Reverse T3 is an excellent marker for reduced cellular T4 and T3 levels not detected by TSH or serum T4 and T3 levels.
High or high normal rT3 is not only an indicator of tissue/cell hypothyroidism, but also that replacement of T4 only (for example Eltroxin), would not be considered optimal in such cases and would be expected to have inadequate or sub-optimal results.