Reverse T3 Serum/Plasma

Reverse T3 (rT3) is an inactive isomer of T3, formed from T4. During periods of stress, illness, or caloric restriction, the body may preferentially convert T4 to rT3 instead of active T3, conserving energy. Elevated rT3 can indicate non-thyroidal illness ('euthyroid sick syndrome') or impaired T4-to-T3 conversion. The ratio of Free T3 to rT3 (e.g., FT3(pg/mL)/rT3(ng/dL) > 0.2) is sometimes used to assess conversion efficiency.