T4, T3

[fiftyone]

What is the difference between Free T3 and total T3; and free T4 and total T4. I presume that my usual thyroid tests are free T3 and free T4, but maybe I'm wrong?? Would thyroxine levels be very different for the 'free' and 'total' blood tests?

[greygoose]

There are two types of T4 and two types of T3 (there are actually three types of T3, but we won't go into the third one at the moment).

Free T4/T3

Bound T4/T3 - bound to proteins, so unusable by the body.

The Free T4/T3 test, as the name suggests, just tests Free T4/T3.

Total T4/T3 tests both the Free T4/T3 and bound T4/T3. But, it doesn't tell you how much there is of each.

So, as the body can only use Free hormone, that's the tests we need done.

If you're in the UK - and probably the rest of Europe - the tests that are done are Free, unless stated otherwise - but the Total T3 is rarely done, anyway. In the US, from what I've seen, they tend to test Free T4 and Total T3, for some reason.

The levels would be different, yes, because the ranges are set much higher and wider. A Free range is comparatively narrow and in single figures. A Total is usually in the 100s, I believe, or double figures at least - although it varies from lab to lab, of course.

[helvella Administrator in reply to greygoose]

The Wiki article on Thyroid Function Tests is quite useful as it shows some "typical" reference ranges in both mass and molar units - that is, USA and rest-of-world units.

en.wikipedia.org/wiki/Thyro...

(As usual, it is Wiki so don't be too trusting.)

[fiftyone in reply to greygoose]

thank you for that.

[fiftyone in reply to greygoose]

thank you for the explanation

[greygoose in reply to fiftyone]

You're welcome.