USA/UK Blood Result Variations?

[HarlesBarles]

Does anyone know if the ranges for TSH/FT3/FT4 are different in the UK and the USA? And why?

On two separate occasions over the last eight months, my FT3/4 results when visiting the USA were just above mid range, but then approx three weeks later when I have had bloods drawn in the UK, my levels were found to be over range. No medication changes before or after the tests and no changes in how I felt.

The USA tests were by blood spot and the UK tests I had blood drawn at the surgery. I saw an endocrinologist that a US friend sees when I was last in the USA and they were happy with my (blood spot) test results and said I could slightly increase my medication as I still have some unresolved symptoms. I didn’t increase as I wanted it to be with the blessing of my UK GP so when I got back home I had the bloods repeated via the NHS and annoyingly the FT3 and FT4 came back over range.

It’s happened twice now so I know it’s not just a fluke. I don’t know which is right but feel reasonably well and if anything slightly under medicated.

Has this happened to anyone else and which country’s guidelines do you follow: the one that says you’re fine and you can increase your medication to try to improve symptoms or the one that that says you’re going to have osteoporosis, a stroke and heart failure if you don’t decrease it???

[helvella Administrator Thyroid UK]

The fundamental issue isn't between the USA and the UK.

It is that thyroid blood tests have not been formally harmonised.

Two labs in the same country, using the same units, will likely produce differing results. More so if done on different analyser platforms.

Add in:

Time of day as there are circadian rhythms of hormone release;

Time between last dose and test;

Interference.

Not sure if blood spot testing generally produces differing results but, at heart, that is just another aspect of harmonisation.

[HarlesBarles in reply to helvella]

If that’s the case (and I’m sure you’re 100% correct) how does one know if they are in range or not? My ability to function in the world is controlled by the medication given to me based on blood levels.

Should I take the USA position that I’m fine and could possibly increase my dose or follow the UK medic induced fear that I’m destroying my future by feeling pretty good now!

[helvella Administrator Thyroid UK in reply to HarlesBarles]

Think of a very simple example. (Just one test to keep it simple.)

Free T4 - reference interval a typical UK one of 12 - 22.

Say your result is 23. And you feel good. Then maybe that is where you need it to be.

Then you get tested in the USA with a reference interval of 0.7 to 1.5.

This time, your result is 1.2. And you feel exactly the same as when you had your UK test.

You simply have to accept the tests are not 100% comparable. It could be that this is just how these tests are for you.

Makes it very difficult to have useful and meaningful discussions, I agree.

But, once you know where you are doing well, if a USA test came out at 1.6, or a UK test at 16, you'd know that they were not where you feel well. If you were feeling wrongly dosed, you'd have some sort of evidence for your self.

It is appalling. It is dire. But that is how the testing world is when there has been no proper harmonisation.

[HarlesBarles in reply to helvella]

That makes sense and is very helpful, thank you. If only my UK GP and Endo thought the same way!! Thank goodness for this platform and so much experience on here (Thank you so much helvella !!).