The Reverse T3 (rT3) Enzyme Immunoassay is a diagnostic test designed to measure reverse triiodothyronine (rT3) levels in serum or plasma. rT3 is an inactive metabolite of thyroxine (T4) that can increase during physiological stress, illness, or altered thyroid function. This assay provides accurate and reliable measurement of total rT3, supporting the evaluation of thyroid hormone metabolism and aiding in the assessment of conditions such as non-thyroidal illness syndrome and impaired peripheral conversion of T4 to T3.