Free T3 : Reverse T3

Introduction

The Free T3 to Reverse T3 ratio gauges how effectively your body converts inactive thyroxine into active triiodothyronine—a critical step in maintaining optimal metabolism and energy balance.

Background of Free T3 and Reverse T3

Your thyroid secretes thyroxine (T4), a prohormone that peripheral tissues convert into metabolically active Free T3 or inactive Reverse T3. During acute stress or illness, conversion may favor rT3, slowing metabolism to conserve energy.

What the Ratio Reveals

• Free T3 : Drives basal metabolic rate, thermogenesis, and nutrient utilization.
  
• Reverse T3 : Competes with T3 for receptor binding but lacks biological activity.
  
• Ratio: Free T3 divided by rT3; a balanced value indicates healthy conversion.

   

Normal and Optimal Ranges

• Standard: 12–20 (Free T3 pg/mL : rT3 ng/mL)
  
• Optimal: ≥20

   

Values below the standard suggest impaired T4→T3 conversion.

Clinical Implications of a Low Ratio

A reduced ratio often signals poor conversion and can manifest as:

1. Thyroid Hormone Conversion Syndrome: Symptoms of hypothyroidism despite normal TSH and T4—fatigue, weight gain, depression. See hypothyroidism.
   
2. Chronic Stress: Elevated cortisol from adrenal stress impairs deiodinase enzymes.
   
3. Inflammation & Oxidative Stress: Systemic inflammation and oxidative stress shift conversion toward rT3.
   
4. Gut Dysbiosis: Impaired GI function reduces T3 production via enterohepatic pathways.
   
5. Catabolic States: Trauma, infection, or starvation elevate rT3 to conserve resources.

    

Clinical Implications of a High Ratio

A ratio in the optimal or elevated range typically reflects efficient T4→T3 conversion and adequate thyroid activity, with lower risk of symptomatic hypothyroidism.

Factors That Interfere with the Ratio

• Medications: T4-only therapy (levothyroxine), amiodarone, glucocorticoids can skew the ratio.
  
• Nutrient Deficiencies: Selenium deficiency impairs deiodinase function.
  
• Acute Illness: Non-thyroidal illness syndrome (“low T3 syndrome”) reduces Free T3.

   

When to Test the Ratio

Consider measuring Free T3 and Reverse T3—and calculating their ratio—in patients with:

• Persistent hypothyroid symptoms despite normal TSH
  
• Unexplained fatigue, weight changes, or mood disturbances
  
• Chronic inflammatory or catabolic conditions

   

Functional Medicine Strategies

A holistic approach addresses root causes:

• Optimize Nutrients: Ensure adequate selenium and iodine intake.
  
• Manage Stress & Inflammation: Incorporate stress reduction, anti-inflammatory diet, and regular exercise.
  
• Personalized Hormone Support: In select cases, a combination of T4 and T3 (liothyronine) may be warranted.

   

Conclusion

In conclusion, a comprehensive evaluation by functional medicine telehealth in Florida facilitates identification of cellular-level and molecular imbalances impacting thyroid conversion. By integrating evidence-based therapies with regenerative medicine medical care services—where hormonal optimization is prioritized—we offer a regenerative, preventive framework to rebuild resilience and optimize wellness. Call (904) 799-2531 or schedule online to request your personalized thyroid evaluation.

Further Reading

• Wolff, J., & Dumont, J. E. (1976). Effect of amiodarone on serum triiodothyronine, reverse triiodothyronine, and thyroid-stimulating hormone in humans. The Journal of Clinical Endocrinology & Metabolism, 42(4), 728-735.  Pubmed

• Klieverik, L. P., et al. (2007). Bexarotene stimulates the peripheral metabolism of thyroid hormones. The Journal of Clinical Endocrinology & Metabolism, 92(8), 3175-3181. Pubmed

• Wilson, J. B., Hoang, T. D., Lee, M. L., Epstein, M., & Friedman, T. C. (2025). Reverse T3 in patients with hypothyroidism on different thyroid hormone replacement. PLoS ONE, 20(6), e0325046.Pubmed

• Gomes-Lima, C., Wartofsky, L., & Burman, K. (2019). Can reverse T3 assay be employed to guide T4 vs. T4/T3 therapy in hypothyroidism? Frontiers in Endocrinology, 10, 815. Pubmed