Thyroid Question

So my acupunctarist recommended thyroid testing (since I have never had a full thyroid panel). My results came in (I just got them over the phone, the actual numbers are coming in the mail) and my dr. said everything looked good. She then said that there was one test that she doesn't order because it supposedly doesn't do anything, it's called the Reverse T3. The upper limit of this test is 350 (upper end of a range), and my test was 380. She said it didn't matter, but it seems like rT3 has to be created somehow and have some sort of relation to T3 and T4. Does anyone know anything about this? I tried looking it up on google, but didn't find anything written in terms I could understand without pulling out my biology book.