Are blood tests any use with combination T4 and T3?

I have read somewhere that blood tests are useless when taking T3 but does this mean when using T3 only or does that also apply to a combination of T4 and T3?

If no, and tests are still a useful guidance to see if T3 needs to be increased and/or T4 reduced, can someone spare the time to point me in the direction of a reliable source that can correctly explain how to interpret TSH, T4, FT3 and FT4 when on a combination dose please?

I've tried using the search button but T3 is rather common and I'm struggling with other sources.

Thank you.

8 Replies

  • Hi No that is not correct, you need fairly frequent tests for TSH, T4 and Free T3. Normally you would expect and need the T4 to be in top third of range and FT3 near the top of range, to feel well. Both need to be considered together with ow you feel.If on T3 . Should have a FT3 test repeated, 4 to 6 weeks after going onto T3. Ask for a print out + ranges, put on here if it helps.The TSh is lowered, a bit, by being on T3 and if T4 too high on T3, often it needs to be lowered.

    Best wishes,


  • Thank you for your response Jackie - I would have replied before now but I didn't get an email saying anyone had responded to my question so I wrongly thought I was out of luck. Didn't get one for Clarebear either!

    I will have to spend a bit of time absorbing this so thanks very much indeed for taking the time to help xx

  • Hi That is fine. Often we seem to have glitches on the system!Wonders of the tecnical world!

    Let us know how you get on. It does take a long time to be sorted.

    Best wishes,


  • I think that there are at least 2 distinctive schools of thought. Some people think you can go purely by symptoms and others advocate the use of blood tests.

    I am on Armour Thyroid and have to say that I use a combination of both. There have been several occasions when I have felt that I could be over-medicated and I have had a blood test (TSH, FT3 and FT4), and on all occasions the results have shown that I have been under-medicated rather than over-medicated. This has then given me the confidence to increase, and put up with the initial horrible side effects that I get.

    However, I am still tweaking things, and much as I would like to have a blood test more frequently, I am now adjusting by symptoms. I had blood tests done when I was feeling pretty good earlier this month and these showed my TSH < 0.02, FT4 to be 16 (12-22) and FT3 to be 5.8 (3.1-6.8). I am glad to have these results as I now know that I feel pretty good at this level. However, since then things have changed and I was uncertain whether to increase or decrease slightly, so without a blood test it has been a case of trial and error. I firstly tried decreasing by 1/4 of a grain, but this made me feel worse, so I have tried a 1/4 increase and am feeling better. I do wish we could have some kind of finger prick test, like diabetics have, to get a quick and accurate reading.

    Sorry I haven't really answered your question and can't point you to a reliable source on how to dose/manage and T3 and T4 combo. Hope your sister is getting on OK? xxx

  • Hi Did you know sometimes hyper symptoms can be identical to hypo. Endo says this is so. I have found it too, ie I am very thin, hypo. I hope this helps. I firmly believe how you are with the bloods are essential. If like me, GP not allowed to do FT3, expensive too!

    Best wishes,


  • Usually, if you are on any form of T3, it is only the FT3 blood test that is of any use, but Most doctors will only do a TSH test and possibly FT4. As long as the FT3 levels are in range, then that is okay.

  • Thank you for taking the time to answer my question Muffy.

    Can I ask? Does this mean that it doesn't matter what the TSH and FT4 results are even if they are way out of range?

    If so, how do you measure the effect of the T4 dose and whether that should be decreased/increased

    The question is for my sister who is on 20 mcg Liothyronine and 175 mcg Levothyroxine. I think her T3 dose needs to be increased and perhaps reduce her T4 by an equivalent amount but I don't feel confident to do it without the support of test results because she is unable to judge or explain what her symptoms are accurately.

    Thanks again so much for your help

  • As you probably know, T3 is the active hormone, so it could well be that your sister needs to increase the T3 and reduce the T4 that she is taking. It really is a case of trial and error so to speak, but if the medication is titrated in very small amounts, then it should be much easier to find the correct dose for her.

    Is she still under the supervision of a doctor?

    As for the blood test results, because the T3 is the active hormone, it by-passes the TSH. Hence the body won't produce that as it has enough active hormone in the body by taking the T3. The FT4 results could be high and maybe she isn't utilising it, so perhaps what you suggest i.e. less T4 and more T3 could well be the answer to your sister's health problems.

    If you do get a chance, you need to speak to someone qualified, so ring or fax your doctor who has been treating her and ask for advice.

    I hope that helps a little.

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