Reverse T3 is a metabolite of T4 produced when the body deiodinates the 'wrong' iodine off the molecule. Biologically inactive but occupies T3 receptors, blocking active T3.
Mechanism — Stress, illness, fasting, calorie restriction, and inflammation shift deiodinase activity away from active T3 production and toward rT3. The 'sick euthyroid' or 'low T3 syndrome' pattern — TSH normal, Free T4 normal, Free T3 low, rT3 elevated.
Lab range — Reference 10-24 ng/dL. Free T3 to rT3 ratio more informative: greater than 20 (in ng/dL units) suggests good conversion; under 14 suggests thyroid resistance.
Caveats — Don't treat rT3 in isolation. Treat the upstream cause: chronic stress, undereating, overtraining, inflammation. Direct T3 (Cytomel) lowers rT3 quickly but masks the underlying issue.