Anyone know about the process of t4 and t3?? Simple terms only lol

Just wondered if anyone could explain to me the process from u actually taking the levo to it converting to t3?

And what does the total t4 and total t3 mean and also ft4 and ft3 mean?

And when i stopped levo for 3 weeks (long story) the ft4 dropped to 5.7 range (11-23) then after 6 weeks of 50mcg went up to 14.6, when it dropped was that just because i wasnt taking it? And that it had already converted to t3...

Really want to understand if im converting or not as been on levo 9 years and my tsh and t4 have only just gone mad!

Thank you everyone i really appreciate everyones knowledge x


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21 Replies

  • ThyroidObsessed,

    Simply put T4 is a storage hormone which is converted in the thyroid, kidneys and liver into T3 which is the active thyroid hormone.

    Total T4 and Total T3 measure the T4 and T3 bound to proteins. Free T4 (FT4) and Free T3 (FT4) measure the T4 and T3 unbound to proteins. Total T4 and T3 have largely been superseded by FT4 and FT3 in the UK.

    Your FT4 dropped because you stopped taking Levothyroxine (T4) and your thyroid gland was unable to produce sufficient natural T4. TSH will have risen to try and stimulate your thyroid gland to produce T4. If you have insufficient T4 T3 may remain stable for a while but will eventually drop without sufficient T4 to convert from.

    If you want to know whether you are a good or poor converter you will need TSH, FT4 and FT3 tested at the same time. Low TSH with high (in range) FT4 and low FT3 indicates poor conversion.

    Patients taking Liothyronine (T3) without Levothyroxine will have little FT4 but should have good levels of FT3 if they are optimally dosed.

  • The Total tests measure the Free hormone and the bound hormone, not just the bound. :)

  • Greygoose,

    Total is bound to proteins and free is unbound.

  • No, Total is both bound and unbound. That's why it's the total, the total amount of T3 in the blood. This has been discussed before. :)

  • Hi clutter thanks for the quick reply always there when i need help ☺...

    So at the moment after stopping levo for 3 weeks and then going onto actavis 50mcg (instead of the lethal teva) for 6 weeks my ft4 increased from 5.7 (11-23) to 14.6 but my tsh still remains over 100! Endo has told me to increase to 75mcg for 6 weeks.. so with ft4 rising and the tsh being high does that mean im converting?

    Baring in mind i have been on levo for 9 years and never had hypo symptoms only started with hypo symptoms when i stopped the levo...

  • ThyroidObsessed,

    You won't be able to tell how well you are converting until TSH is normal, around 1.0, and you have an FT4 and FT3 result. FT4 14.6 is a rapid improvement and I'm surprised your TSH has remained so high. Usually it is the other way around with TSH improving quickly and FT4 lagging. It will be interesting to see how your TSH has improved on 75mcg.

  • Well i have too tried to work this out, and think the only thing that could explain it is because the lab that test the tsh doesnt measure the tsh past 100 and that maybe when it was over 100, 6 weeks ago that it may have been 190 180 170 and so on and that it has come down but we cant see that because its still over 100 do u understand what i mean??

  • ThyroidObsessed,

    That is one possible explanation. Another is that you have heterophile antibodies to TSH and a third is if you are taking Biotin or B complex vitamins and the lab uses biotin in its testing that can skew the test and present false high TSH.

    When I resumed 100mcg after 4 weeks off Levothyroxine my TSH dropped from 107 to 22 in a couple of weeks but FT4 remained well below range. It took eight weeks for TSH to drop to 0.16 and FT4 to get to 14 or 15.

  • I dont take any vitamins at all, in fact i dont like taking any tablets at all i wont even take a paracetamol if i have a headache, i even came off the contraceptive pill in case that intefered with the levo!... like i said before iv never had hypo symptoms and always been ok on levo for 9 years it was only in october of last year when i started teva new formula that i started having bad anxiety and tsh shot up when i stopped taking them but anxiety calmed down and now it just lingers or more unstable moods should i say now.. just cant work it all out? Endo said if tsh doesnt start dropping after 6 weeks of 75mcg then i will have to go in hospital for a few hours and take a tablet that stops me needing levo for a week! to check if im converting or not.. do u know what kind of test this is, its freaking me out to be honest!..

  • ThyroidObsessed,

    I've heard of patients suspected of non-compliance (not taking Levothyroxine daily) being asked to take a week's worth of Levothyroxine in front of their doctor but not of a tablet which has to be taken in a hospital setting that would tell whether or not you are converting.

    If TSH isn't dropping after 6 weeks mention the heterophile antibodies and ask for the blood to be sent to a different lab for testing.

  • That's a very useful bit of information you've given us Clutter, about the Biotin, thanks

  • OMG!! Clutter this could explain my high tsh and and normal t4 last august!! As i was taking biotin drinks at the time!!?...

  • ThyroidObsessed,

    What about more recently? Usually stopping biotin a few days before the blood test is all that is required. If the analysis doesn't use biotin it won't be a problem anyway.

  • No i stopped the biotin drinks, the ft4 and tsh were both high in jan of this year.. endo says this can only happen when u stop levo and resume at a higher dose and the tsh and ft4 dont catch up..

    Though he said if my tsh doesnt start going down on the 75mcg in the next 6 weeks then it may be a conversion problem, though im skeptical about that...

  • Looking at all my previous thyroid test i have always had a tsh of just below 1 and a ft4 just slightly above range but never had ft3 tested before as never had hypo symptoms really so what would u say this indicates? ..

  • ThyroidObsessed,

    It indicates you were adequately medicated. Without FT3 done at the same time it is impossible to say whether or not you are a good or poor converter.

  • If a large part of the T4 is normally converted to T3 by the thyroid gland, and the latter is dysfunctional, can that affect the conversion process negatively? I mean, if the thyroid is unable to produce sufficient amounts of hormones to begin with, maybe it's also unable to convert enough T4 to T3...? Could that explain why so many thyroid sufferers seem to require additional T3 to rid themselves of all hypo symptoms? Or how much of the conversion process actually takes place in the thyroid, compared to the other organs? I admit I have never seen any figures about this.

  • Anna69,

    At least 20% of T3 is converted in the thyroid gland so a dead or missing thyroid will certainly affect the conversion level.

  • Interesting, thanks!

  • Dont worry about taking a large T4 dose. I know there are sone on here that take a weeks worth at a time and don't suffer side effects. It you think about it T4 is a storage hormone and so it is only called upon to convert some to T3 when your body needs it.

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