Free Thyroxine Index (FTI)
The free thyroxine index is calculated from total T4 and T3 uptake and estimates free T4 when binding protein levels are altered. It was widely used before reliable direct free T4 assays and remains useful in pregnancy, estrogen therapy, and other states that affect thyroid binding globulin. Abnormal FTI parallels free T4 changes in hyper- and hypothyroidism.
1 providers include this
Thyroid Function
1.4-3.8
What is Free Thyroxine Index (FTI)?
The free thyroxine index is calculated from total T4 and T3 uptake and estimates free T4 when binding protein levels are altered. It was widely used before reliable direct free T4 assays and remains useful in pregnancy, estrogen therapy, and other states that affect thyroid binding globulin. Abnormal FTI parallels free T4 changes in hyper- and hypothyroidism.
Why It Matters
Adjusts Total T4 for binding protein variations. Useful when binding proteins are abnormal (pregnancy, estrogen use). Correlates with metabolic status.
Which Providers Test Free Thyroxine Index (FTI)?
Full Provider Comparison
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | — | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | — | $349 | 65 |
 Labcorp OnDemand | — | $398 | 40+ |
| — | $486 | 40+ | |
| — | $449 | 288 | |
| — | $349 | 100+ | |
| — | $829 | 54 | |
 Function Health | — | $365 | 160+ |
| — | $250 | 65 | |
| — | $495 | 80+ | |
| — | $895 | 100+ | |
| — | $1950 | 130+ | |
| — | $375 | 95+ | |
| — | $Varies | 75+ | |
| — | $190 | 100+ | |
| — | $99 | 50 | |
| — | $124 | 60 | |
| — | $349 | 130+ | |
| — | $349 | 80+ | |
| — | $375 | 85 | |
| ✓ | $700 | 128 | |
| — | $99 | 100+ |
Frequently Asked Questions
What is Free Thyroxine Index (FTI)?
The free thyroxine index is calculated from total T4 and T3 uptake and estimates free T4 when binding protein levels are altered. It was widely used before reliable direct free T4 assays and remains useful in pregnancy, estrogen therapy, and other states that affect thyroid binding globulin. Abnormal FTI parallels free T4 changes in hyper- and hypothyroidism.
What is the optimal range for Free Thyroxine Index (FTI)?
The standard reference range for Free Thyroxine Index (FTI) is 1.4-3.8. Optimal ranges may differ based on individual health goals and expert recommendations.
Which blood test providers include Free Thyroxine Index (FTI)?
1 out of 22 blood testing providers include Free Thyroxine Index (FTI) in their panels. This biomarker has limited availability.
What category does Free Thyroxine Index (FTI) fall under?
Free Thyroxine Index (FTI) is categorized under Thyroid Function. This category includes biomarkers that help assess related aspects of health and wellness.
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Medical Disclaimer
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
Last reviewed:2026-04-12