Difference between free T4 and total T4

[mb008]

Hi again,

Sorry another question.

I've just looked this up but brain's not working well - can someone explain (simply) the difference between these two measures and what we should be looking out for when reading our test results.

Thanks

[jimh111]

Most T4 (and T3) is bound to serum transport proteins (TBG, TTR and albumin) and not available for use (a sort of storage process). It is the free T3 and free T4 levels that matter. The only time total T4 might be used is if a doctor wants an idea of how much hormone the thyroid is putting out, whether its capacity is diminished. Really its the fT3 and fT4 figures that are useful, the total figures are rarely of any use. The free figures tell you how much hormone is available.

[mb008 in reply to jimh111]

Perfect, thanks Jimh111.