Very, very roughly, a healthy person might convert one third of the T4 they produce into T3, one third into reverse T3, and one third will be excreted.
The third that is excreted is (mostly) sulphated or glucuronidated - which makes an inactive substance that can be excreted without affecting the gut.
Your body has some control over the amount of T4 that goes in each direction.
But you cannot use "bucket chemistry" to explain thyroid hormones. It isn't a case of one third of the T4 being converted to T3 and that is all there is to it.
Some of the T4 is transported into your brain and locally converted into T3. Indeed, many of our tissues can do some local conversion. But, for one example, the heart cannot.
All these things interact in ways that are very complex.
One thing to be aware of is that suppose you are being treated with T3 only, prescribed by your doctor, or you are buying T3 online.
Then suppose that your doctor decides you aren't allowed to have T3 any more, or the website you were buying T3 from disappears, then you don't have a reservoir of T4 to fall back on, and could run out of thyroid hormones altogether in a worst case scenario.
By taking T4 you might be saving yourself from running out of thyroid hormones even if you run out of T3.
Good thyroid hormone function is balanced for best outcome not just by thyroid hormone levels and ratios but transport, signal transduction, excretion (as explained by helvella above) and the effects of genetic variation, (plus other factors that I can't think of at the moment😬).
It is when we ourselves (or doctors) give the wrong meds/dose, fail to optimise cofactors or address coexisting conditions that negatively influence thyroid hormone physiology that good balance can not be found. A very small proportion appear to function well on T3-only meds but most require T4 meds because each tissue has a different expression of transport that prefer to transport T3, T4, or both into the cells.
My own example relates to a reversal of an osteopenic diagnosis 12 years ago. Bones have T4 receptors and use thyroid hormone transporters for ALL thyroid hormones and metabolites meaning they ALL have a job to do. There used to be a website called ’TiredThyroid’ which is now sadly defunct but it had a page listing essential reasons for mediating Levo which for some with genetic impairments needed combining with T3 meds.
'While T3 appears to be the most metabolically active, all thyroid hormones (T4, T3, T2, T1, T0) have non-genomic effects many are not aware of. All this means is that they can exert an effect on the cell at the plasma membrane (surface) or cytoplasm level, whereas the primary effects of T3 are at the cell’s nucleus (after conversion from T4). In other words, T4 exerts these non-genomic effects outside of the nucleus, and before its conversion to T3. So to say it is a prohormone (storage hormone) with no effect is a false statement, because it does have an effect in its unconverted state, as T4'.
Hello Radd, Lovely to hear from you, and thank you for explaining so gently.
Maybe I would have done better with Levo not NDT....I still have not found a version that doesnt make me itch and scratch. Maybe the T3 in NDT is too much at its ratio.......dont know anymore.
I do know im better with 'some' T4 ......its just working out how much before I shut down my own production.........difficult to know from blood tests if T3 (in NDT) suppresses TSH and T4 readings.
Ive taken NDT before to a level where TSH is still detectable (happy Endo!) but then T4 is below range (unhappy Endo) and I still felt rubbish, Then adding additional T3 to this takes TSH and T4 even lower, but I felt a bit better...but only a bit.
I remember Tired Thyroid too, will check out my files of stuff I printed off 10 years ago and re read.
Thank you for taking the time to reply, I hope you are doing well, and send you my best wishes. G
I agree with the above and suspect, with no evidence, that t4 like many biologically active compounds has many effects and that we need a certain amount of t4 in the brain. This might explain why some people get pretty unwell on t3 monotherapy. However, others are fine possibly because they produce sufficient t4 and/or the t4 critical pathways can function on a lower level of t4.
As I said this is a hypothesis and might well be completely wrong!
What you say makes complete sense to me,... individuality. Which makes a complete mockery of this ' box ticking fit the ranges' nightmare that physicians insist on today and terrify us into compliance........apologies for the rant.
Thank you for taking the time to reply and explain. Every best wish. G
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