Blood test protocol

Hi, I've been reading on here suggestions that we should starve and not take Levo before blood tests. I just want to clarify this. I have read elsewhere the following advice: "T4 has a five to nine day long half-life, which means that once you have become stable on it, it will take as many as 9 days for 50% of the dose to clear the body....Whether you take T4 right before your lab tests, or haven’t taken it for up to 48 hours, your TSH value should be the same. Thus, you will get an accurate representation of your TSH value whether or not you take your T4 medication before a lab test....Same goes for free T3 levels when you take a T4 medication. The free T3 level will also be relatively constant. Levels of free T4, however, will show a peak 2 hours after your T4 medication is given.

... Your thyroid labs may show that your T4 is falsely elevated, and this may result in your doctor lowering your medication, when in reality, your T4 levels may be within range for the rest of the day.....In most cases, taking a T4 medication the morning before your lab test will not be an issue, as most doctors adjust the dosages according to the TSH, which stays stable after T4 dosing. However, to get a reading of your T4 levels that is reflective of most of the day, you would want to postpone your medication until after the lab test."

I take my Levo usually between 5am & 6:30am. I can't get an early morning appointment because I'm busy doing the school run. This means the earliest appt I can get is 9:30/10am. If T4 peaks after 2 hours, then does that mean I'm safe to just take my dose as normal, as by the time I'm seen it's more like 4 - 5 hours later?

7 Replies

  • Mandshef,

    T4 can peak for up to 6 hours after taking a dose but the peak will be highest 2 hours after ingestion. Why not just take it after your 9.30am blood draw.

  • I guess I wasn't sure of the benefits (if the Dr is only going to look at TSH, then from what I can see it won't make much difference). Also, I suffer from low blood sugar, so I'm a bit reluctant to fast until an hour after I've taken the Levo, which could push a delay on eating anything 'til 10:30/11 (I go a bit rubbish and dizzy!!). My Dr hasn't given me any advice to fast or delay medication, but then she has also started me on 25mcg for 3 months, when I know according to this forum I should've been started on a slightly higher dose and be tested after 6 - 8 weeks! I find the lack of info from Drs, and conflicting info from the internet, a bit confusing! My next blood tests will be the first since starting treatment, so I want to make sure I get it right :)

  • Mandshef,

    Advice given to fast is because TSH drops after eating and drinking. If your GP is only testing TSH you can take Levothyroxine before the test but if the lab does test FT4 it may show high if you've taken Levothyroxine. It won't hurt to take Levothyroxine within a few minutes of food on one occasion or you could take it 2 hours after you've eaten.

  • Thanks for your replies everyone. So, my previous TSH results were taken after I'd had food, as I was unaware of these issues before diagnosis. If I start to fast before the test, having previously not done so, would that make my result look artificially high by comparison? Or should I just be trying to be consistent from here on in and not worry about past tests? Sorry for so many questions!!

  • Mandshef,

    If you want a dose increase or you want to avoid a dose reduction arrange early morning and fasting (water only) blood draws when TSH is highest.

  • Unfortunately, doctors appear to be the last to know how to advise hypo patients. They only concentrate on the TSH (not symptoms) and if its anywhere in range as far as theyr'e concerned anything else we complain about is unnconnected. Not true in most cases. In most cases food/levo before tests can skew results and not in the patients' favour.

  • P.S - we don't recommend to starve before a blood test, just that breakfast be taken later as taking either levo or food before can skew results.

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