Reverse T3 (rT3)

Reverse T3 (3,3',5'-triiodothyronine) is the biologically inactive isomer of T3 produced when 5-deiodinase type 3 (D3) cleaves the inner ring of T4 instead of the outer ring. Plasma rT3 is measured by radioimmunoassay or LC-MS/MS, with typical reference values around 10-24 ng/dL (0.15-0.37 nmol/L). Levels rise sharply in non-thyroidal illness ("euthyroid sick syndrome"), starvation, severe trauma, sepsis, glucocorticoid, amiodarone or propranolol therapy, and during hepatic dysfunction. Although popular in functional medicine for diagnosing "rT3 dominance" or guiding T3 therapy, the American Thyroid Association, AACE and Endocrine Society do not recommend routine rT3 measurement; reviews and consensus statements conclude there is no validated clinical role for it outside research and rare presentations of consumptive hypothyroidism or thyroid hormone resistance. The T3/rT3 ratio is similarly unvalidated for healthy outpatients.