The "off-switch" thyroid hormone — high Reverse T3 blocks T3 receptors and signals chronic stress, illness, or overtraining.
Reverse T3 (rT3) is an inactive isomer of T3 produced when the body shunts T4 conversion away from active T3. It binds the same nuclear receptors as T3 but doesn't activate them — effectively blocking T3 signaling. Elevated rT3 is the body's "metabolic brake" during prolonged stress, severe caloric restriction, chronic illness, or trauma.
The Free T3 / Reverse T3 ratio is more informative than either alone. A ratio below 0.2 (or some clinicians use 20 if both in the same units) suggests conversion is being shunted toward inactive rT3 — often the cause of unexplained fatigue, cold intolerance, and body temp drops despite "normal" TSH.
Reverse T3 is influenced by: chronic stress (cortisol drives the shunt), severe caloric restriction (extended dieting), acute illness (universal rise), inflammation, low selenium/zinc, beta blockers, amiodarone, pregnancy, and overtraining without adequate recovery. Returns to normal as the trigger resolves.
For users running tight diets, intense training, or stressful life seasons — track rT3 with Free T3. Persistently elevated rT3 suggests pulling back training volume or increasing recovery resources. Some clinicians use low-dose T3 to help "clear" rT3-blocked receptors when conversion is impaired. Not commonly moved by peptide protocols themselves; more reflective of the broader stress environment.
Upload any lab PDF and MyProtocolStack maps your values to Reverse T3 and 40+ other biomarkers. StackAI interprets the trend in context of your protocol.
Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.