I am at a loss to understand something, and I have been thinking about it for some time.

We are told that the TT4 to TT3 ratio in our blood is about 20 to 1 (very round numbers here). i.e. at any given time we expect to see a lot more TT4 than TT3. We are also told that a measure of efficiency of T4 to T3 conversion is given by the ratio of TT3 to TT4.

However, we are also told that this ratio should be greater than 17-20. I am not sure if “they” mean it should be a number between 17 and 20, but never mind that for a minute.

If we have more TT4 than TT3 shouldn’t this be the other way round? i.e. TT4/TT3?

The “traditional” way of getting the diagnostic number is to use the units in which they were measured, i.e. ng/dL for TT3 (the numerator) and mcg/dL for TT4 (the denominator), this gives you the number which is (hopefully) greater than 17-20. BUT this appears to me to be upsidesdown – unless I am missing something very obvious. Has anyone else noticed this?

Example: I have TT3 = 106 ng/dL and TT4 = 5.1 mcg/dL. This gives me a TT3/TT4 of about 21, which is fine.

If you convert the units to be the same (which is what you should do for a ratio) you get .021 (you need to * by 1000 to get back to the 21 – maybe that is another tradition).

If you do the TT4/TT3 ratio which is intuitively more correct, you get 47.6 – which is on no-one’s scale.

I am so confused.