Safety of testing 8-12 hrs after last T3

[HotelHurricaine]

I'm trying to understand the rationale behind testing 24 hrs after last levo and 12 hrs after last T3.

I realise it gives a higher TSH and lower T3.

My endo asks me to test 4 hours after T3. He has never discussed splitting dose so assume he thinks it will be my full dose (in my case 10mcg).

If we test only 12 hrs after last dose, I assume my T3 will be much lower in range than it is 4 hours after last dose, which I understand might be it's peak of the day.

So is advice here, to test 12 hrs after last dose, based on the idea that it's safe to be over range for some of the day?

I did a test yesterday as he asked (though he will not see these, they are just for me to understand what's happening) - 4 hours after my full dose, and T3 has come in over range: FT3 - 7.7 (3.1-6.8).

When I test for him, I take half dose 4 hrs before test as a little compromise.

I just want to check it's safe for me to be over range at 4 hours post full dose.

Thank you for your wisdom!

🙏

[HealthStarDust]

I’d like to still understand this too.

I can understand we may want to skip a dose to get the highest TSH of wanting to increase dose, but, I’ve also wondered if it would benefit me to know what my levels look like after having my dose at some point.

I don’t think I like the idea of being over range even for part of the day anymore.

[SlowDragon Administrator]

personally I would take that as evidence it might be better to always split the dose of T3

[greygoose]

I realise it gives a higher TSH and lower T3.

No, it doesn't. The timing of your last dose of T3 has absolutely no effect on the TSH because the TSH cannot move that fast. What affects the TSH is the time of day. And that is highest before 9 am.

OK, so you have proved to yourself that having the blood draw four hours after half your dose of T3 (how much are you taking?) gives you an FT3 slightly over the top of the range. But, what you haven't shown is how long it stays at that level. That is the peak - or is it? How do you know that the peak wasn't three hours after taking it and it's already coming down? So very many unknowns in all this.

And, in any case, such a slight amount, temporarily, over the top of an arbitary range, is hardly the end of the world. Ranges are just rough guides, not lines you must not cross on peine of death. And there are some people that always have their FT3 that high because it's the only way they can be well.

Also remember that what you are doing is a blood test. And a blood test only shows you what is in the blood. And T3 in the blood doesn't do anything. It doesn't become active until it gets into the cells. And that temporary excess T3 is not going to have time to get into the cells before the peak is over.

So, what we try to do by having the blood draw 8-12 hours after the last dose is measure the average circulating level of T3 in the blood. It's a compromise, but all blood tests are compromises. It's just that when you always do it the same way you see patterns, and associated patterns and learn to judge what is too much and what is too little.

But, one thing is certain, if you have the blood draw four hours after taking your full dose, you are going to find yourself with a reduction in dose. And I often wonder if that's not exactly why endos tell you to do that.

[HotelHurricaine in reply to greygoose]

Thank you greygoose . This test was 4 hours after current full dose of 10mcg.

Usually when endo is going to see the test result I have been taking half dose (then 2.5mcg) 4 hours before blood draw.

Thank you for explaining further.

I understand from SlowDragon that by splitting the dose we avoid high peaks which makes sense. I've just been struggling to manage on splitting the dose, feel more energy on 1 full dose, but that may change if endo increases my dose.

Do we have any credible research on effect of T3 being over range? I understand you say the range is arbitrary.

[greygoose in reply to HotelHurricaine]

I don't know of any research, no. And I doubt anyone would even bother to research the effects of something that is only going to exist for a very short while. Plus, as I said, there probably aren't even any efffects because T3 just doesn't do anything in the blood, it's not active. And if you're only taking 10 mcg, you're not going to have a wild excess in the cells, are you. My feeling is that none of this is really worth worrying about.

Have a read of this to get a better grasp on ranges:

helvella.blogspot.com/p/the...

[HotelHurricaine in reply to greygoose]

Thank you I will have a read of this.

I may go up on T3 over next few months - I have only recently started trying it - so it's helpful for me to understand the concept / any risks of being over range.

[FancyPants54 in reply to HotelHurricaine]

If you were over range all the time then it might be necessary to adjust something. But with your low dose and your results, that is not an issue.

[rjb112 in reply to greygoose]

"What affects the TSH is the time of day. And that is highest before 9 am."

greygoose, I only take levothyroxine: Besides the time of day, how does the time that I took my daily levothyroxine affect TSH? Also I'm a nightowl: how does the time I go to sleep and time I wake up affect TSH?

Thanks very much.

[greygoose in reply to rjb112]

The time you take your levo has no effect on TSH, That's not how it works. And I'm pretty sure the time you go to sleep doesn't have any effect, either.

[tattybogle]

if you are getting fT3 7.7 4 hrs after 10mcg, i think it's fairly safe to assume it won't be so high 4 hrs after taking 5mcg ( cos you've only had put half as much into the blood).... so when split dosing , you probably won't be over range for any of the day .

[tattybogle in reply to tattybogle]

This might help : thyroidpatients.ca/2019/11/... free-t3-peaks-and-valleys-in-t3-and-ndt-therapy/

[HotelHurricaine in reply to tattybogle]

Agreed! But I don't split my dose.

