Is it always best to not take Levo before a blood test, would it give a high T4 reading ? Thankyou

[Val-55]

[helvella Administrator Thyroid UK]

Correct.

I take my levothyroxine at bed time. But when I am going for a blood test, I defer that until after the blood draw in the morning. One reason I always try to get that blood draw done as early as possible.

Rod

[nostoneunturned]

It is definitely best not to take any levothyroxine on the morning of the blood test draw, you are right , the T4 level will be temporarily raised, just take it after the blood is drawn. Also have the test done early as possible as the TSH is higher then.

[Apples]

I don't understand this advice, surly the point of the test is to see how you are doing on your medication? If you have not taken it you are not giving a correct reading. I can see that being tested very close to taking your dose might give a dodgy reading but skipping it 12 hour before?

[editfmrt in reply to Apples]

I have asked myself the same question and understand where you are coming from with this. However, my GP (not that he is much good at anything) also told me not to take my meds on day of the test because it temporality raises the circulating levels of thyroid hormones in the blood.

Logic says that you wouldn't eat or drink anything high in sugar before a glucose test for example and that's why you have fasting tests to get the true picture of blood sugar levels. Although not entirely comparable with thyroid testing because it takes much longer for TSH levels to alter from medication. Nevertheless, it does affect T4/T3 levels and can give an artificially high result. This is certainly true if on T3 meds.

In truth though, thyroid function tests do not reflect what is happening at a cellular level and that is why they should only be used as a guide and only when taken in the context of symptoms.

I think consistency is more relevant i.e. taking the test at the same time of day each time blood is drawn, preferably earliest in the morning and without taking meds either the night before or if taken in the morning on that day.

[mandy-white in reply to editfmrt]

I am on T3 only and took my usual evening dose before I went to sleep. When I had my blood test the next morning my TSH was high and my GP accused me of not taking my medication. I tried to explain that T3 doesn't stay in the body long but he still wasn't having it. When should you take T3 if having a blood test please.

[editfmrt in reply to mandy-white]

Hi Mandy. I don't know the answer as treatment with T3 is different.

Hopefully, someone with experience may see this and answer your question or it might be worth posting it as a separate one which would be useful for T3 only or combination of T4/T3 people?

[susymac in reply to Apples]

Taking your meds can cause a spike in your levels therefore not giving a true indication of how well you are doing. Levo is a slow acting med, not taking it until after your blood test will not affect the results anywhere near as much as taking it will. Therefore for a truer indication of your overall levels will be if you dont take them. We are not talking days here, only a few hours or so

[tegz in reply to susymac]

T3 can have a bigger individual effect on TSH dosewise -and is quicker acting also.

Studies,as we know, often can determine only averages on multi person trials and this data is working from deviations from the log linear model.

In other words -patients vary a lot and they're still finding out why!

2010 report: European Journal of Endocrinology (2010) 162 323–329 ISSN 0804-4643

We encountered similar problems in the T3 intervention

arm. Although the log-linear relationship between

TSH and T3 was significant for all 11 subjects together,

this was not true for each individual. We selected the

doses of 25 and 50 mg T3 in view of the biological

potency ratio of 5:1 for T4 and T3 on aweight basis (20).

It would be of much interest to see whether the

sensitivity of the pituitary to suppress TSH is similar

for equivalent doses of exogenous T4 and T3. In the

study of Fish et al. (3), suppression of pituitary TSH

secretion was more clearly related to serum T3 than to

serum T4,

[mandy-white in reply to tegz]

Thankyou for replying. I am sorry though but that was too confusing for me!! Does it mean I should take some T3 before having my blood test to make the TSH better in my GPs eyes? Sorry!!

[tegz in reply to mandy-white]

This is not direct research on the question [that I've qouted] - just an indication that where people here have said 'don't take T4 before blood draw' ,T3 is even more likely to affect results and drive TSH down.

I'm unable to give direct advice as to individuals versus their health advisors, Mandy- so you have to read between the lines and decide for yourself.

Everyone is different, as stated- but most people arrange things to get the best result to present a case to their GP- if there is contention.

[mandy-white in reply to tegz]

Thankyou very much for your help tegz

[Apples]

Thank you, as an early morning tablet taker I shall either get a late afternoon blood test or skip my tablets until after an early one.