Totally confused over TT3/TT4 ratio…….or is it?

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.


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10 Replies

  • The TT4 and TT3 are useless tests, anyway. They don't give you any useful information. What you need are the FT4 and FT3. But, can't help you with the maths! I'm hopeless! lol

  • Heavy going but quite interesting

  • Thank you very much for this reference. I have only read the abstract so far but I intend to plow through it in great detail, it is very interesting - however, while the healthy people will be very happy with their conclusions us hypos and hypers will no be so happy. I will get back to you, thanks.

  • Total hormone measurements are strongly influenced by how much T4 and T3-binding proteins you have in your circulation. The total hormone represents essentially the reservoir of hormone bound on the proteins, ready to be released as free when required by the cells. Reservoir capacities for T4 and T3 can vary in individuals by as much as a factor of 10. Some have low capacities because they totally lack the strongest binder in their blood. Others have 4 times the amount of the strongest binder than the average. So to get the same FT4 or FT3, you can have a 10 fold difference in total T4 or total T3 depending on your reservoir size. Now, there's an extra complication in that T4 and T3 bind to the proteins with different strengths. So any difference in protein amounts affects the T4/T3 ratio as well as the total amounts of each. This means it isn't as good a measure as FT4/FT3 because this corrects for the binding protein differences from person to person.

  • Thank you very much, that puts a much different light on things. Pardon my ignorance but are the binding proteins you refer to the same as what is called the thyroid binding globulin? I agree with you that FT4/FT3 is a much more meaningful ratio and I will stop worrying about TT4/TT3 whichever way up it is calculated. And btw, I think I understand why it is done upside down, it has been calibrated against the correct way of doing it and a far simpler scale was discovered which enabled doctors to do an instant calculation using the units in which they had just done their measurements, in physics it would be known as changing the frame of reference.

    But do let me know about the proteins vs the TBGs, I am not very biological, I am just trying to survive without a thyroid (I had thyroid cancer).

  • There are 3 such proteins. The most important is thyroxine binding globulin. On average this takes about 2/3 of the bound T4 onto it and about 60% T3. The other two are called transthyretin and albumin. On average they bind 25% and 10% of the T4. T3 hardly binds to transthyretin at all but similar to T4 for albumin. You can see how complicated the relationship is between FT4 FT3 and the totals with these quite random differences person to person.

  • Wow Diogenes, that's amazing stuff, thank you, I'm glad I asked. It gives us a whole new set of variables to add to the ones we are already trying to understand and balance. I hope you do not mind but your answer raises another question in my mind. It is said that we shouldn't take blood pressure (BP) lowering drugs because they use the TBGs to "travel" to the cells of our bodies and thereby leaving a deficiency for our thyroid meds. Do you know of any BP reducing drug which doesn't use the TBGs that maybe use one of the other proteins (the transthyretin or albumin.)?

  • It won't matter; if something interferes with T4 binding and cell transport, the thyroid and pituitary simply readjust to keep the status quo. There may be a really short term effect but it's soon over

  • Perhaps you should convert the m/gs to n/gs e.g. 5100/T3

  • If converting units fascinates you the way it fascinates me then give this calculator a whorl, you can have hours of fun with it. Units are the heart and soul of thyroid measurements and ratios - I've never seen so many different ones.


    You will be using the "density" tables - yes, you can see that from the url, BUT…..if you ever lose that, it's a long way back.


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