Blood test and T3/T4 timing advice

Hi everyone. Normally I would leave 24 hours between taking Levo and having a blood test. A few months ago however I started taking T3 also, and I have to split it half/half around 8am and again at 3pm, I would leave 12 hours before a blood test with T3. But at my last Endo visit I was prescribed a new brand (Prothyrid) which has both T3 and T4 combined, which has been great, but now I find myself not knowing how/when to take my dose before my next blood appointment which is next week.

I do still have some old T4 and T3 tabs that I could take separately, but the point is to see if this new brand is working for me and I don't want to do anything to skew my results.

I can take my blood any time from 8am to 12pm on the day of my test. Should I just take my meds as usual and do the test right at 8am? That would leave 17 hours from my last dose.

Any advice would be appreciated.



12 Replies

  • dang I have read differing views on this.

    As you know, it's 24 hours for Levo and 12 hours for T3. I take both, first thing in the morning in one dose. When I test, on the day before I take my morning Levo as usual and half my T3 dose. I take the second half of my T3 dose about 8pm. That way I've followed the recommendation.

    When taking a combo T4/T3 in any form, I've seen it said to leave just the 12 hours and not 24 as you would do normally for the T4 element.

    One member mentions that if you leave 24 hours then to add 20% onto your FT3 result to give a truer picture.

  • Hmmm... I'm not sure if I could do 12 hours, since that would mean taking my second dose at 8pm, normally if I forget to take my second dose right at 3pm I'll start falling asleep and getting cold, I've never taken it later than 5pm... Maybe one day this week I'll test doing this schedule and see if my body can handle it or not.

    I was more-so worried about not leaving 24 hours between t4 and bloods, I'm trying to get my FT4 down and don't want Levo in my system to show a false FT4 reading. Do you think that would be an issue?

  • dang Well, the difference between 17 and 24 hours isn't that much and to be honest as T4 has such a long half life I doubt it will make that much difference to your FT4 result. It's not going to be easy with a combo tablet, you can't satisfy both recommendations so you'll have to decide which is more important.

  • Ok thanks for your advice!

  • Can I ask how easy you found it to get Endo to prescribe something other than Levo? Do you live in U.K.? Thanks

  • I was sent to an Endo because T4 alone doesn't work for me, when my TSH is in range on Levo alone I have some bad side effects and feel uneasy, after a blood test they saw my FT4 is over range. They tried reducing my Levo but my TSH went up and my FT4 barely went down at all. So I started asking for alternative treatments and at this point my GP referred me to Endo. I went through 2 other Endos before I found the one I like (other ones just told me to stay on Levo and to ignore my FT4). This whole "battle" took about 4-5 months, and I was on the phone almost daily calling up every Endo in the city, asking other doctors I knew for a good recommendation etc, let's just say I found REALLY hard to find my Endo.

    I live in Germany.

  • DO NOT take your meds before a blood test. Blood tests only measure whats in the blood not whether its working!

  • Thanks, but I don't think you read my post before replying. My situation is a little more complicated. Normally you would leave 24 hours between Levo and bloods, and 12 hours between T3 and bloods. But if you read my post you'll see I'm taking a combined pill which contains both T4 AND T3 at the same time, meaning I can't separate them to take them 24 and 12 hours before bloods respectively. So my question wasn't IF if should take my meds before bloods, but rather WHEN and HOW to take them the day before my bloods.

    If you have recommendations towards that I would love to hear them.


  • Most of the peak caused by taking oral thyroid hormones fades by 12 hours. Indeed, yesterday I was reading that between about 12 and 24 hours there can then be a rise in thyroid hormones due to any remaining working thyroid pushing out what it can.

    I think I would just try to make a 12 hour gap, or as long as possible, and very much avoid the first eight hours.

  • Thanks. If I take my meds as usuall it would leave 17 hours from the last dose of t4/t3 so it seems like enough time for the T4 to not peak (I assume).

    Do you know why people recommend 12 hours between t3 and bloods? (I'm still new to taking T3). Would there be an issue with me waiting more than 12 hours? Like it would falsely show my FT3 is even lower than usual?

    Am I overthinking this? I have a feeling I am...

  • You are NOT over-thinking this.

    The root problem is that doctors do not know how to interpret blood levels in the context of treatment. Even if they understood the pharmacokinetics in general, it would be difficult-to-impossible to make appropriate allowances/adjustments.

    If you wait longer, yes, you could see a lowered FT3. I'd rather see a low FT3 test result than a high one. As soon as FT3 is high, doctors scrabble to reduce dosing. Even if your FT3 were only a little high for five minutes a day! Because they would not realise that. If FT3 is low, few doctors feel they have to push a dose increase - more likely they would ignore it, or make a suggestion of a small increase.

    Longer term, whatever you decide to do, make sure you at least note what you do. Then you will be in a position to consider what effects timings have had.

    I think I would take the 08:00 tablet straight after blood draw.

  • Luckily my Endo is open to raising my T3 (even told me I could do it on my own depending on how I felt), he considers how I feel more important than my results. He also told me to take my medication whenever I want and just let him know if I did/didn't take it before bloods so that he could interpret it.

    I'm going to do as you say and take it after my blood draw at 8. Thank you so much for all the info, I really needed another opinion.

You may also like...