Do I need to fast before blood Tests?

Hi everyone,

I have managed to get one of my G.P's to test me for Thyroid antibodies, Vitamin B12, follate, and Ferritin. He is unable to request free T3 and T4 and Vitamin D3 as the lab will not allow it. I have made an appointment to see a Professor Bowen Jones in the Wirral as I have had enough of feeling this way. If anyone has seen him could they private message me? Reading other posts I noticed that it was recommended that bloods be taken after fasting. Is this the case, and if so Why? Thanks

5 Replies

  • Mumljh, You don't need to fast for antibody, B12 and folate tests, unless TSH is also being tested.

    An early, fasting test (water only) is recommended because TSH is highest early morning and drops after food and drink. It can be helpful in persuading a GP to increase dose, or to avoid a dose reduction.

    You can order private thyroid panels and vitD from Blue Horizon and Genova, or a vitD test from City Assays via

  • Thank you mumljh for asking question about fasting before TSH test! I have gotten conflicting info for years. And thank you Clutter for your answer. I will fast on my next TSH, T3, T4 test.

  • There have been a couple of updates recently with regard to blood tests. First bloods should be taken as early as possible, and fasting but you should drink a reasonable amount of water as that also makes it easier to draw bloods. You should also take your last dose of thyroid hormones approx 24 hours before the blood test, take them afterwards.

    These three things, early testing - fasting before it and taking hormones afterwards helps to keep the TSH at a higher level as the doctors are apt to sometimes adjust our doses of hormones to try to keep the TSH within a range. We may then develop more clinical symptoms. Most of us feel best when TSH is around 1 or lower.

  • Thank you Clutter and Shaws. I had my TSH levels taken after fasting so happy the 4.31mu/L is fairly accurate. Just wanted to check that I was doing everything I could to get the most accurate results.

  • Yes you should fast before the tests

    Your GP and the labs are totally and utterly wrong refusing to test free t4,free t3 and vit d3 its disgraceful

    No wonder so many people dont get diagnosed

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