Advice on timing of NDT dose prior to blood test


I've searched the forum for advice on this but there seems to be conflicting information .

I'm due my first blood test post stabilising my dose of Thyroid-S at 2 grains. Based on temp and symptoms I think I need a small increase and would like some blood tests that support this, but am concerned that I will get a distorted view of FT3 because I mis-time my dose.

I normally take 2 grains c5am. From what I have read consensus is that you shouldn't take a dose just before the test, but I have also seen that leaving anything more than 12 -15 hours between a dose and testing can lead to understated FT3.

Can anyone suggest the best approach based on experience -

A. OK to have test 24 hours after meds

B. Split dose and take one grain as normal and second grain c12 hours before test

C. Delay full dose until 12 hours before the test

D. Another option I haven't thought of ...

I know the test should be done as early as possible and fasting.

Thanks in advance!


Last edited by

6 Replies

  • Kiwidel,

    I would delay taking your usual 2 grain dose until after your blood draw. FT3 result will show a bit low as it will be slightly more than 24 hours after last dose. If you extrapolate your FT3 result by +20% that will give you a reasonable estimate of your normal circulating FT3.

  • Thanks - sounds sensible. Out of interest, what is the 20% adjustment based on - research or experience?

  • Kiwidel,

    Someone else's research. I'm sorry I don't have a link to support it.

  • No worries, thanks for coming back to me. 🙂

  • Have you seen the graphic on this page?

    Based on that information, I vote for choice B. Choice A gives you an artificial low, and Choice C may not allow you to sleep that night. I personally take my meds 4x/day, so test right before my lunchtime dose.

  • Delay your thyroxine till AFTER your blood draw for a 'true' read out of thyroid bloods.

You may also like...