What rT3 is
Reverse T3 is the inactive isomer of T3, same chemical formula, different arrangement. Produced by deiodinase D3 from T4 instead of active T3 via D2. rT3 doesn't activate thyroid hormone receptors but does occupy them, partially blocking T3 effects.
When it's elevated
- Acute illness (appropriate adaptive response)
- Chronic stress (HPA dysregulation)
- Caloric restriction / dieting
- Severe inflammation
- Liver disease
- Some medications
- Selenium deficiency
Free T3 / Reverse T3 ratio
The ratio captures the balance between active and inactive conversion:
- Calculation: free T3 (in ng/dL or pg/mL, match units) / rT3 (ng/dL)
- Healthy ratio: >20 (using pg/mL for both, with appropriate conversion)
- Suboptimal: 15-20
- Poor conversion: <15
Interpretation
Elevated rT3 with normal TSH and T4 indicates the body is converting T4 toward inactive form. Symptoms can mimic hypothyroidism despite "normal" labs.
Treatment
- Address chronic stress (sleep, lifestyle, HPA support)
- Adequate caloric intake (avoid extreme restriction)
- Selenium adequacy (200 µg daily often)
- Treat underlying inflammation
- For severe cases, T3 supplementation can bypass conversion problem (specialist territory)
The clinical pearl: Elevated rT3 is a sign that the body is putting brakes on thyroid signaling. The fix is usually upstream, address the chronic stress, caloric restriction, or inflammation driving the conversion shift.
Bottom line
Reverse T3 reflects conversion balance. Elevated rT3 indicates dominant inactive conversion, common in chronic stress and illness. Free T3 / rT3 ratio quantifies the imbalance. Treatment addresses underlying cause.