Thyroid UK
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Blood tests T3 and free T3

For my next blood test should I ask for T3 and T4, and Free T3 and Free T4? Is there a difference between T3 and Free T3?

I will ask for TSH and TPO and TG as well as vitamin D3 and B12, and ferritin and folate. Anything else?

I'm recovering from high TSH which led to GP increasing my Levo to 125 mcg, and from deficient in D3. My Iron and B12 tested OK (within range) six months ago but i wonder if I still suffer iron and B12 deficiency too. I read the comments hrre and see that symptoms of high TSH and low D3 or B12 can be similar. Any guidance gratefully received.

6 Replies

Hi Adam,

This is a link and I've found it helpful. It is a bit of a mystery trying to wade through unfamiliar things.


Thank you shaws, very useful link, it feels like rocket science at the moment


There is a difference. Free T4 and Free T3 measure the actual active fraction of hormones that are entering your cells and controlling your health. The totals are the reservoirs of hormone that circulate in blood, bound on to transport proteins specially designed for the purpose. These release T4 and T3 into the free fractions as your tissues need them. Problem is that people are very variable in the amount of transport proteins they have, and thus the hormone reservoirs. They can change by at least 2-fold from person to person, and in rare people even more than that. For example a pregnant woman has twice the level of proteins, and twice the total T4/T3 that she has when not pregnant - but the same or even slightly less free T4 and T3. Summarising, its not the reservoirs that matter but the supply of ready available hormone to your tissues.


Thank you Diogenes for the explanation and greater clarity


Hi Diogenes,

So does this mean it's better to get free t3 and free t4 tested (rather than t3 and t4)?

And can conventional drs do the free t3 and free t4 test?

If u could let me know I'd really appreciate it.


Starbys :-)


Yes, FT4 and FT3 tests are readily available. Its only if you are totally average that total T4 and T3 tests are equivalent. If you are different from the average in the transport protein levels in your blood, then FT4/FT3 tests are definitely better as diagnostics.

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