If a large % of our T4 gets converted to T3, what happens to the remaining T4 and what action does it have on our body before it get expelled please?
Im curious to know why some of us need some T4 (despite the DIO2 fault) and what its action is and to which organs in particular it assists.
Any advise greatly appreciated, forgive me if this sounds scrambled, im barely with it at all today....yes still searching to get well.... kind regards, and every best wish, G
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Gillybean1
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Only a tiny bit of T4 gets converted to T3. If you look at the total T4 and total T3 reference intervals you can see the relative proportions in the blood. Going from memory we have about 60x as much T4 as T3.
A much higher fraction of T3 is unbound than T4 which is why the difference between fT4 and fT3 levels is not so great.
T4 is able to bind to receptors on the cell membrane but I don't think anyone knows much about it.
So, some T4 is converted to T3 or rT3 but most of it is broken down into other substances and eliminated.
I have a feeling your figures are very different to mine!
Normally, about one-third of T4 is converted to T3 and about one-third to rT3. The remainder of T4 is metabolized by different pathways, in particular glucuronidation and sulfation.
Metabolism of Thyroid Hormone
Robin P Peeters, M.D. Ph.D. and Theo J Visser, Ph.D.
The faster processing of T3 would result in a lower serum level.
And a significant portion of T4 does so much get broken down but added to - sulfated or glucoronidated, then excreted. Though there is also the possibility of recirculation via these substances and the gut.
Yes, after I posted I remembered the half life of T3 is one day v. seven days for T4. Thus, T3 would disappear 7x as quickly and rT3 even more quickly. I should have written 'only a fraction gets converted to T3'.
The one third to T3 would presumably apply to each T4 molecule created which is a little different to the T4 (total) we measure in the blood.
I think your TT4:TT3 has got the decimal point I the wrong place? nmol / pmol mix up?
Anyway I would go with one third or a little less being converted to T3. The important point being that T4 goes multiple ways, not just to T3.
T4 also seems to have some actions other than acting on thyroid hormone receptors either directly (less than 1%) or indirectly by being converted to T3. I don’t think these other actions are crucial as I and others have been on high dose T3 therapy with little or no T4 in our blood and not noticed anything unusual.
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