Confused about FT3 result because of different lab range - over range?

Hi all,

I'm still not feeling great and have continued to raise dosage of T4 and T3 according to my endo's instructions.

Currently taking 88.5 mcgs levo and 20 mcgs T3.

I had my bloods done at a different place and the lab range is different so I'm confused about the result because the FT3 is showing high. Yes, I did leave about 18hrs before having bloods done.i find it very hard to believe that I'm HYPER because I feel VERY hypo still. I have NO hyper symptoms. I was on 2 grains of ERFA a few years back so I CANT be over Medicated?!

TSH. 0.02. ( 0.40 - 5.00)

FT4. 13.4. (9.0 - 19.0)

FT3. 5.76. (2.60 - 5.70). *high

I usually have them done using different lab ranges where the FT3 range is ( 3.1 - 6.8 )

So now I'm wondering if I am over range?

Or would my FT3 of 5.76 STILL BE 5.76 on the 3.1-6.8 range which is the range my endo uses?

Sorry really confused about this..... Sorry if it's a stupid question and why can't they just standardise these ranges!!!

Thanks everyone.x

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32 Replies

  • Joesmum, FT3 5.76 is mildly over range but having missed doses for 18 hours prior to testing your normal circulating FT3 could be +15% higher ie 6.6. People often don't feel hyper when results are over range. I felt right as rain with FT3 8.4 24 hours after last dose. I really wouldn't worry about being so slightly over as you don't feel hyper.

    Labs use different machines to analyse results. Ranges are based on statistical population averages which is why they vary from lab to lab. Results must be interpreted within the specific range used. If you'd tested at your endo's lab the result would be different but it should be over 6.8.

  • And it is not wise to go on the blood tests alone. It is how you feel and what signs and symptoms you have. People who are on t3only treatment cannot rely on ft3 blood tests to guide their dosage as they are notorious for being variable as we use it up so much more quickly in our bodies. Paul in his book about medication on T3 only talks about this. He found if his doctor medicated on this blood test result his medication was all over the place and he felt unwell.

    So I agree with Clutter I would not read too much into the slightly above range FT3 blood test result.

    Out of interest Clutter can you say more about the 15% you curious and hadn't heard of this before. Thanks. Am always learning on this site :)

  • Waveylines,

    T3 has a half-life of about 1 - 2 days which would make exponential decay analysis applicable. Half-life is the time for decay to reduce a level by half.

    For example, if the half-life is a day, 1 day after stopping or missing a dose, the amount is reduced by half.

    However, this would only be applicable on the initial dose as T3 has a cumulative effect so previous doses need to be considered in the total left circulating in the blood.


  • Flower,

    I don't think that exponential decay analysis is applicable to biological systems. The rate at which your body uses up T3 is not solely dependent on the amount present.


    I am not surprised you are confused.

    Let me throw in another element of confusion.

    If you take a levothyroxine tablet, you absorb that and your body is able to convert some of it to T3. If you continually measured T3 levels in the blood, you would see that the peak from one levothyroxine tablet usually occurs around 48 hours after taking it.

    If you take levothyroxine at 08:00 every morning and have your blood test as early as possible, then your T3 level could well be showing a peak from the levothyroxine you took two days earlier! This will only be a low-ish peak but might be enough to take you slightly over-range.

  • Yes Rod,

    I agree "exponential decay" is too scientific an answer.....(..sorry waveylines & joesmum..)..

    However, I don't think it incorrect as it describes the fall to one half of its initial value...half life.

    I also agree it doesn't "totally" depict the rate at which your body uses up T3 but as previously discussed with you before...we need a baseline to work from...otherwise we'd be taking random guesses.

    I explained in my previous answer that length of time medicated on T3 should be considered and you rightly pointed out, if T4 was being taken as well.


  • Thanks flowerpot .... am capable of absorbing scientific information if explained. However am aware of the dropping by half etc... But not the 15% described. Am also aware that the shelf life of T3is much shorter in the body then T4. Just not aware of a very accurate measurement of this drop in the individuals body.

    Am also aware that some people require more t3 than others due to not only thyroid dysfunction but ability by the body it is presumed to absorb into cells which is not testable. This has nothing to do with blood levels.

    Paul Robinson & his doctor have never found the Ft3 a useful way of managing his needs as t3 fluctuates far more widely then T4 in the body. Am therefore always slightly bemused by this test as a mechanism to optimising dose.

  • Hello wavylines,

    I don't know about the 15% decrease.

    I have the Paul Robinson book too and he gives very clear explainations regarding the fluctuation and conversion of T3.

