Thyroid UK
84,232 members99,231 posts

Thyroxine before blood test - how much difference does it make?

I had my last blood test before finding this site (which is brilliant by the way). I had never been told not to take my Thyroxine before my blood test.

I take it at around 6.30am and had a blood test about 9.15am. My TSH was 0.95 (0.5-5.5) so all seems OK but I have symptoms and I am wondering if the result could be flawed by me taking my Thyroxine and if so by how much (I take 75 mcg).

I also had B12, Glucose and I think Ferretin tested which were all OK but I don't know the figures. (I never asked because I would not have understood them at the time).

2 Replies

Usually we expect TSH to change slowly - which is why so many people are told to wait 8 weeks or more before a repeat test. So not very much impact on that. (The time of day is more likely to be important. So if you want your TSH to show its highest value, have the blood drawn as early as possible. As you did.)

However, if you have a Free T4 test, that most definitely can look much higher within hours of taking a levothyroxine tablet. As you can never be sure if they are, or are not, going to do an FT4 test, best be prepared and avoid taking in the, say, 12 hours before having blood drawn.

It is impossible to say by how much - we are all different and the relationship between taking a tablet and exact value of FT4 is not straightforward.

Please do get your other test results - and post them.



Even 12hrs can cause a higher T4 reading. Mines been lower since I stopped taking Thyroxine before (which is somewhat working in my favour with getting a higher thyroxine doseage now). I take it at night so would have around 12hrs inbetween dosing and blood test.


You may also like...