3,3',5'-Triiodo-L-thyronine (also known as Reverse T3 (rT3)) is an inactive mirror-image of the active hormone T3. It is formed from T4 hormone and binds to T3 receptors, thus blocking the action of T3. The ratio of rT3 to T3 is a valuable biomarker of the metabolism and function of thyroid hormones. An elevated ratio of rT3 over T3 is a sign of “sick euthyroid” syndrome and helps to exclude a diagnosis of hypothyroidism, particularly in critically ill patients. The rT3 test is performed by ELISA method.