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.
Not really, the conversion problem is all. There's not really much clinically proven that could be done if you end up with more RT3 than T3. Usually that test isn't ordered that much, unless you're dealing with E. Denis Wilson's sychophants as far as Wilson's Syndrome is concerned. Of course, in numerous ways, that's been proven to be quack medicine (I'd love to talk to an impartial party that's actually been helped by his "get rich quick scheme").
I'm surprised usually that any doctor would order the RT3, and yes it does tell you something, but really there isn't that much that could be done about it.