    I like my new


  • What was your name before? Flower 007 is a great name!! x

  • called me Flowerpot

    Which I quite


  • Whoops -sorry -the wonders of predictive text on!! I like Flower 007 -sounds like the nature world of 007!! 😊😊

  • Made me laugh so that has to be a good thing! Keep meaning to ring you waveylines but things hectic at the moment! May calm down a bit in a few days.

  • Do you remember.Bill and Ben. The flower pot men. There was a flower in that.

  • Waveylines, I was told I could estimate my normal circulating FT3 to be +20% of the result if I left 24 hours between last dose and blood draw so I extrapolated it to +15% after 18 hours. I have no evidence to back it up so it could be true or complete fiction.

  • Thanks Clutter. Appreciate the feedback. i was just curious 😊

  • Hi clutter - I had an unexpected blood test 2 hours after taking my 2.25 grains of NDT. My fT3 was massively over range at 14 or 15 (3.1-6.8). I explained the situation to the doc who ordered a repeat test. That time I left a 24 hour gap between my dose and the blood test. FT3 was back to my normal level of 5. So not sure about the 20% figure :D

  • Clarebear, If FT3 is 5 after 24 hours it's not your normal circulating level. The +20% is to estimate normal circulating level, but as I've said, I can't cite evidence for it.

    That's a helluva spike after 2 hours. Do you *feel* it?

  • I know I was rather shocked! No I don't feel any spike at all (I take all of my dose in one go). When I was on a lower dose I used to feel the T3 wear off but I feel fine now on this dose :)

  • Thank you all,

    This all makes very good sense but goodness is it frustrating. Thank god I have an endo who doesn't seem to care about blood results!

    I do, however have a GP who believes that Moses carted them round as the 11th commandment! 😩

  • Well, thank you joesmum, your last comment has given me the first good laugh of the day :-)

  • LOL.

    This explains pooling of T3 and perhaps may explain.

  • Ha ha Joesmum -spot on!!! Thank goodness for the endo. Your GP needs to discover that your endo is Jesus reincarnated so he can ditch Moses!!😊😊😊

  • were both blood test taken at the same time of day...consistency in that is important too....if they were I agree this is a little odd....and wonder if you don't need more t4 and less t3 hence you may be a excellent converter- converting the t4 to t3- so you don't need as much t3...but I wouldn't make drastic change because you are just at the top end....I do know with blood test it is good to always take at the same time of day with the same protocol as the last test...fasting or not, they are comparable.

  • On the subject of the half life of T3 I have read numbers between 4 and 24 hours. I believe it is on the shorter end of that range. For myself I take my NDT (which has T3) at 8:00am and then I "crash" about 3:00pm i.e. 7 hours to drop below my optimal level of T3. This doesn't mean that this is the half life of T3, it means that it's level has dropped beyond the critical point that I need to operate on. This suboptimal level could be in advance of the T3 half life, or it could be beyond it.

  • Don't quote me on this but I thought T3 had a shelf life in the body of around 5-7hours. One of the reasons why levels fluctuate so much.

    Leah's am like you I get a slump in the afternoon which is why I split my NDT into three doses per day.

  • Wavey, T3 will be out of the blood about 6 hours after last dose and out of the system in 3-3.5 days after last dose.

  • Which is more relevant Clutter-in the blood or in the system? Not sure what I'm trying to say here but it's difficult to get a regulated day so I suppose I'm asking if it's still in the system does that matter?

  • Silverfox, What you feel in the blood is the peaks after taking a dose. it's only relevant once it gets into the cells which takes up to 36-48 hours.

  • Thank you!

  • Clutter -How do they evaluate it being out of the system?

    I certainly find I get a slump everyday if I don't take a top up -surely if t3 lasted that long I wouldn't. Paul Robinson on T3 only also splits his dose to avoid peaks & troughs. There are some who take their t3 in one dose but many don't.

  • Waveylines, I don't know whether what's in the cells can be evaluated. The dose is working for up to 3 days but will wear off unless topped up. What you are feeling is a slump as the T3 stops peaking in your serum. The peaks and troughs happen whether you feel them or not. Multi-dosing evens out the peaks.

  • Yes -absolutely Clutter -that is my point as this happens hours after taking my dose not days which is why my dose is split into three. And it also why when on a combination or t3 only therapy testing ft3 is so unreliable. Whereas t4 levels change far more slowly & therefore can be more reliably measured.

    I have found these peaks and troughs are far less on a NDT then T3/Levothyroxine. So something about T4/T3 conversions must vary for me between the two formats. Nevertheless I do believe the main effects of t3 is hours not days.

    There is no mechanism to test uptake in cells -it is theory Shame there isn't a test!

  • Your TSH is a bit too low. On combination T3+T4 therapy, I find I do not do well when TSH drifts below 0.1. To help people give you advice, you might post the units used for FT3 and FT4; the units used in UK and USA tend to be different.

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