It's a measure of your conversion rate from T4 to T3 in a sense, and can be used with T3, and T4 to determine whether one problem exists. There are two forms of T3, regular T3 and reverse T3. Chemically they have the same content, atoms wise, but they are put together differently. Regular T3 is the only one that is put together so the body can use it. Reverse T3 is not usable by the body. If there is more RT3 than T3, then it can point to a problem.