When to do tests when on T3 only?

Hi, I am on only T3 for 3 years, self-medicating. I take 100 mcg. Recently I wanted to finally do some tests. When I took the test 4 or 5 hours after I took my morning dose of T3 my FT3 was way over the top, FT4 was below lower reference range and TSH 0.001. Few weeks later I stopped taking T3 for 24 hours and when I did test on FT3 it was within reference range. I wasn't repeating FT4 and FT3 since it would be too expensive. So which result is relevant now? Thank you.

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12 Replies

  • Stroganoff,

    You can extrapolate the result you had 24 hours after last dose by +20% to estimate normal circulating T3 levels.

    T3 can peak in the serum for up to six hours. I would leave 8-12 hours between last dose and blood draw in future.

  • If I add 20% then they would be over the range. I feel completely normal, no signs of hyper.

  • Stroganoff,

    If FT3 is over range you are biochemically over medicated whether or not you feel over medicated. I suggest you retest FT3 8-12 hours after last dose and if it is over range reduce dose.

  • Basically, neither. T3 peaks 2-6 hours after taking it, so on your first test you were measuring the T3 you'd just taken so it would be high. On the second test, leaving 24 hours after your dose, was too long. It's generally recommended to leave 12 hours after a T3 dose for a blood draw/fingerprick.

    TSH and FT4 will always be low when taking T3.

    Who are you using for your tests? If you are in the UK then Medichecks charge £29 for FT3 only and £39 for TSH, FT4 and FT3 fingerprick tests medichecks.com/find-a-test/... and Blue Horizon charge £29 and £49 for the same tests plus they give a donation to ThyroidUK bluehorizonmedicals.co.uk/P...

  • After 24 hours my FT3 was in the reference ranges but closer to the higher range. So I was not low with it at that point. I did my tests in a private lab, blood was drawn from my vein on both occasions, prices were about £30 or something for each but when I did test the second time I was not very keen of giving £60 or more to repeat TSH and FT4.

  • I just wanted to point out that Free T4 will always be low when taking T3 only, and could even get down to zero.

    You may have a thyroid still capable of producing T4, but if you give the body enough T3 it doesn't need to waste resources on producing T4.

    I'm on T3 only. My lowest Free T4 so far was 1.94 pmol/L (12 - 22).

  • I have a bit more of FT4 than you :) Anyway, now I am not sure what's the purpose of doing tests when on T3 only since basically they can't show us anything?

  • I think there is a point to testing Free T3 when taking T3 only. It's surprising how many people come along and say they feel absolutely fine with very high levels of Free T3 e.g. double the top of the range.

    I must admit though, I hardly ever test my thyroid any more.

  • May I ask what's your FT3?

  • On the day I got that low Free T4 my Free T3 was 4.1 pmol/L (3.1 - 6.8). I felt under-medicated at the time. I have problems taking enough thyroid meds (of any kind) because of high cortisol issues, which means I get symptoms like palpitations, fast heart-rate, BP which goes a bit too high for my liking, even when I'm clearly still under-medicated.

  • Thank you.

  • Humanbean, I have the same problem with T3. I can feel undermedicated even when my FT3 is right at the top of the range, but recently although my T3 level dropped a bit I was getting palpitations and an uncomfortably fast pulse, though my blood pressure was low. My FT4 was mid range. I am guessing here, but I recently caught a nasty virus and was very ill. I am only really getting over it, and wondered if the stress of it had affected my adrenals.

    I stopped my T3 for 24 hours and things settled down. Ive started back on my normal dose today. So far no adverse effects

    I am on 100mg levo and 25mg T3, and recent bloods suggest I am not pooling.

    What woukd be your take on this?


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