This is a thyroid test that reportedly many fibromyalgia and CFS patients are failing. This test is also known as R-T3. Many Endocrinologists don't seem to be aware of this test.... but this is something that may be of interest to you if you have thyroid symptoms and/or thyroid problems.
http://www.holtorfmed.com/nss-folder/printable_pdf/thyroid/reverse_r3_%20thebest.pdf --- "Reverse T3 is the Best Measurement of Tissue Thyroid Levels"
http://thyroid.about.com/library/glossary/bl-reverset3.htm --- Reverse T3