Reverse T3
What it measures, and what changes the number.
Reverse T3 is an inactive form of triiodothyronine. The body makes it by removing an iodine atom from thyroxine (T4) at a different position than the one used to make active T3, so it has almost no thyroid hormone activity. The test measures its concentration in serum.
It is typically read alongside TSH, Free T4 and Free T3 (or Total T3), since it only makes sense in the context of how much T4 is available and how it is being converted.
The assay method matters. Some labs use liquid chromatography with tandem mass spectrometry, often written LC/MS/MS on the report, while others use immunoassays, and results from different methods are not directly comparable. Reports may list it as T3 REVERSE, LC/MS/MS or T3 reverse.