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 The most reliable diagnosis of RT3 problems is by looking at the ratio of RT3 to RT3. This is made more complex by the many different units that are used for these measurements by different labs. The first step is to make sure than both measurements are in the same basic units which fall into 2 categories, metric, and molecular weight. Examples of metric units are:- T3 REVERSE     552     90-350 pg/mL     T3 FREE     3.23     2.50-3.90 pg/mL FER 62 24-336 ng/dL Examples of molecular weight units are:- Reverse T3 800 pmol/L (170-450) FT4 22.0 pmol/L (9.0-22.0) FT4 15.4 pmol/L (9.0-22.0) You need to have measurements that are all metric or all molecular weight, mixing them stops a direct calculation The Molecular weight ones all have "mol" in them. These are mainly used in Europe   Whatever units are used you need to make sure that the FT3 number is about 10 times the RT3 number and move the decimal point if need be to achieve this. An example would be FT3    3.23, RT3    500 in this case the FT3 is a lot smaller and you need to move the decimal point 3 places to the right making it 3230 then 3230 / 500 =  6.46 = a horrible ratio, this needs to be 20 or better Other ways you may see the same numbers are FT3 323, RT3 0.5, in this case you need to make the FT3 into 3.23 and that gives 3.23 / 0.5 = 6.46, the same sum with the decimal point moved on both units If you are finding this difficult while brain fogged then post your lab results on the RT3 group and one of the mods will calculate it for you. If you are not already a member of that group then you can click here to join There is more on these calculations on this link in the files section of the yahoo RT3 group (you need to be a group member and signed in to use this) There are a few files there about calculating the RT3 ratio; the one most often used is "Calculate RT3 Ratio in this Excel File"