Thyroid UK
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What's the difference in 'normal' t3 and t3 from hyperstimulated remaining thyroid tissue?

I don't know if anyone knows but if we are producing lots of t3 via the peripheral tissues because of a low t4 is this the same 'quality' as the t3 that would have been produced from a normal amount of t4?

I ask because of my hpo symptoms and because I read that,

'However, serum T3 measurement, whether total or free, has limited utility in hypothyroidism because levels are often normal due to hyperstimulation of the remaining functioning thyroid tissue by elevated TSH and to up-regulation of type 2 iodothyronine deiodinase (75)’.

I don't really understand this? It says this type of t3 has limited use - so could this account for hypo symptoms despite high levels of t3 being available?

It implies that the t3 produced from overstimulation is somehow different from the t3 produced by usual conversion methods.

Can anyone help?

4 Replies

The T3 itself is no different - I think the issue is whether the process of measuring it is worthwhile.

They are saying that when people go hypothyroid, the body responds to that by increasing the amount of T3 it produces. So the measuring of T3 doesn't say much about your condition - because it is being measured in an odd state where you are producing more T3.

The Wiki article on the deiodinases (D1, D2 and D3) is here:

Not wonderful but might help. Don't expect to understand everything - and certainly not at first reading! :-)



Interesting reading Rod.

If you have ft3 and ft4 readings only (opposed to t3 and t4) could a ratio still be arrived at? They are measured in the same units but of course in differing ranges. I'm presuming its all still relative.

So, ft3 6 (range 0.5 - 5.7) and ft4 at 11.7 (range 9 - 24). Can this be converted into a ratio of ft3/ft4?


Not sure what you mean by "t4" - almost all T4 measurements are Free T4.

Similarly, most T3 measurements are Free T3 - but some labs do Total T3. (At least one lab does FT3 when processing tests for one hospital and TT3 for another hospital - both in the same connurbation.)

And they are not the same units! Usually in the UK:

FT4 is in pmol/L; TT4 is in nmol/L

FT3 is in pmol/L; TT3 is in nmol/L



Sorry Rod, maybe I'm confusing the issue. The wiki article talks about the ratio between t3 and t4 as being pretty relevant and can indicate the level of hypo with a cut off around 18ish. Above looking like one condition and below looking like another.

As we don't normally have t3 and t4 tests but free t3 and free t4 then what I was wondering is whether the ratio calcualtion can be done to these figures instead to arrive at a ratio of ft3 to ft4.

The two free figures are of course in the same units as each other pmol/L - as are the two total figures as you say nmol/L. I'm working on the two free figures only so that's ok as they are in the same units.

This must mean that as long as we convert each of the free figures into the same type (i.e. it is possible to turn each into a percentage in its own right) then we can ratio one against the other.

So, the ft3 figure I have of 6 from a range of .05 -5.7 would come out at aorund 110% and the ft4 figure of 11.7 from a range of 9 - 24 would be around 24% (my maths is pants). That gives a ratio of around 110:24.

What I don't get is how this ratio is then converted into a single figure as seen at the end of the wikki article above.

Is that any clearer or have I made things worse?


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