Can I take Levothyroxine night before Thyroid blood test or should I miss a dose?

[Milburn13]

I take Levothyroxine at night. Getting a Medichecks thyroid test at 9am. Can I take Levothyroxine the night before or can I miss a day?

I'm just starting to feel better after being unwell for several months, so I don't want to make things worse.

What do you advise?

Thanks 😊

[Jaydee1507 Administrator]

The recommended timing to show stable blood levels of free hormones is to leave 24 hours between your last Levo dose and the blood test.

This would mean skipping your usual Levo dose 2 night before test and taking it the next morning instead.

There is a pinned post with timings here: healthunlocked.com/thyroidu...

[Milburn13 in reply to Jaydee1507]

Thanks, that's so helpful.

[tattybogle]

Hi Milburn , the time of last dose only affect fT4 result , it doesn't affect TSH result .

So if GP is only testing TSH , then it doesn't matter anyway.

if fT4 is being tested then time of last dose will affect ft4 result

but as long as it's 12 hrs from last dose , the difference it makes to fT4 result is going to be extremely small. The biggest effect on fT4 result is seen during (approx) the first 6 hours after taking the tablet, when the fT4 level peaks quite sharply and then falls again ..... by the time you get to 12 hrs the ft4 level is more or less back to it's settled level . So in my opinion even though 24 hrs is recommended , 12 hrs is 'good enough'.

for more details of how the time of last dose affects fT4 result , see the graphs in my early replies to this post : healthunlocked.com/thyroidu... suggested-thyroid-hormone-test-timing-protocols

missing the tablet the night before and leaving a 36hr gap will give a 'false' low ft4 result.

So you could either bring the night before dose forward just a little so it's at least 12 hrs from tablet to time of test .

or you could do something rather more complicated and adjust doses for few days before hand to get a 24 hr gap (details in that post above) . Admin usually recommend this method , but personally i find it to complicated , and i think the level after 12 hrs is close enough.

No method is perfect :

eg .if you follow the more complex method of delaying doses for a few days to get a 24 hr gap, you actually end up testing with 'one dose less' in your system at the time of the test (which you then take immediately afterwards).

whereas if you keep doses more or less as normal and have a 12 hr gap until test , you get a very slightly higher fT4 result than you would with a 24hr gap .

personally i'd just stick to taking it the night before , and do all your test consistently this way as 12 hrs . it's a lot simpler to remember what you did , so it makes it less likely you'll accidentally mess it up , or accidentally do something different in future tests .

[Milburn13 in reply to tattybogle]

Thanks, that's amazing. 12 hours sounds good to me & will be more consistent with previous tests. I'm doing the test mainly to get my T3 and vit d levels as NHS won't do them.

[jimh111]

As long as there is a four or five hour interval it should have a very small effect on fT4. We store around 10 days worth of T4 in our blood.

[Milburn13 in reply to jimh111]

Thanks