Do you need to have max 12h between t3 and test?

[Clara9]

I am trying to convince doc to test me before apt. But since I don't know if she will in not very eager to split my dose since I'm usually taking all 15 in the morning. Will this be OK?

[SeasideSusie Remembering]

Clara9

8-12 hours is the recommended time for last dose of T3 before blood draw. Any closer and you will get a false high FT3, any longer and you will get a false low FT3. You need to know your normal circulating hormone and testing 8-12 hours after last dose gives this.

I usually take mine all in one early morning dose, but I do split the dose the day before my blood test and take my last dose around 9pm the night before as I know I do my tests around 8.30am.

If your GP doses by TSH then you need your blood draw as early as possible, no later than 9am, as TSH is highest early morning and lowers throughout the day, although when taking T3 it's often the case that TSH is suppressed anyway. Also, leave breakfast until after the blood draw if TSH is important, and water only, eating and coffee can lower TSH.

[jimh111]

You need to have the blood taken around half way between doses to get a rough idea of average fT3 and TSH levels. If you take your 15 mcg before the test the results will be very wrong, if you leave 24 hours it will also be wrong. If you have time can you switch to taking the L-T3 in the evening? This would make it easier to have the blood taken in the mornings.

[humanbean in reply to jimh111]

jimh111

So, if Clara gets tested halfway between doses she'll be doing a blood draw at, what time? late afternoon?

TSH fluctuates with time of day, and is likely to be close to its lowest in late afternoon.

Jim, you are the only person I've ever read about on thyroid forums who got treated for hypothyroidism with a TSH of just over 1 with little difficulty, and you seem to be able to swing dose adjustments whenever you ask for them. For the rest of us this simply doesn't happen. So, trying to make people do what you feel is right is likely to get many people being treated by doctors end up denied treatment at all or having dose increases refused or doses reduced. Whether you're right or wrong isn't the issue. The issue is that many of us struggle to get treated at all. And yet your posts on how and when to test seem designed to sabotage the treatment that people do succeed in getting!

[jimh111 in reply to humanbean]

I wrote 'If you have time can you switch to taking the L-T3 in the evening? This would make it easier to have the blood taken in the mornings.'.

Interpreting fT3 is very difficult for someone taking T3 medicine. The best we can do is to have the blood taken around half way between doses, I will put futher details below.

I had extreem difficulty getting a diagnosis. For many years nobody suspected any form of hypothyroidism. I had progressive memory failure, IBS, gradual weight gain and bile reflux. The latter led to attacks of biliary pain which was very severe. For some time my gastroenterologist and myself held off going for a cholestectomy because we feared the removal of the gall bladder would make the bile reflux worse. After many years of research which is too complex to go into I concluded my problem was due to hypothyroidism even though at the time I had few hypo symptoms. I had the usual blood tests from my GP and saw a very nice endocrinologist who felt I couldn't be hypo because the 'blood tests said so'. During the next year I became very hypo with obvious signs and symptoms. At one stage when going to the local shops for a loaf of bread and a newpaper I had to write a shopping list, I would not be able to remember. I manage to get a private referral to the late Dr Skinner who said he thought I might be hypo. After starting on levothyroxine I improved a lot but needed high doses of liothyronine to get better.

My GP was very reluctant to treat and I had to use every ounce of diplomacy I had left to convince him. Eventually he accepted the diagnosis as he could see I was getting better but he pushed very hard to lower my dose. This was excellent care, a GP should not be a pushover but should be willing to try treatments. I have been able to make dose adjustments because I am on much lower doses and my GP is aware that I am keen to stay on the lowest effective dose.

All this had an enormous effect on my life. I became irritable and 'asocial' as Dr Skinner describes it. I had to give up my job in computing, I was really good at it at an exciting time with international teams were being set up. Fantastic opportunities which I had to miss. I estimate that leaving my job had a net cost of around £250,000 in terms of lost net income and pension rights. This is just putting a number on it, the loss of doing a very exciting job with highly capable colleagues was much greater.

[humanbean]

There was someone on the forum (Clutter) who used to say that leaving a 24 hour gap between previous T3 dose and blood draw was okay (but not ideal). If you do this you would have to add 20% to the result you got to cater for the long gap.

[SeasideSusie Remembering in reply to humanbean]

I remember that HB, and I admit maths isn't my strongest point, but on which figure is the 20% added? Probably obvious so guessing if the result is 5 (3.1-6.8) and we add 20% to the 5, are we looking at 6? Personally, I'd rather test at the correct time than mess around with a possible 20% difference.

[humanbean in reply to SeasideSusie]

Yes, your example is how I think it was supposed to work. But I agree - I'd rather test first thing in the morning with a 12 hour gap between previous dose and blood draw (for T3 testing) the way that you describe.

