With expanding knowledge of thyroid function complexities, it is becoming clear that reverse T3 (rT3) can serve as an excellent marker of tissue hypothyroidism which is not usually detected by standard TSH or serum T3 and T4 levels.
PHYSIOLOGY Under any kind of physiological or emotional stress, illness, inflammation, dieting, depression and numerous other conditions, T4 is preferentially converted to rT3 which acts as a competitive inhibitor of T3, reduces metabolism, suppresses T4 to T3 conversion and blocks T4 and T3 uptake into the cell.
CLINICAL RELEVANCE Numerous publications consider rT3 as a valuable biomarker of the metabolism and thyroid function because the process of conversion of T4 to T3 and rT3 to 3,3’-T2 is inhibited in a number of non-thyroidal conditions combined by the name „Sick euthyroid” syndrome (ESS) or “Low T3” syndrome.