With T4 doses can be alternated between days, like 100mcg one day and 75mcg the next and back and forth. Can anybody here tell me what would happen if that was done with T3? Let's say one day 5mcg are taken and the next 10mcg. My brain can not figure out the consequences of this. Does T3 build up in the body like T4 does? I know it is absorbed really fast and has a much shorter half life than T4, but I don't know what that actually translates into other than a blood test after the higher dose would likely be different from a blood test after the lower dose (Maybe?). But how different exactly? Is there an answer to that? Has anybody ever tried that? Or had any experience with that? Any thought would be welcome, because I'm all out of thoughts. 
Alternating T3 doses: With T4 doses can be... - Thyroid UK
Alternating T3 doses
Buddy because of its short half life and the way the body uses t3 it needs to be taken as the same dose each day. You cannot alternate daily doses.
Hi buddy
Alternating T3 would not be a good idea. You can get away with alternating doses with T4 but not T3.
I expect others will be along who actually take T3, but from reading up this is the one with the shortest ‘half life’. Half life is a measure of how long any given substance takes to reduce by half in any given system. It’s an indication of reactivity.
T4 has a much longer half life. Estimates vary but I’ve seen 7 to 9 days quoted. So once your dose has built up in your body it provides a fairly steady baseline and it is considered the storage hormone.
The T3 is more reactive, it is the bioavailable thyroid hormone and has a shorter half life of hour rather than days so it is readily available to the bodies cells.
So T4 is converted to T3 in various organs of the body but T3 is ‘ready meal’ so to speak.
Many people on combo split the T3 dose prescribed within the day because it is so potent.
Consistency is important with T3 same dose daily as a minimum from what I gave read on here and as the half life is so short I think alternating doses would definitely have an effect.
Are you grappling with doses at the moment? Tell us a bit more buddy 🤗
So here is what I think I know: When T3 is taken there is a "spike" within a couple of hours and then it decreases. That's why some split the doses to have these little spikes for sustained energy throughout the day. Where does the T3 go? I read it is "absorbed" (95% within 4hrs). What does that mean? Does it all attach to the T3 receptors and is therefore not free anymore? Or does the body break it down? Pee it out? Turn it into RT3? Or does "absorbed" mean it is now in the bloodstream? I don't understand what exactly goes on with the T3 in the body.
Many medical websites say that taking 1 dose once a day is the way to go. Is it then assumed or proven that, despite the fairly immediate action of the T3, there is still enough to have sustained benefit for the rest of the day? If so, what is the benefit of compounded slow release? Izabella Wentz, for example says that it does not work for many of her patients. It sure did not work for me. The reason I was given is, that my gut did not absorb it (here is that word again). My thinking is, that what is not absorbed goes out and what is absorbed attaches to receptors. Any thoughts on that?
When I take 1x 5mcg my FT3 after 10 hrs is 68% through the interval. The benefit is minimal. When I take 2x5mcg with a time span of 6 hrs in between doses I feel great but overshoot the upper range, not by much but still. Either way the TSH is below range which does not concern me after what I've learned here; more below range with option 2, though. It is still above 0.03 either way. Now, because of what I learned about the nature of T3, I assume that the longer before a blood test I take the T3 the lower the FT3 result would be. I also assume that TSH would not be influenced as immediately as the FT3. Now the half life of T3 is 2.5 days (give or take). Does that mean that, if there is a continued supply of T3 (daily) it builds up in the body? After all only half of it would be gone after 2.5 days if not replenished. But what happens if it is replenished daily?
Somehow I am not able to fully comprehend what exactly happens with the T3 the moment it goes into my body and what the processes are after several doses. Therefore, in my mind, if FT3 builds up in the body, doses could be alternated with a slower build up than sustained higher dose, especially if split to avoid a higher spike. And that's where I feel that my thinking is taking a wrong turn somewhere, but can not figure out where it is going awry.
