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Question: "What result on the Free T3: Reverse T3 ratio is suggestive of conversion issues? Your range seems to be 100X larger than the one I was taught!"

The clinical significance of the Free T3:Reverse T3 ratio

The clinical significance of the Free T3:Reverse T3 ratio is a ratio value below 10, which is a sign of growing Thyroid Hormone Conversion Syndrome i.e. the peripheral conversion of T4 into the more metabolically active T3 is decreased. A high ratio is not clinically significant other than being an optimal finding!
You asked about the 100X/ There is an issue in the literature around the actual range for the ratio. This is due to the fact that Free T3 is reported in a couple of different units: pg/ml ( the range we use in the software) and pg/dl. In order to convert pg/dl into pg/ml, for the use in this software, you must divide by 100, i.e. 300 pg/dl is 3.0 pg/ml and vis versa, to get from pg/ml to pg/dl you must multiply by 100, i.e. 3 pg/ml is 300 pg/dl. This is where that 100X comes in!
In the software, we collect free T3 in pg/ml but the calculator we use for the ratio uses Free T3 in the larger pg/dl. We do the conversion automatically but the result is going to be larger by a factor of 100 over the range used by some people, i.e. 0.1 becomes 10 and 0.2 become 20, etc.