When getting my blood tests I know that if it’s been 24 hours since my last Levothyroxine, my T4 result will show the lowest figure over that time.
But if I split my T3 dose so that I take half the daily amount 10 hours before the blood is drawn, will the T3 result be representative of the average figure or again the lowest figure over the 24 hour period ? (I’d usually take my T3 only once per day.)
I’m a bit confused about this
It is a confusing one and I often say it depends who you are testing for as to which timing you use
Medics don't believe fT4 fluctuates which is somewhat true for many but for some the hours after taking T4 can cause a slight rise so 24 hours after gives a much clearer reading, it has a much longer half life as a storage hormone
When it comes to T3 it bobs up and down quite quickly and medics often request 3 hours after dose to catch the peak which rapidly drops off, it does have a half life of around 2-3 days so there is a carry over hence the testing recommended 8-12 hours after last dose to give a good baseline so you could call that your 'average'
I'm like you and dose once a day and have experimented with various timings (for my own knowledge) and find obviously a spike 5 hours after full dose which drops back to 60% after 24 hours compared to splitting the dose and leaving 12 hours results in 80% though split dosing I find far less effective symptoms wise
The thing about blood tests is that hormone in our blood isn't actually doing anything but acting as a reservoir and it needs to enter our cells to become active and take effect
Hope that helps 😵
I am seeing a private doctor and he recommends testing 15 hours after last dose. Hope that doesn’t confuse things!
Sorry to jump in here annesarah but I'm in a similar situation. Do you mean 15 hours after last dose of T4 or T3 or both please?
Thankyou for your reply TiggerMe, yes that does help. That’s good to know.
15 hours after both though I guess he is more interested in the T3 result.
Very odd! It would give you a false low fT3 result though wouldn't make much difference to your fT4
This is a brilliant post from radd
How much T3 are you taking
Is it capsules or tablets
If possible I would split as 3 doses day before, with last dose 8-12-hours before test
I was taking only 7.5mcg T3 per day, in tablet form , all at once, no splitting. But I’ve now had my test results back which show that I need more T3, so I’ll now increase to 10mcg daily and see how that goes. (I get my T3 tablets from Roseway and so I’m sure they will be ok with that).
My latest blood results showed my FT4 at 53% and FT3 at 56.8%.
I take 62.5mcg Levothyroxine daily . My T4 result is also probably a bit low isn’t it …. do you think I should keep this at the same dose in case the extra amount of T3 causes my FT4 to increase a bit, until my next blood test in 6 weeks time? Or increase now to 75mcg at the same time as increasing my T3 dose by 2.5mcg daily?
How long before test was last 2.5mcg dose of T3
I would be tempted to initially increase Levo first to 75mcg daily
Retest in 6-8 weeks
it was 10 hours SlowDragon, but the amount was half my usual daily dose (so a tad more than 2.5mcg). My T3 tablets are 5mcg each so before my blood test I cut one tablet in half and another tablet into quarters, and so took a half and a quarter.
I was thinking it was more important to get my T3 raised than my T4?
My cortisol (random) blood test result (included as part of the blue horizon bronze kit) was 461.0 : range 73.8 - 507.0
So this is quite high within the range ….. I don’t think I’m stressed …..do you know if this level could have an effect on my T3 and T4 in any way?
T3 dose day before bloods 
Why splitting dose before blood test?
Can I use total t3 to judge an increase in T3 meds?
Test result help please?: Low TSH and Low T3