And I may yet increase it.

So trying to understand if there are any risks to being over range and by how much etc.

[tattybogle in reply to HotelHurricaine]

oh right ~sorry, misread your post.

so it's in range when you test 'for him' 4hrs after half dose ?

as for the risks of being over range in the blood for part of every day , nobody knows the answer to that question , and it's going to be pretty difficult to find out .... presumably it would not be easy to get ethical approval for an official large scale trial that deliberately gave people over range fT3 levels .

my thinking is that if it's possible to keep fT3 more or less within range all day, by splitting dose , then it would be smart to do so .... but if it's not possible ie ,if you have already tried splitting the dose and couldn't feel well doing that , then so be it...... you just have to choose between having the safety net of staying within range all day versus. the better quality of life you get from single dosing.

The medical profession are quite happy for us to accept the unknown risks of running relatively higher fT4 levels / relatively lower fT3 levels on levo monotherapy.........

[HotelHurricaine in reply to tattybogle]

This is so true!

I hadn't thought of the benefit of splitting dose being to keep within range, only for symptom control, so that makes a lot of sense - I now have a better idea of which risks I'm balancing!

Thanks so much for your time. As I go up in dose (if he does put it up!) I will try splitting dose again.

Thanks again.

[TiggerMe in reply to HotelHurricaine]

I've tried various timings of testing and splitting/ not splitting various doses and as   greygoose says it's all only a very vague guide as we only know it is getting to our cells by how well we feel... or not

I've found 15mcg works best for me one dose am and this produces results

4 hours post 140% post 10 hours 75% 24 hours 60%

So it spike and falls pretty quickly...

If I split dose I need to almost double the dose to feel well 25mcg with 12 hour post 60%

So it seems sensible to stick with the lower dose once a day and if I need to produce a 'happy Endo' result it's easy and reliable at 24 hours post T3 & 4 😅

It's Endo's that get hung up on blood tests so I'd suggest finding your happy place (dose, timing, splitting or not) and check your levels for your information and then working out what you need to do to produce the 'right results' so they leave you alone and don't mess it all up 🤗

I have to tell my NHS Endo that I split my dose as that's what she wants to hear 🙄

[HotelHurricaine in reply to TiggerMe]

Very helpful thank you! I see I’m only at the start of my testing!

[TiggerMe in reply to HotelHurricaine]

It's a tool to get you what you need 🙃

[helvella Administrator Thyroid UK]

I have tried to explain in my blog:

helvella - Scheduling Blood Draws

Factors to consider when choosing time for blood draws. Includes links to several documents/webpages and previous HealthUnlocked posts.

Last updated 18/07/2024

helvella.blogspot.com/p/hel...

[HotelHurricaine in reply to helvella]

Thank you this is really helpful.

[SmallBlueThing]

I had a consultant have me tested about five hours after my dose, when taking a combination of levothyroxine and one grain of desiccated thyroid extract (so about 8 mcg T3), and my fT3 level was similar to yours. When I tried to manage my dosing more intelligently, my surgery refused the same testing, apart from on one occasion.

When experiencing thyrotoxicosis after COVID-19, I tested at about 9pm, when most likely to have symptoms. My fT3 was about the same but my fT4 was now over-range at 29 pmol/L. The symptoms I would describe as dysautonomia. I had an ECG, echo, 24 hour blood pressure monitor, and blood test for heart failure, none of which found anything. My walking would turn into a stagger within a few minutes, which lasted for over 18 months. I felt cold, constantly, with regular drops to 35.3°C.

Several years ago, when I was taking desiccated thyroid extract only, my upper arms would feel a bit weak when chopping veggies at about 6pm, while on 4.5 ~ 5 grains.

During another phase of thyrotoxicosis (pre-pandemic) I lost a lot of upper arm strength. I know, because I had to carry buckets of 5 ~ 10 kg of solid fuel and quite suddenly I could only manage the lower weight. My strength quickly returned, but i think I have undiagnosed myopathy, affecting hips, shoulders, upper back and neck.

During undiagnosed hyperthyroid phases before treatment, heat intolerance, restlessness, racing heart, loose bowels and dry mouth were among the symptoms.

When tested during a hyper phase before treatment, my highest fT3 was 8.4, five months after feeling much worse, though untested. My fT4 stayed within range, at 17 ~ 19 pmol/L. Over the course of a year my TSH went from 5.5 to 0.42 to undetectable.

[ParlourPalm]

Hi there - this is just to share my personal experience , not meant as advice. I ended up in A&E with racing heart, tight chest, shaking and faintness and intense anxiety. I had been on NDT and following protocol of taking half dose 12 hours before testing, so whenever it was tested my T3 looked OK. My T3 was tested in A&E (about 4 hours after taking the dose) and was found to be too high. My endocrinologist then had me test 2 hours after a dose and I was hugely out of range. I hadn't been feeling well for ages, and feel much better now it's sorted out. I guess it shows we are all different - just sharing in case it helps.