Reverse Triiodothyronine
Reverse T3 is an inactive form of triiodothyronine produced when your body converts T4 down a non-active pathway. It is a less commonly used thyroid measurement.
Your body can steer T4 toward either active T3 or inactive reverse T3. Reverse T3 often rises during illness, fasting, or stress as a normal adaptation, which is why it is sometimes measured when results are hard to interpret. Its clinical usefulness in routine testing is debated among experts.
A high reverse T3 commonly reflects the body's response to acute illness, injury, or caloric restriction rather than a thyroid disorder itself. It is not a standalone diagnosis, and interpretation should be cautious. Your clinician can place it in context.
A low reverse T3 is generally not considered clinically concerning on its own and is rarely acted upon in isolation.
Reference ranges vary widely between labs, and reverse T3 is not part of standard thyroid screening. Your clinician can advise whether it adds value for your specific situation.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.