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Thyroid UK
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Test for T3

Hi, I'm due to have blood tests tomorrow & have a queery re T3. I've been taking 12.5mcg T3 together with 75mcg levothyroxine & am going to check my levels to see if I need a dosage adjustment.

My question is this: If T3 stays in your system for 6-8 hours & you take it twice daily, this means that your levels fluctuate. So if my last dose before I have the blood tests tomorrow morning is 3pm today, how can the test for free T3 levels be accurate?


5 Replies

T3 levels are not a good indication once you are on t3 of any sort. Neither is TSH. The levels of t3 fluctuate too much. Any results should be taken with symptoms in mind. If you have left several hours since your last dose, there will still be t3 in your blood. 6-8 hours is just the half life so t3 will be in the system for a long time after that. The t3 test may be useful as a guide for you to know at what levels you feel well but isn't great for determining correct dosage. You definitely don't want to test right after a dose of T3 as it would give a falsely high reading.

Having said all that, as you are also taking t4, the variation in t3 level shouldn't be as much as for someone on t3 only. You may find the result a useful guide for your own records but it shouldn't be used in isolation.

I hope I haven't just confused the issue!

Carolyn x


Thanks Carolyn that makes a bit more sense now : )


T4 is particularly high until 9 hours after ingestion, so don't take your thyroxine either before the blood test.

1 like

Thanks Muffy : )


Rebecks, if you test FT3 12 hours after your last dose you can add 20% to the reading you get to get an idea of your 'true' FT3 at the time of testing. T3 fluctuates throughout the day, lagging about 90 minutes after TSH. If you test shortly after a dose of T3 you will get a reading which measures the peak level of the T3 you have recently ingested, not the circulating FT3 available.


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