Blood test fasting or not?

I have been Hypothyroid from may 2013,currently on 100mcg of Levo and i have a blood test on Tuesday morning.Can someone please tell me the best way this test can be done,should i take my Levo as normal in the morning and have my breakfast or should i totally fast with no Levo and have it when i get home? i have not been told to fast but will this give a better and truer reading?

11 Replies

  • Levo AFTER blood test definitely.

    I have never fasted for a TFT but I'm surgically hypothyroid and as such my TSH is suppressed and doesn't really fluctuate.

    This article may be of interest

  • You should not take your meds before a blood test as it skews the results completely. Take it afterwards. If you have taken breakfast before going to the GP, you have to leave two hours between breakfast and meds..

    I read today that if you eat before a blood test, it can reduce your TSH which you may not want as some GP's reduce dose according to the TSH result.

  • Hehe! I have a fasting blood test, TFT, vitD, B12 & folate for GP who reduced Levo at my request and I've been a leetle bit concerned endonob will castigate him as it's no longer suppressed.

    So, after the fasting test I shall stop off for steak and chips then onto the hospital for endo's TFT & thyroglobulin. Such a pity one set of tests can't do all, but the systems don't speak to each other and GP can't get FT3 & FT4! Still, it'll be interesting to compare TSH taken hours apart, pre and post fasting.

    Must remember to take levo before bed tonight too. Reverse usual advice to suppress TSH.

  • Clutter, this was posted earlier today and, as you will see from the graph, the TSH varies over 24 hours, so no wonder Dr S, Dr P and Dr Lowe said the TSH should be used as a tool and not a diagnosis.

  • Heavens, yes, the variation is considerable. Thanks for this (I know Helvella originally posted it). I shall make sure endo bloods are geared for 1pm when TSH should be lowest.

  • Sorry to hijack but I've also read the opposite about fasting so I'm a little confused...

    This an extract from this page

    TSH decreases when fasting. [17]  Most patients do their lab tests in a fasting state, because other labs like glucose and cholesterol require it.  But this may result in an artificially low TSH that does not reflect true thyroid levels.  In fact, TSH has a circadian rhythm, with a peak around midnight (with much variability between individuals), and a low in the afternoon; fluctuations are normal.  The change in TSH from peak to trough is approximately 72%.  Free T3 levels also show a similar circadian rhythm (with a smaller amplitude) with a time lag of approximately 90 minutes behind the TSH curve.  The Free T4 curve did not follow the TSH curve at all. [18]  Labs drawn in the morning could be significantly different from labs drawn in the afternoon after lunch, with one TSH in "normal" range and the other in the hyperthyroid or hypothyroid range...

  • Thank you. Lunch after endo TFT then :(

  • Sorry clutter, didn't mean to put you off your plan. I just don't know which site to believe!!! There is much contradicting info out there.

    My first tsh was fasting but in late morning and it was lower than the one I had earlier in morning without fasting. So I'm left wondering if it was the earlier time that made the difference or it was the lack of fasting or both!?!?

  • Pah! You're not helping :-D I shall have the fasting tests later than originally planned, get to endos tests earlier than planned but near as poss for draw at 1pm then lunch on the way back. Sorted.

  • Thank you for the replies.

  • I do a fasting one but no meds at all. X

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