# Reverse T3 (rT3)

Reverse T3 (3,3',5'-triiodothyronine) is the inactive twin of T3. (T3 is the thyroid hormone that drives your metabolism.) It forms when an enzyme called D3 (5-deiodinase type 3) trims the inner ring of T4, instead of the outer ring. Labs measure plasma rT3 by radioimmunoassay or LC-MS/MS. Typical reference values are around 10 to 24 ng/dL (0.15 to 0.37 nmol/L). Levels jump in several states. These include non-thyroidal illness ('euthyroid sick syndrome'), starvation, severe trauma, and sepsis. They also rise with glucocorticoid, amiodarone, or propranolol therapy, and in liver disease. Functional-medicine circles like rT3 for diagnosing 'rT3 dominance' or guiding T3 therapy. But the major bodies disagree. The American Thyroid Association, AACE, and Endocrine Society do not recommend routine rT3 testing. Reviews find no validated clinical role for it, outside research and rare cases of consumptive hypothyroidism or thyroid-hormone resistance. The T3/rT3 ratio is just as unvalidated for healthy outpatients.

## Sources

- Peeters RP. (2017). Subclinical Hypothyroidism. New England Journal of Medicine. https://doi.org/10.1056/NEJMcp1611144
- Warner MH, Beckett GJ. (2010). Mechanisms behind the non-thyroidal illness syndrome: an update. Journal of Endocrinology. https://doi.org/10.1677/JOE-10-0017
- Krasner AS, et al.. (2020). Trust your Endocrinologist - Report and Recommendations on the Ordering of Reverse T3 Testing. Annals of Clinical and Laboratory Science. https://www.annclinlabsci.org/content/50/3/383.full

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_Canonical: https://longevity-germany.com/en/glossary/reverse-t3 · Part of Longevity Cities · Updated 2026-06-22_