I've tried to gain more understanding by reading up at thyroidpatientcanada, but that is just blowing my brain to bits. I find some research papers easier to comprehend. And maybe this whole thing is far too complicated for a simple straight forward explanation. Maybe one of you lovely people wants to try it anyway? Thanks to all (even if I never manage to wrap my head around it 
)
OK buddy .. i'll have a bash at explaining what (i think ) i know so far
The spike you see 'in the blood' after taking a dose fT3 does not necessarily equate to what is happening 'inside the cells that use it'
It doesn't 'do' anything in the blood , it just goes round ... it is only 'active' once it gets inside a cell and fits onto the T3 receptor inside that cell. then that cell 'does' whatever it does , depending on what sort of cell it is ,... heart/ muscle etc etc etc .
just because 'the blood level of T3 goes up' .. that is not the same as 'the action of T3 in the cells going up'... the times of these happening will be different.
when you see the 'half life of T3' being mentioned ,, (i think) it means "how long it lasts in the blood" .... the action of T3 'once it's inside the cells' is what is said to last 'up to 3 days' on that individual cell receptor.
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"Does it all attach to the T3 receptors and is therefore not free anymore? Or does the body break it down? Pee it out? Turn it into RT3? Or does "absorbed" mean it is now in the bloodstream? "
Free T3 is 'free' while it is in the blood, as in "it is not bound" to the thing's that transport it round in blood (~ ? thyroid binding globlulins, and some other things )
T3 cannot be turned into Reverse T3... you can only make reverse T3 out of T4.
T3 can be turned into T2.. (there is more than one type of T2)
T2 then gets turned into T1 . then T0
T4 is 4 iodine atoms , T3 is 3 ,T2 is 2 ,T1 is 1, the de-iodinases are what remove these atoms of iodine, and some of that removed iodine gets recycled by the body.
The word 'Absorbed' can be used to mean 'absorbed from the gut into the bloodstream' . or it can be used to mean 'taken into the cells and fitted onto the T3 receptors'.. so you always need to look at the context of how the word has been used.
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I assume that the longer before a blood test I take the T3 the lower the FT3 result would be.
I also assume that TSH would not be influenced as immediately as the FT3.
Both Correct.
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Now the half life of T3 is 2.5 days (give or take). Does that mean that, if there is a continued supply of T3 (daily) it builds up in the body? After all only half of it would be gone after 2.5 days if not replenished.
The half life is about 1-1.5 days in the blood
The action is said to last up to 3 days in the cells
They are Different things
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Corrections welcome if anybody has any
Thank you so much, tattybogle. That was very comprehensive. I will let my brain chew on that for a while (and then might come back with more confusion 
)
This is very useful, thank you for asking the questions my poor befuddled brain can’t quite begin to formulate yet! I’ve followed this thread with interest and tattybogle ’s explanation really helped me get my head around some of the absorption and ‘free’ and conversion issues. Very interesting. 🌱
Yes, tatty did a great job. My brain could totally grasp what she was saying. I still have to put all the ducks in a row (even if the row might be a bit crooked ). But it took me a while to figure all the other stuff out, why should this be any faster?
it might help you feel better about how long it takes to get your head round this stuff if i tell you it took me about 8 months before i realised i was saying deiodinase wrong , (let alone remember what they actually do) .
i was calling them dioedinase for ages until the penny dropped that it is:
de _iodine _ ase
as in
"to remove some iodine "
Only 8 months?!!!! I don't want to go into a competition here, but for some stuff it took me decades to figure it out (like "doctor does NOT always know best" ). And who knows where else I'm missing the boat.
Yes just finished the thyroid.ca paper on this. I feel more informed by it and your (Tattybogle) explanation. I will have to reread both some more. I feel apprehensive after reading what has NOT been researched about those of us on T4 therapy only and still no real explanation why my body won’t respond well to T3. So much for everything being known about thyroid workings. Literally millions of us being treated by people who don’t know what the hormones are doing and them just praying most of their patients keep silent on their treatment outcomes. Surely it has to be better than this?
So much for everything being known about thyroid workings. Literally millions of us being treated by people who don’t know what the hormones are doing and them just praying most of their patients keep silent on their treatment outcomes. Surely it has to be better than this?
Bingo ... you've understood how endocrinology works .
A ) they know basically nothing because they have not been interested enough to be very observant
and b) we're all so different anyway that everything comes down to individual trial and error anyway .
Just a side note.
Quite a number of people distinctly dislike any form of alternating dose even for levothyroxine-only.
Indeed, there some who prefer split-dosing despite its long-ish half-life.
I guess it is the stony path of figuring out what works best (bleeding feet and all ). I've run into so many dead ends that I have asked myself whether I would be better off not seeking knowledge and just go with whatever any given doctor decides at any given time and suffer silently. But, of course, I have to be like a dog with a bone, trying to get to the bottom of everything. Sigh!
Test Results and Symptoms 
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