Thyroid UK
90,518 members104,886 posts

When to stop meds for blood test

My last blood test was 7 months ago but over the last few months I have experienced some new symptoms-all of which appear to be linked to being Hypo. Should I do another test?

If so, I would be really grateful if someone could tell me when exactly to stop my meds as my feeble brain can't work it out :-( I take T4 & T3 all together in the early hours of the morning. I think I'm supposed to stop Vit B too so do I stop everything else as well? I'm particularly concerned about B12 as I have developed Tinnitus and have read on here it may be connected to low levels.

Thanks a lot.

6 Replies

infomaniac You should take your last dose of Levo 24 hours before the blood draw, and take your last dose of T3 12 hours before the blood draw. This may mean adjusting the time of your doses on the day before your test. Then take that day's dose after the blood draw instead of the early hours of the morning.

Biotin (B7) in a B Complex or as a stand alone supplement should be left off for a few days, and if testing ferritin no iron supplements for 5-7 days. Any supplements that you do take, do so after the blood draw.

If you're taking B12 then you have to be off that for about 4-5 months to get a baseline result, testing whilst supplement just tells you that you are testing, although that can be useful as you can see if you're taking enough.

Don't forget that thyroid tests should be done as early as possible in the morning, no later than 9am, and fast overnight (water allowed).


I'm glad I asked-I didn't know you had to leave B12 so long! I'm a bit confused...if I went without B12 wouldn't I be deficient anyway after all that time?


I was never very low in B12, around the 600 mark, but I wanted to get it to the top of the range so I supplemented with 1000mcg sublingual methylcobalamin daily. A few months later I did a test bundle that included B12 and it was showing at around 1200 I think. I reduced to taking the methylcobalamin 5 times a week and the next time I tested it was down to around 1000. So, as far as I'm concerned, even though testing whilst supplementing won't tell you what you're holding onto, it does seem to reflect whether the amount you take is keeping you at the level where you want to be. I'm happy with that.

1 like

T4 should be stopped 24 hours before; T3 8 to 12 hours; B complex, and anything containing biotin, a couple of days; other supplements the day before (I think). But, no point in retesting B12 if you are taking B12. You would have to stop if for 5 months to get an accurate reading.

1 like


T4 can peak in the blood for up to 8 hours and T3 for up to 6 hours so you'll be find if you leave 8+ hours between last dose of Levothyroxine and T3.

If you take biotin or B Complex don't take them on the morning of the blood test. If you're testing B12 take it after your blood draw.

1 like


Here you go, Lovely xx


You may also like...