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.
Optimal range: who says what
No institution or named practitioner we track states an optimal target for Free Thyroxine Index (FTI). How we verify ranges.
- General guidance, no named sourceWithin referenceCalculated legacy index; no separate optimal. Largely superseded by direct free T4 assays
- Lab reference interval1.4-3.8What a lab report flags against. A lab interval is not an optimal target.
1 providers include this
Thyroid Function
Why Free Thyroxine Index (FTI) 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)?
Of the panels we itemize, only Vitality Blueprint Elite ($700/year, 138 biomarkers) includes Free Thyroxine Index (FTI).
1 panel that includes it, cheapest first
| Provider | Annual Cost | Total Biomarkers | $/Biomarker |
|---|---|---|---|
| $700 | 138 | $5.07 |
$/Biomarker is annual cost divided by the panel's advertised biomarker count, not the price of this one test. We track 23 blood test providers (32 plans); 21 of those plans are itemized marker by marker. Compare all of them.
20 itemized panels do not include Free Thyroxine Index (FTI)
Superpower · WHOOP Advanced Labs · Labcorp OnDemand · Life Extension Elite · Everlywell · Mito Health · InsideTracker · Function Health · Marek Health Base · Marek Health Comprehensive · Marek Health Complete · Marek Health Executive · Blueprint · Empirical Health · Oura Health Panels · SiPhox Health · Hims Labs · Healthspan · Vitality Blueprint Standard · Ultrahuman Blood Vision Annual
11 providers not itemized in our matrix
We have not itemized these marker by marker (some sell tests one at a time), so this page cannot say whether they offer Free Thyroxine Index (FTI). Check with them directly.
Life Extension Standard · Quest Health · Ultrahuman Blood Vision · Lifeforce · Hundred Health · Personalabs · Walk-In Lab · HealthLabs · WHOOP Specialized Panels · Marek Health Essential · Vitality Blueprint Upload
Frequently Asked Questions
What is the optimal range for Free Thyroxine Index (FTI)?
No institution or named practitioner we track states an optimal target for Free Thyroxine Index (FTI). General guidance without a named source: Within reference (Calculated legacy index; no separate optimal). Lab reference interval: 1.4-3.8. That is what a lab report flags against, not an optimal target.
Which blood test providers include Free Thyroxine Index (FTI)?
1 of the 21 blood testing panels we itemize marker by marker include Free Thyroxine Index (FTI) in their panels. This biomarker has limited availability.
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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 Aug 26, 2026 · First published Nov 30, 2025