very long winded process.


No thyroidal T3 does exactly the same as T3 produced by cells from T4. There's no difference in the accessibility of either form in and out of cells.


Could you please point some reference to that?

I was not able to find anything.


It's just that whether T3 is produced into the bloodstream by the thyroid or produced in cells by T4-T3 conversion, the T3's mingle because there is a movement in and out of cells by T3 which mixes the two together.


But how do you know that?

Have you read it somewhere?


Because cells are not closed, where what is formed inside can't get out. The cells and body fluids like blood, lymph simply equilibrate many substances (fatty acids, glucose, amino acids. vitamins etc etc) and thyroid and other hormones are just the same. It's just straightforward biochemistry.

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