Saw a comment on an online article a while back (unfortunately can't remember where) that suggested if you are taking T3 your body doesn't tend to store T4 as much as it's job is being done. They were suggesting this in the case that you were taking T4 and T3.
Sounds a bit strange to me but it would be interesting to hear what the more knowledgeable posters here think! 
It's true, TheHoggle. T4 is a storage hormone which converts to T3 and rT3. If you are taking T3 direct via T3 or NDT you bypass the need for high storage levels for conversion, plus direct T3 may improve the conversion of T4, further lowering FT4.
If you look at the FT4 results of members on T3 only you'll see their FT4 is usually below range. Members on T4+T3 tend to have lower FT4 and higher FT3 than those on Levothyroxine only.
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I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.
Hi Clutter,
Thanks for the info. What happens to the FT4 that is not then stored. Is it's half life then shortened? I would imagine some is kept for RT3 as you state.
TheHoggle, the half life of Levothyroxine is 7-8 days. Stored T4 would soon be used up wthout replacement from Levothyroxine or NDT.
____________________________________________________________________
I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.
I'm on shaky ground here, so take this with a pinch of salt. I'm not medically trained.
I think that if people take enough T3 to satisfy their body's need for thyroid hormone, then making T4 is unnecessary and would make people feel ill.
The T4 (if it was made by the body) would either be made into more T3, which could make people feel overdosed, or it would be made into reverse T3 which would block the effect of any T3, so that would make people feel underdosed.
I take T3 only, and I feel better on this than on levo or NDT. I have wondered what would happen if I had some severe illness or had a serious accident and didn't have any T3 for a few days. I assume it would take my thyroid a while to start producing TSH and T4 again. I've been considering whether to get a medic alert bracelet or pendant.
I have two choices. Stick to T3 which makes me feel better than any other kind of thyroid hormone, and just hope I never get into a situation where I won't be getting my T3. Or spend my life feeling awful on Levo or NDT, just so that I have some T4 available in the event of something going seriously wrong with my health.
I'd get a medic alert bracelet - either that or a tattoo on your chest that says 'I'm taking T3' - I'd go for the bracelet though, it's the more sensible option. I'm on steroids and I was given a little blue card with the week that I'm on and how much I'm taking. I update it every week and carry it in my purse - I have to say I do wonder what would happen if I was in an accident and didn't happen to have my purse with me 😉
Hi humanbean.I am also on T3 only.Its about 10 weeks since I had T4 in my system.Not only do I feel better,many symptoms are subsiding:-weight & bloat in reverse.When I was on the combo for a bit,I thought I would lose weight & bloat,but it kept increasing.Hair shedding is more or less normal now-wonder if I will ever get thicker hair again,though.Lump in my throat on swallowing has almost gone(had that for years,improved in weeks)Solid pads on the front of my ankles have gone.Still much fatigue,but not of the overwhelming crushing kind & am doing more.
I am replying to you because I have been on"Tired Thyroid"and she is very scaremongering about the dire consequencies of T3 only replacement-dementia,muscle wasting etc.
Any thoughts?
I've been on T3 only since late January or early February this year, so not that long really. I know some people have been on it far longer than this. I know that some people say they have dementia-like symptoms when they don't get any T4 (I think the Tired Thyroid author is one of those) and I have no reason to doubt them, but it hasn't happened to me. I hadn't read about muscle wasting before but I haven't suffered it so far.
Personally, I think the differing experiences of T3 only, good and bad, must have something to do with the proportions of the different deiodinases in the brain and body. But my science knowledge is too rudimentary to really understand it thoroughly :
medscape.com/viewarticle/72...
I would love to be someone who does well on T4 or NDT, but I can't make it happen just by wishing.
One thing I do know... Before I went on T3 earlier this year I had had several years of chest pain for which no cause could be found by the doctors. I discovered that part of the problem was very low iron, but low T3 was a big contributor too. I spent a couple of months on T3 late in 2013, where my pain levels reduced for a while. But over the last few years I have called ambulances or taken myself to my GP surgery or A&E about 7 or 8 times. I haven't needed to go to hospital or use A&E at all this year and my chest pain has become a thing of the past.
Thank you for that link,humanbean.
I have only been off T4 since early August,so about 10 weeks & I feel as if brain-fog is lifting further,so I must be one of those who do better on T3 only.I do have muscle fatigue in upper arms(becomes very noticable in my gentle yoga class)Worst is hip & thigh weakness,but that was getting worse & worse on T4.
The scariest part about starting T3 was all the dire warnings about heart issues.On introducing T3 in 5mcg doses 3 times a day,I ended up with one of my supraventricular tachycardia episodes(been having them since early 20s)After 5 hours at double speed,went to A&E,kept in O/N.
Waited 3 months,tried introducing T3 again but had been slowly reducing T4.Increased T3 extremely slowly.Decided to reduce T4 to zero to follow P.Robinson's protocol for adrenal healing.
Now,I would say my heart symptoms & anxiety symptoms are very much improved,also mood(very linked to chronic anxiety with me)
My memory & concentration are extremely poor,but this has been happening over a very long period of doing badly on T4,I can't blame T3
If you want something serious on the subject of low T3 and the heart (note that it is written specifically in connection with people with heart disease, but still interesting) :
circ.ahajournals.org/conten...
I've had tachycardia and had some brief stays in hospital because of it. It doesn't happen any more. I think both iron and T3 were needed to make that go away for me. I hated the beta blockers I was given and took them only when needed. If I took them all the time I got so weak and tired I couldn't function at all. ( I take half a beta blocker about once a month now.)
Somebody on the forum wrote a post recently asking if others had "heavy arms". I did, but don't any more. Sounds like you might have them too.
I did have to raise my T3 levels very, very, very slowly. I increased by 1/4 tablet at a time, and sometimes I dropped back again for a few weeks if it didn't go well. I think this may have been because of adrenal problems rather than anything else.
I'm borderline ha ha.
The True Cost of Thyroid Blood Tests
Does my neck look suspicious? 
Poor memory and I'm only 47! Is this normal?
A little bit of advice
