Understanding Reverse T3 (rT3): Normal vs Optimal Ranges

?What is Reverse T3 (rT3)?

Reverse T3 (rT3) is a biologically inactive metabolite of thyroxine (T4), produced by an alternative deiodination pathway. While active T3 is the primary hormone acting on cells, reverse T3 competes with T3 at thyroid hormone receptors without activating them. Elevated rT3 can functionally antagonise active T3 action.

!Why It Matters

Elevated rT3 occurs during physiological stress, severe illness, calorie restriction, high cortisol, or conversion problems. It can cause hypothyroid-like symptoms (fatigue, weight gain, brain fog) even when standard TSH and Free T4 appear normal. The Free T3/rT3 ratio is used by functional medicine practitioners to assess thyroid function beyond standard tests, though its clinical utility remains debated in conventional medicine.

Reference Ranges

Lab normal ranges may vary between laboratories. Optimal and longevity targets are based on research literature and should be interpreted with your physician.

Symptoms of Imbalance

• Fatigue, weight gain, brain fog despite normal TSH and T4
• Low body temperature, cold intolerance

How to Improve Your Levels

• 1Address underlying stressors — high cortisol drives rT3 production
• 2Optimise selenium levels for proper deiodinase function
• 3Adequate caloric intake — severe restriction elevates rT3
• 4Treat underlying illness driving the conversion shift

When to Test

Functional thyroid assessment; unexplained hypothyroid symptoms with normal TSH/T4; severe illness monitoring.

Related Biomarkers