How long should I go without Levothyroxine prior to blood test?

[Piglet1956]

Hello. Looking for advice/suggestions. I have blood tests next week for thyroid function. I currently take 50mcg Levothyroxine at 10pm and my tests are at 9am. Should I forgo meds for 11hours or 35 hours?

[SeasideSusie Remembering]

Piglet1956

Last dose of Levo should be 24 hours before blood draw, any longer and you get a false low FT4 and too close you get a false high FT4.

It would be better to adjust the timing of your Levo beforehand to give you the 24 hours.

Let's say your test is on Monday, you should take your Saturday evening dose on Sunday morning at 9am which will be your last dose before the tet (24 hours). Have your test on Monday at 9am then take Sunday evening's dose after the test. Then take your Monday evening dose at 10pm as normal and you're back on track.

[Piglet1956 in reply to SeasideSusie]

Thank you for this. Sounds like a good plan you’ve worked out for me.

[jimh111]

Around 12 hours is ideal as it will give an accurate indication of average T4 levels (and also average fT3, TSH).

We store about 10 days worth of levothyroxine in our blood due to its long elimination half-life (seven days) , so a single tablet will make little difference. However, there is a peak in T4 levels during the first few hours after ingestion so it's best to wait for at least five hours before taking the blood.

See Fig 1(b) scihub.wikicn.top/10.1055/s... for a picture of how fT4 varies over time after taking levothyroxine. This scanned copy is not very clear but it's the upper dotted line that corresponds to patients on levothyroxine only. You can see that after about eight hours you get an average fT4 level, i.e. the mid-point of the area under the curve (AUC). In any event after five or six hours any variation is less than 5% which is smaller than the accuracy of the assay.

[Piglet1956 in reply to jimh111]

Thank you for your reply. It’s very thorough- I’m going to print off the paper and slowly read it. Brain fog....

[jimh111 in reply to Piglet1956]

I only looked at the picture!

[Ellie-Louise in reply to jimh111]

So have I been doing it all wrong then?Susie says 24 hrs., you say 12 hrs. I have been way over both every time I get tested.

I take my levo at around 2am before bed, so have left it off the night before as per previous advice here, and usually ask for my blood test appointment to be around 8:00-8:30am.

Should I do things differently next time, I’m due a test in May.

Thank you.

[SlowDragon Administrator in reply to Ellie-Louise]

If you normally take levothyroxine at bedtime/in night ...adjust timings as follows prior to blood test

If testing Monday morning, delay Saturday evening dose levothyroxine until Sunday morning. Delay Sunday evening dose levothyroxine until after blood test on Monday morning. Take Monday evening dose levothyroxine as per normal

[Ellie-Louise in reply to SlowDragon]

Thank you for that, it sounds easy enough to remember.So I’ll take my levo after the blood test and then go back as normal that same night/ early hours.