To T4 or not to T4 that is the question? - Thyroid UK

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To T4 or not to T4 that is the question?

Shazy-B profile image

So you lovely people I am having my T3 tested next week and was wondering should I take my Levothyroxine before I have it done...bearing in mind that I have started taking it before I go to bed (so my HRT doesn't interfere with it)

:) thanks a squillion in advance

15 Replies

I would err on the side of caution and not take the one the evening before.

I take it that they are actually testing FT4, TSH AND FT3?

It is better not to have any of the tests done in isolation as you can't very well put an FT3 test together with a FT4 test that was done a month ago... does that make sense..?



Yes Louise that makes perfect sense but I'm not sure if the doctor has requested TSH & T4 again until I collect the blood forms; my last one was only done 31/1/14 TSH 11.2 (0.30-6.00) T4 15.8 (10.00-22.00) it's been a battle to get the lab to agree to test my T3. Initially they declined but my GP called them and they said they would do it using the sample that was taken 31/1/14 when I phoned for the results yesterday they'd said the amount was insufficient to test :(

Have a look at this extract for a well known thryoid site.

Serum T4 concentrations peak 2 to 4 hours after an oral dose and remain above normal for approximately 6 hours in patients receiving daily replacement therapy.

So in my book, and this is what I have always done, if you are night time dosing there is no reason to stop taking it the night before a test - I could be wrong but it has never caused me any problems taking my nightime dose and then having a test early in the morning as leaving a 10 hour gap is well past the 6 hour range.

This is the full article.

Moggie x

helvella profile image
helvellaAdministrator in reply to Moggie

I think it is a difficult call - I also take mine at bed-time (23:00 or so) but do defer my dose until after my blood draw at about 08:00. (I then take it as I leave the phlebotomy department!)


Moggie profile image
Moggie in reply to helvella

I agree Rod that it is a grey area but I wouldn't be happy with stopping my thyroid meds for 24hrs - maybe a compromise for this lady and others is taking it just that little bit earlier the night before testing making sure you leave 10+ hours between taking a nightime dose and testing.

Moggie x

Moggie profile image
Moggie in reply to helvella

Unless there is evidence to say differently - and if anyone has evidence it is likely to be you Please share if you have.

Moggie x

helvella profile image
helvellaAdministrator in reply to Moggie

I absolutely understand - and no, have no decent evidence. In my case, the gap between one dose and the next ends up being about 33 hours instead of the usual 24 and I cope fine with that. But if I were under-dosed, maybe it would have more impact?

Your suggestion about taking the dose earlier the evening before possibly does make sense. :-)

I found myself having to have a private test and and NHS test within 24 hours of each other so I took my t3 as usual the day before but left out the evening t4 and had the private test at 4pm. (Result: everything within range.) I was on the floor by the the evening so took a single dose (12.5mcg) of t3 around 5 and had my NHS test in the morning around 10. Result: t3 too high and everyone in a kerfuffle.

I'm increasingly convinced that the t3 test is useless when you're on t3.

T3 is a totally different kettle of fish anf does need to be stopped a day or so before testing if you want your levels to please the GP - yes you will feel awful but, as you have found out, if causes mayhem if you dont.

Not to sure why you think that the T3 test is useless unless you are on T3, surely if you want to know if you are converting T4 to T3 then this test is a good indicator.

Hope you managed to smooth it over though.

Moggie x

HI Moggie. No - not useless UNLESS you are on t3, useless IF you are on t3 (caps only for clarity/emphasis, not shouting :-) ). I think I got the wrong end of the stick here, mixed up the facts of 1) taking t4 and 2) testing t3 and thought this question was about taking t3 as well as t4 or something. Duh, sorry. :-)

I'm kind of on my own with my t3 so I haven't sorted it out as such. Was seeing a private doc and very occasionally an NHS endo, neither of which really suits. Need to find a good doc really.

I did as you are normal so sensible - maybe a bad brain fog day!!!!!

Cant help out with the good doc problem sorry.

Moggie x

Yes - for both of us! xx

I'm puzzled when to take levothyroxine when having blood test....I take mine at 7.0 am.... every day....if my blood test is around 10/11am...and I haven't taken meds won't it give a false reading and suggest low result???isn't it better to take as normal at least it will read what you are on a normal day....

That's exactly what I thought Lucy12......I'm myself and don't understand the logic behind not taking it before hand :(

kosmickaz profile image
kosmickaz in reply to Shazy-B

Hi Bellaboo,

You are effectively dumping the T4 in the bloodstream, so could give a false high reading. T4 has a long half life so even though its taken late on the day of the draw it won't affect your general situation . But it could make it look like you need to reduce when you don't need to or may in fact need more. When Dr's generally are blood test obsessed and don't look at the clinical picture - one can't take any chances!! ;-)

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