Test Query

[A-Little-Life]

Hi Guys,

Can someone tell me why I shouldn’t take my Thyroxine before the blood test? I thought the result whilst the Thyroxine is in the system would be the most important?

Thanks

A Little Life

[fuchsia-pink]

Because your GP won't understand that the blood result you get this way is the highest possible, so it's safest to get your "mid" reading, after you have metabolised some of your meds, ie which shows the levels you have most of the time.

[A-Little-Life in reply to fuchsia-pink]

Thank you so much! ❤️

[StitchFairy]

It's a bit like measuring your appetite just after you've had breakfast. You won't theoretically be feeling hungry at that point. BUT that doesn't mean you have enough food in your digestive system to last you the whole day. If your appetite was measured before you had breakfast, it would give a different reading.

But this only matters with respect to the FT4 reading. It doesn't have much effect on the TSH reading.

[A-Little-Life in reply to StitchFairy]

Thank you so much! 😀

[SeasideSusie Remembering]

A-Little-Life

Levothyroxine (T4) has an elimination half life of 7 days, so if you take 100mcg today, there will be 50mcg of that dose left in 7 days' time.

T4 is a storage hormone (some of which converts to T3) so you will build up a store of T4 when taking Levo regularly (a bit like stocking up your pantry).

You want your blood test to measure your normal circulating amount of hormone.

If you take your dose of Levo before the blood test, this will spike in the blood 2-4 hours after taking it then that spike will start to gradually decrease.

To avoid measuring that spike, and therefore show a false high FT4 level, we take our last dose of Levo 24 hours before blood draw and that then measures the normal amount of circulating hormone.

[A-Little-Life in reply to SeasideSusie]

Thank you so much, that totally clarified things for me! ❤️