T3 dose day before bloods : Hello, I normally... - Thyroid UK

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T3 dose day before bloods



I normally take 25mcg T3 in one sitting in the morning. The day before my latest blood test I split into 12.5 in the morning and 12.5 12hours before the blood draw. Labs showed FT3 at 5.6 (3.1-6.7).

So if I normally take 25mcg at once, 24h later the next morning there should be 12.5mcg left in the blood if half live is 1 day. So when splitting dose the day before bloods there will be 9.375mcg left in the blood (12.5*0,5 from previous morning + 12.5*0,25 from 12 hours before ) right ? So when not splitting the dose labs should show higher FT3 because 12.5mcg>9.375mcg in the blood ?

And people say splitting the dose because T3 will be gone from the blood after 24 hours but I doubt that my FT3 results would show 5.6 only from 12.5 mcg 12 hours before ? There needs to be some left from 24 hours before ?

14 Replies

It's more complicated than this ;-) If you take 25 mcg daily just after taking your tablet you will have 25 + 12.5 from yesterday + 6.25 from day before yesterday + + +. i.e. about 50 mcg in your blood (assuming 100% absorption). (It gets even more complicated with levothyroxine with seven day half-life and daily tablets!).

We don't really know how tissues respond to T3 levels, we know receptors have to be saturated for several hours in T3 before they respond. Do the receptors respond to average T3 levels or to maximum T3 levels? It seems the best we can do is have a stab at average fT3. I'd prefer to take L-T3 twice daily and take the blood half-way between doses. This does away with all the calculations and avoids the taking the blood early am to get a high TSH sillyness (at this time TSH is varying rapidly, the results are unreliable).

In your case I would add about 10% to your fT3 figure to get a very rough idea of your average fT3. The 10% is a pure guess, it's not worth trying to do an accurate calculation as there are too many variables and the result is good enough.

Bear in mind that the true calculation is even more complicated than mentioned above. You are taking L-T4 also and your thyroid might be secreting some T3. For absolute precision you would have to factor in the T3 converted from the L-T4 you take plus T3 converted from T4 from the thyroid plus T3 secreted by the thyroid. An approximate fT3 is fine, it doesn't tell you tissue T3 levels anyway.

greygoose in reply to jimh111

This does away with all the calculations and avoids the taking the blood early am to get a high TSH sillyness

At the risk of being called 'rude', again, I have to say that you still don't get it, do you jim?

Maybe it is 'sillyness' (sic) but no-one is going to thank you if they take your advice and get their dose reduced. We patients didn't make this situation, doctors did, with their lousy education and obsession with the TSH. We just have to learn to play their game to get what we need. Nobody cares about us, and making us well, so we have to care about ourselves. And, if that means getting blood tests at the crack of dawn in order to get an increase in dose, so be it! You are being extremely disrespectful to us 'silly' women!

jimh111 in reply to greygoose

Philipp's TSH is already 0.02 so time of day is unlikely to make any difference. I do not know of any study that shows a diurnal TSH fluctuation in subjects receiving exogenous thyroid hormone at a dose that brings TSH close to zero. Once on a full replacement dose getting up at the crack of dawn in the hope of a high TSH seems silly, this is not a gender specific issue. In terms of getting an initial diagnosis (of primary hypothyroidism) with an elevated TSH the most important factor is to choose the right day. For this you need to be a pre-menopausal woman frontiersin.org/articles/10... , with luck you can raise your TSH from 2.0 to 4.5 (Figure 1 Case A).

greygoose in reply to jimh111

Your avoiding the point I was making. I was not talking about the PO's TSH specifically, I was talking about your generalised comment that it is 'silly' to try and get your TSH as high as possible. That adjective was uncalled for. We all know your opinions on this subject, but you could express them in a less perjorative manner. The 'silly' was totally uncalled for.

jimh111 in reply to greygoose

I'm happy to withdraw the mispelt adjective 'sillyness', the point I was trying to make is that it is often not a wise strategy. You are measuring TSH at a point on a slope so the result will vary from day to day. Also, many patients I see on this forum have severe symptoms with a non-elevated TSH. If a highish TSH is achieved it leads to a diagnosis (of primary hypothyroidism) but that will in most cases be the wrong diagnosis. It's more likely their hypothyroidism is due to TSH being too low for their corresponding fT3, fT4 or due to peripheral resistance to thyroid hormone (PerRTH). Patients wrongly diagnosed with primary hypothyroidism will receive inadequate therapy and other causes of hypothyroidism will be overlooked giving the impression they do not exist. In the long term it is better to get accurate data, that's the only way progress can be made.

fortunata in reply to greygoose

Well said, Greygoose.

greygoose in reply to fortunata

Thank you. :)


The test measures what's in the blood, not what's in the cells. T3 is not detectable in the blood after 12 hours although it will still be in the cells.

I don't have the original link to where I read this.

jimh111 in reply to SeasideSusie

T3 is detectable in the blood after 12 hours. After 24 hours half of the T3 will be in the blood and after 48 hours one quarter (assuming no additional dose). Measuring T3 in cells is difficult, even in animal experiments. I suspect T3 enters and leaves the cells quickly, at least not taking days. For example, if I forget my L-T3 and then take it later I notice the effect within an hour. What is relevant is that the effects of T3 on cells can last much longer, the proteins produced can hang around or have effects that last.

so confused right now whether to take my dose at once the day before or split. Normally I’d want to see where I’m at 24h post taking T3 to see if I need to increase, if I split that’s not my normal dose is it ?

I would take split doses all the time because you get more stable levels, closer to what happens naturally.

If you want once daily dosing then stick to it. If you have the blood taken about 24 hours after the last dose (and L-T3 is the only source of your T3) then half the T3 will have been eliminated. If you were to have the blood taken about 12 hours after taking the L-T3 about 71% of the T3 would still be around. So, if you have the blood taken 24 hours after the last tablet and do a rough calculation 70 / 50 x fT3 you will get a rough idea of your average fT3 level. i.e. multiply your fT3 result by 1.4.

We can only get a rough idea because each of us will eliminate T3 at different rates and the elimination of thyoid hormone varies with levels, more hormone in the blood quicker elimination.

taking another T3 dose in the pm is difficult for me since I eat more calcium rich foods around this time which will interfere with the T3.

And why multiplying FT3x1.4 when taking it 24h before ?

I take L-T4 at breakfast and bedtime but it's up to you.

Assuming you want an idea of your average fT3 levels. Once daily dosing: if you have the blood taken 12 hours after the tablet there will be 71% of the T3 left in your blood. After 24 hours there will be 50% left. So, if you have the blood taken 24 hours after the last tablet multiplying by 71 / 50 will give you an approximation of what the level was after 12 hours, i.e. half way between doses. 71 / 50 = 1.42.

Dare I throw another spanner into the works. I have been taking Thyroid S for nearly 4 years now. When I first started on a very small amount I soon increased to taking twice a day ( when I remembered). Since then I've heard the suggestion a few times that Thyroid S is slow release which I tend to agree with is I feel consistently the same throughout 24 hours. Whereas the 30+ years on Levo I could tell by lunchtime if I had forgotten my medication so if I am feeling now consistent throughout the day then I don't think it matters whether I leave 12 or 24 hours after medication for testing, not that I've tried that out.

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