Thyroid UK
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Fasted blood test on T3?

I have a blood test later this morning and the advice I have been given on here (not by my Endo) is to have my bloods taken fasted. Which I have always done.

Should this apply even when taking a combination of T4 and T3? Given the T3 will have left my system, as it's fast acting, won't being fasted skew the reults to look like i am under medicated?

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I've got blood test today too, last took T3 & T4, yesterday morning, normally take T3 twice a day. Will take meds as soon as blood test done.

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Forgot to say, taking meds before a blood test can skew results.

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I last took T3 yesterday afternoon. If T3 is fast acting won't it have all left my system by the time i have my bloods taken later rhis morning? Therefore potential showing a low FT3 than I actually have on my current meds? This is the bit I don't get.

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Don’t take your meds before your test. Only after. 🙂

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I'm hoping my TSH might rise a bit, has been around 0.02 - 0.05, my GP will poop herself again, despite endo happy with it cause T3 in top part of range!

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Yep, my GP was obsessed with TSH too.. despite my Endo not being concerned... mine was 0.016... but a recent thyroid scan showed that it had atrophied significantly... so surely a low TSH is no issue given the Thyroid is effectively dead 😂

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I don't know, I always wondered this too. However recent blood tests have shown my T3 dropping, because it's cold, despite me continuing to fast before my blood tests.

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I'm sat in the Drs waiting room now and fasted so let's see... But it is confusing given T4 and T3 have such dramatically different half lifes.. fasting T4 makes sense... but T3 less so....

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The advice is to leave 24 hours between the last dose of levo and the blood test but you only need to leave 12 hours between the last dose of T3 and the test.

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Ahhh... thanks for letting me know Maggie0652.. is there a link where I can find this?

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Hi. Sorry, I don’t know how to put links in, but if you put ‘time between last dose of T3 and blood test’ in the search box at the top of your HealthUnlocked home page they’ll come up.

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Why didn't I think of that!! Thanks!

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