How many hours do we need to fast before blood ... - Thyroid UK

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How many hours do we need to fast before blood tests.

lucylocks profile image


I have repeat blood tests on Thursday morning as I had weird results a few weeks ago.

Do we have to fast for 12 hours before tests or is 8 hours enough.

It is that I like a small protein snack before bed to help with blood sugar levels so find the 12 hours difficult

Thank you.

10 Replies
SeasideSusie profile image

You should be fine with that, it's just overnight and don't eat breakfast before your test, water only.

Many thanks SeasideSusie.

I've never been told to fast before thyroid tests. Is it usual to have to fast? Wonder if this could be why both my son & I don't seem to benefit fully from thyroxine

DawnD profile image
DawnD in reply to Lindamt

I have also never been told to fast before any blood tests, whether they include TSH and T4, or just full blood count, CRP, iron, etc. So I don't!

Lindamt profile image
Lindamt in reply to DawnD

I've got my blood test tomorrow but don't think I'll be bothering to fast either then. Thanks DawnD.

lucylocks profile image
lucylocks in reply to Lindamt


Doctors don't tell you to fast, but the knowledgable members on here really do advise it. It can make a big difference to your thyroid results.

If you fast you will get a more accurate result. It is always advisable NOT to eat breakfast and DO NOT take medication before the test, take after you have had it done.

Leave 24 hours from last does of levo. before test and if on NDT like I am, I take my afternoon dose the day before but do not take my next dose until after bloods have been taken. This means I do not take NDT around 17 hours before my blood test, although I think 12 hours is O.K.

Thanks for that. I had read about fasting before a test and asked the doctor, who said there was no need. I think I will, just to be on the safe side as my first full function thyroid blood test is this week!

lucylocks profile image
lucylocks in reply to DawnD

Hi DawnD,

please see my reply to Lindamt.

I've never fasted just for thyroid tests but I had a whole range of blood tests yesterday at my local Hospital (full thyroid, cortisol, gluten intolerance, not sure what else!!) at 9am and the nurse double checked that I hadn't had anything since midnight. So 8-9 hours I think should be fine. Good luck with your tests.

Many thanks,

yes it is always best to fast.

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