Why splitting dose before blood test? - Thyroid UK

Thyroid UK
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Why splitting dose before blood test?

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Hey there,

Why should I be splitting dose the day before bloods? I take 112 mcg t4 and 20 mcg t3 in one go, my ft4 is mid-range and ft3 is over 70-75% after taking my dose 24 hrs before the test. I have heard it is recommend to test ft3 8-12 hrs after the last dose.

I think jimh111 you have mentioned based on half life of t3 the actual reading difference would be minimal, like adding around more less 10% on top of the result (of course everyone metabolises it differently), tsh would be a tad lower, not sure about ft4.

When someone splits a dose day before testing and let's say they take it twice a day and maybe twice a day their ft3 is 75% range, and someone who tests after 24 hours with same result 75% range, does it not mean that possibly the person that splits takes way too much, should they not have slightly lower levels and more consistent maybe, but if they took all in one dose surely they would be over range in the morning? I can't seem to be able to figure out the rationale of it, need some help please :) or I'm starting to suspect bloods are quite unreliable for ft3 levels.

Thank you!

R

11 Replies
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SeasideSusie
SeasideSusieAdministrator

ritatita

I can't quite get my head around what you're saying, possibly because it's nearly bedtime and my brain isn't fully functioning, so I'll leave others to comment on that, but I did notice this:

my ft4 is mid-range and ft3 is over 70-75% after fasting for 24 hrs.

Are you seriously saying that you fasted for 24 hours before testing? If so why? This is not suggested on the forum and no-one has to deprive themselves of food for 24 hours before a thyroid test. All that is suggested is that breakfast is delayed until after the test because it's possible that eating could lower TSH and that coffee is said to affect TSH so water only to drink before the test is advised.

Hidden
Hidden in reply to SeasideSusie

Sorry SeasideSusie, my bad, I meant medication fasting , not sure why I said it this way 😂

SeasideSusie
SeasideSusieAdministrator in reply to Hidden

Oh good, I couldn't imagine how anyone could do that, I feel deprived after a few hours :D

Hidden
Hidden in reply to SeasideSusie

Haha sorry! I'd be dead not eating 😂🙃😀

GKeith
GKeith in reply to SeasideSusie

I fast quite a lot & always have so maybe because I'm used to it, it doesn't bother me. I believe a 24-hour fast, just water, on an adult will not hurt the majority of us but do not suggest anything that may bother you physically. Peace be upon you.

Thyroid Uk and Stop the Thyroid Madness both state NOT to take your a Thyroid medication before a blood test. Always leave until after the blood test as levels will appear raised.

Hidden
Hidden in reply to Bearly56

Hi bearly, I meant splitting the dose the day before the test of you take t3 and t4 meds. It is recommended on here to test ft3 8-12 hrs after the last t3 evening dose. But I don't split, I take it all in one go.

SlowDragon
SlowDragonAdministrator in reply to Hidden

If you leave it 24 hours then your Ft3 results in reality are likely much higher than test shows

Graph showing why to take T3 approx 12 hours before blood test

healthunlocked.com/api/redi...

Research Paper data comes from

ncbi.nlm.nih.gov/pmc/articl...

Hidden
Hidden in reply to SlowDragon

Thanks for the graph and study. Seems the graph is based on taking 50 mcg t3 in one go at 6 am, however, taking 25 mcg t3 would produce a smaller spike as they mention in the study and taking 10 mcg t3 would be a within range spike. I'd say it's individual as well, taking Levo would also create spikes and convert to some t3 so won't look exactly like it.

When you take t3 it peaks, which is correct, however, it's the effect it has on cells, not in the bloodstream, therefore there are people here taking various and certainly some higher doses of t3 in one go and I'd say their bloods would indicate they may be too high, while they feel well. I guess I'm trying to understand why the result of ft3 is important just precisely after 8-12 hours, not 6 (closer to its peak) or 14 if you get me? And how accurate is it to split your dose three times,especially if doses are not equal (higher morning dose, half of it in the afternoon), this could mean that your ft3 might be much higher in the morning after 8 hours , but you only measure the effect of the last dose.

Sorry just trying to wrap my head around it, but I still don't get it :) if I split my dose before the test, but I go back to taking it to once daily, my results wouldn't be a true reflection of what I take if it makes sense?

SlowDragon
SlowDragonAdministrator in reply to Hidden

Mainly this advice is purely to keep medics happy and produce a result that doesn’t result in dose reduction....

If you took 25mcg T3 in one go and measured blood levels 4 hours later.....dose would almost certainly get reduced

Hidden
Hidden in reply to SlowDragon

Makes sense! Thank you SlowDragon :) maybe this is why my doc doesn't mind me testing 24 hrs after. I did however overdose, so learning now :)

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