I think looking through Clutter's posts to find where she discussed the 20% idea might be difficult - I think she must have written 1000s of posts over the years.

[Stourie in reply to humanbean]

I take my t3 all at the same time so leave 24 hours between last dose and blood test. Jo xx

[jimh111]

Clara, this is a general comment, I'll reply to you next.

I posted some figures on the effects of leaving a 24 hour gap. T3 has a half life of about 24 hours. If you take a T3 medicine once a day then after 12 hours you will have 70% of the initial dose in your blood. After 24 hours you will have 50%. So, if you wait 24 hours instead of 12 hours you would need to scale up the fT3 result by 70/50. i.e. multiply the fT3 result by 1.4. However, it is not as simple as this. If you take lots of T3 the half life reduces as the body tries to get rid of the excess. If you are not taking large dose then the T4 you have will be producing some T3, this fraction will vary according to how much residual thyroid function you have, your TSH (TSH promotes T4 to T3 conversion in some tissues) and how much T4 you are taking. It's just too complex to work out. The best you can do is to have the blood taken roughly half way between doses and accept that this will give you a rough guide to average fT3 levels.

TSH has circadian variation, higher in the early hours overnight and lower during the daytime. TSH is a good marker for primary hypothyroidism but unfortunately doctors place too much reliance on it and completely fail to acknowledge other causes of hypothyroidism. If you have a marginally high TSH I can see that having an early morning blood draw might up your TSH a little and help achieve a diagnosis. Nobody knows how this circadian variation is altered by thyroid hormone treatment, especially when TSH gets low. If you have a TSH of e.g. 0.3 will taking the blood at 7 am make any difference? If it does and you get a TSH of 0.4 will it make any difference to your GP?

There are disadvantages of playing this game. During the morning TSH is changing and your body clock will vary from day to day. If you have the blood drawn at these times it will be more difficult to compare sets of results due to the random varition at this time compared to later in the day when TSH is more stable. You also reinforce the doctor's misconception that TSH can always be relied upon, after all you are getting better and have not suppressed your TSH (early morning). So the TSH really does work. The main problem is that you may be paying lots of money for blood tests that aren't much use, epsecially if you need to look at your history at some time in future.

This game is based on the assumption that the person concerned has a failing thyroid and this is why they are hypothyroid, or still have symptoms after treatment. I have noticed that a large number of patients with ongoing symptoms at diagnosis have a normal TSH and low normal fT3 and fT4. This does not indicate a failing thyroid, rather it suggests the pituitary is underperforming due to various reasons. These patients have poor T4 to T3 conversion which is not surprising as TSH promotes deiodinase. Over the last year or so I've kept a copy of patient posts that fall into this category and so far I have about 80 examples. These patients often have quite severe symptoms. If you artificially obtain high TSH results you overlook these patients. It's not a question of trying to fiddle the TSH result, we should aim for accurate data because that is the only way we will get accurate diagnoses, progress thyroid science and have data that is useful for case histories.

[jimh111]

Clara,

If you can I would switch to evening dosing and have the blood taken the next morning. In the longer term it would be better to split your liothyronine into two daily doses as this will give you more stable levels.

Looking at your very first post before you took any thyroid medicine your results were: -

Ft4 11 (12-22), Ft3 3.1 (3.6-6.3), TSH 3.9 0.4-4

This very much falls into the category I described earlier, a TSH that is too low for the given low levels of fT3 and fT4. In fact both your fT3 and fT4 were below the lower limits of their reference interval. For most people their TSH would be very high in this situation, certainly above 20 and perhaps around 100. Generally as fT4 falls TSH shoots up and this stimulates more T4 to T3 conversion allowing fT3 to remain stable until fT4 falls further and there simply isn't enough hormone to maintain fT3 levels. Your pituitary isn't responding adequately to your low fT3, fT4 levels. This is called 'secondary hypothyroidism' or 'central hypothyroidism'. Doctors tend to be unaware of this condition, they are aware of secondary hypothyroidism caused by e.g. a non-cancerous pituitary tumor but in these cases the TSH is very low, around 0.01 rather than 3.9 as in your case.

There can be a number of reasons why TSH can be inadequate. It can be caused by very strict dieting, depression, severe illness or trauma to the head. You suffered from anorexia and it is quite possible that this has caused you to have insufficient TSH. There could be other reasons but this is the obvious one in your case. Sometimes the TSH picks up and sometimes it doesn't. In your case I would point out your initial blood test results that show the low fT3 and fT4 with a normal TSH. This shows your 'axis' is dysfunctional and TSH cannot be relied upon. I suspect you will need more TSH to get better. Unfortunately if you are given levothyroxine it has the effect of lowering your TSH further which reduces your T4 to T3 conversion. So as you take more levothyroxine your T3 levels fall. Hence you need some liothyroine and some levothyroxine.