Timing of t4 testing

[redhead41]

Is there any evidence that you need to leave 24 hours between last dose of levothyroxine and the bld draw? Also why is it advised to do bld draw on empty stomach, only water on waking before bld draw? Thanks Sue

[jimh111]

We store around ten days worth of T4 so the timing of the last tablet makes very little difference, I'd just avoid taking levothyroxine within a few hours of having the blood taken.TSH is higher in the morning and the idea is that by getting a higher TSH result it will be easier to get a doctor to treat you. There is some validity in this because most studies take the blood at this time. In practice it makes a small difference because most surgeries don't open in the wee hours of the day.

There was a badly conducted study that seemed to show having.breakfast affected TSH. The idiots took the blood before and after breakfast. At different times of the day. Other studies found no difference, except for one Chinese study that claimed to have suspiciously low p-values, a degree of certainty well beyond previous studies.

I'm about to go on holiday so don't have time to cite the research, sorry.

[tattybogle]

see replies (3rd -6th ) to this post for research / evidence :

healthunlocked.com/thyroidu.... list-of-references

[Tina_Maria]

Peak plasma concentrations of levothyroxine are reached around 3 hours after taking the tablet. If you take your medicine in the morning and have a blood tests after 3h, you would actually measure your levothyroxine at its highest concentration, which is not reflective of your general level. It would give you a false high, and if a GP would measure your T4 at that point, he might decide to reduce your levothyroxine on the basis of that test, which could leave you under-medicated, as this peak level does not correspond to your general T4 concentrations.

I have attached a publication looking at pharmacokinetics of levothyroxine, hope this helps.

ncbi.nlm.nih.gov/books/NBK5...

Therefore it is advised to not have your medication in the morning when you are having your blood tested.

[jimh111 in reply to Tina_Maria]

Note that this study in Figure 1 references (11) a study that used a single bolus dose of 600 mcg. In practice this variation in TT4 will be much smaller although it still makes sense to avoid having the blood taken within a few hours of taking levothyroxine.

[redhead41 in reply to jimh111]

I get most/ all of this but the recommendations from this group state that last dose of levo needs to be 24 hours and 12 hours for t3 before bld draw - just wondering where that information has come from. Would leaving 12 hours for both levo and t3 be acceptable? Thanks Sue

[Tina_Maria in reply to redhead41]

T3 is broken down more quickly than T4, therefore the recommendation is to leave 12h between taking T3 and the blood draw. If you leave a longer gap, your T3 may show a false low, conversely, if you leave a shorter gap, your T3 might show higher than it actually is. Hence the timing is important.

[jimh111 in reply to redhead41]

I take T4 the night before and T3 half way between dosea to get the most accurate result.

[redhead41 in reply to Tina_Maria]

Yes but why does this group say to leave 24 hours between last dose and bld draw? And why bld draw is fasting?

[Stourie in reply to redhead41]

I think the 24 hours gap is because if you take t4 every 24 hours you still want a good result with it so you still feel good. If you only left a few hours then your result could be too high and the doctor might want to lower your dose. That’s putting it simply Jo xx

[Tina_Maria]

As I mentioned earlier, peak plasma concentrations are reached after 3h and decline slowly, so after 24h you would have the concentration your body would normally have without the new dose. This would give you the most accurate level of levothyroxine you have accumulated over the 6-8 weeks you have been taking your medication.

Depending on what kind of foods you ingest before having your blood drawn, the nutrients in the food may impact on your TSH and free thyroid hormone levels. In addition, if you are eating food that contains biotin (eggs, mushrooms, bananas, avocados), this could influence the assay that measures your thyroid hormones. Even if the influence might be small, it would be better to avoid any interference so you can be sure you get the most accurate reading for your TSH as well as T4 and T3 (if the GP measures them).

Hope this helps.

[tattybogle]

you can test with any gap you like ... 24 hrs ,,, 12 hrs ... even 4 hours if you wanted to .... it's just a matter of understanding what effect it has , and taking that into account when interpreting them :

at 24 hrs , you will see the lowest level your FT4 gets to on that dose

at 12 hrs , you will see 'nearly but not quite' the lowest level.... there is not much differnece between the level @12 hrs and @24 hrs in the studies i've seen.

between about 1 hour and about 6 hours ... you will see a higher level than you have for most of the day .... but as fT4 goes up and down pretty rapidly within this time frame , you won't ever be sure of you caught it at the highest 'peak' or not.

between about 6 hours and 12 hours ... the level is falling from it's highest peak and is now on it's way back down to your settled level ... but again , how fast it falls will be individual , so it's hard to know what point you are looking at when comparing it to other tests.

So "24 hrs" is simply logical and convenient ... it means you know what point you are testing with a degree of certainty ..... the lowest point it gets each day ..... just before you take next dose .

And since many people take levo in the morning , and since TSH is highest early am ... advising 24 hr gap / 9 am blood test makes it easy to get consistent and comparable TSH / fT4 tests done.

the fasting advice is simply because one piece of research has found that in some people ( but not all ) eating breakfast lowered TSH by up to 30% ..... ... so advising to fast is just one more variable we can control to try and get consistent comparable tests done .... if that study is correct , and if you were one of those people .. and if you ate different things for breakfast before each blood test ... how can you compare it properly to the last one ?

you don't have to fast at all .... but if you are not consistent , any variations in your TSH level become harder to interpret ... ie has your TSH really gone up this time meaning a dose increase is worth considering ... or was it just your lack of/ different breakfast compared to last test when you had a full english ??

[jimh111 in reply to tattybogle]

Agree but what nobody tells you is time oof the month has by far the biggest effect doi.org/10.3389/fendo.2017.... .

[tattybogle in reply to jimh111]

ah yes .... and that too

[helvella]

I look at the issue of fasting like this:

Fasting has sometimes been shown to make a difference - though usually quite modest.

Imagine you don't fast and build up a history of test results. Then, for your next set of tests, you absolutely HAVE to fast. Your result comes back a little different. You are now left wondering if that difference is due to a real change - or that you fasted. You have no way of checking and determining which it is.

If you always fast, you have a consistent basis.