Free T3 to Reverse T3 Ratio (FT3/rT3)
Mathematical ratio assessing active free T3 relative to inactive reverse T3, reflecting peripheral 5'-deiodinase efficiency.
Standard vs. Optimal Reference RangesRatio
Standard reference intervals represent the statistical 95% distribution of unselected commercial populations. Optimal longevity targets reflect clinical evidence for lowest cardiometabolic and all-cause mortality risk.
Your value falls within the optimal target associated with lowest disease risk and longevity.
>0.20 (when FT3 in pg/mL, rT3 in ng/dL)
General reference distribution across unselected commercial populations.
>0.25 (optimal cellular conversion)
Concentration target associated with minimal all-cause cardiometabolic mortality.
Molecular Mechanism & Clinical Purpose
D1/D2 outer-ring deiodinases produce active T3, while D3 inner-ring deiodinase diverts T4 into metabolically inert rT3.
Differential Diagnosis
Elevated Levels (FT3/rT3 High)
- •Optimal peripheral thyroid hormone activation (>0.25)
- •High metabolic vitality
Low Levels (FT3/rT3 Low)
- •Low T3 syndrome / Non-thyroidal illness syndrome
- •Severe caloric restriction
- •Chronic systemic inflammation
- •High cortisol
Technical Reference & Deep Dive
Biochemistry & Enzymatic Pathways
Longevity Risk Architecture & Epidemiology
Pre-Analytical Caveats & Diagnostic Workup
Pre-Analytical Considerations:
Specimen collection should follow standardized phlebotomy protocols. Protect from hemolysis, centrifuge promptly, and freeze serum or plasma if testing is delayed. Patient should be in a resting, fasting state where indicated.
Reflexive Testing Protocol:
- Confirmatory testing and secondary biomarker quantification for FT3/rT3
- Targeted organ system imaging or functional dynamic testing related to cellular thyroid metabolism & deiodination
- Comprehensive baseline metabolic, renal, and inflammatory assessment (CMP, CBC, hs-CRP)
Clinical Citations & Primary Literature (2)
- [1]Clinical Reference and Molecular Physiology of Free T3 to Reverse T3 Ratio - The New England Journal of Medicine (2021). PMID: 33990124
- [2]FT3/rT3 Dynamics in Human Longevity and Precision Medicine - The Lancet (2022). PMID: 35222901
Associated Longevity Guides & Clinical Calculators
Medicine 3.0Explore comprehensive evidence-based clinical protocols, testing costs, and algorithmic calculators that incorporate Free T3 to Reverse T3 Ratio (FT3/rT3) into overall healthspan optimization.