Free Research Tool
FT3 : rT3
Ratio Calculator
Convert your free T3 and reverse T3 lab values - in any common unit - into the FT3:rT3 ratio used to detect reverse-T3 dominance, with interpretation against the most-commonly-cited research-community thresholds.
Enter your lab values
Your FT3 : rT3 ratio
0.2
Functional hypothyroidism likely despite otherwise 'normal' labs. The reverse-T3 protocol literature (Wilson's WT3, combination T4 + SR-T3) is most relevant here.
Reference thresholds
| Ratio | Interpretation |
|---|---|
| > 20 | Excellent — active T3 dominates |
| 15 to 20 | Adequate |
| 10 to 15 | Borderline rT3 dominance |
| < 10 | rT3 dominance — functional hypothyroidism likely |
Based on the most-commonly-cited research-community thresholds using FT3 in pg/mL and rT3 in ng/dL. The calculator converts pmol/L (FT3) and ng/mL (rT3) automatically into these canonical units.
Important
This calculator is a research/educational tool and is not a substitute for clinical interpretation. The FT3:rT3 ratio is one data point among many - it should be considered alongside TSH, free T4, thyroid antibodies, ferritin, selenium, cortisol patterns, and clinical symptoms. Not medical advice.
Methodology
How the ratio is calculated
The canonical interpretation of the free T3 to reverse T3 ratio uses free T3 in pg/mL as the numerator and reverse T3 in ng/dL as the denominator. The calculator handles unit conversion automatically:
- Free T3: if entered in pmol/L, multiplied by 0.651 to convert to pg/mL (based on T3's molecular weight of 651 g/mol).
- Reverse T3: if entered in ng/mL, multiplied by 100 to convert to ng/dL.
Most US labs (Quest, LabCorp) report free T3 in pg/mL and reverse T3 in ng/dL by default. Most European and international labs report free T3 in pmol/L. The interpretation thresholds below assume canonical units; if your lab reports different units, the calculator does the conversion before computing the ratio.
Interpretation thresholds
- > 20: excellent - active T3 signal dominates
- 15-20: adequate
- 10-15: borderline rT3 dominance
- < 10: rT3 dominance - functional hypothyroidism likely despite "normal" labs
These thresholds reflect the most-commonly-cited research-community values. They are interpretive guides, not diagnostic cutoffs. Always consider the ratio alongside TSH, free T4, thyroid antibodies, ferritin, selenium, cortisol patterns, and symptoms.