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Calculates the free thyroxine index (FTI or T7) from total T4 and the T3 resin uptake, an indirect estimate of free thyroid hormone used when direct free T4 assays are unavailable or when binding-protein status is in question.
FTI = Total T4 (µg/dL) × T3 Resin Uptake (%) ÷ 100
Adult reference range is typically ~1.0–4.5 (assay-dependent; usually interpreted with the T4 Uptake)
The free thyroxine index (FTI = total T4 × T3 uptake) estimates the free (active) T4 fraction when direct assays are unavailable. It is a well-established, long-standing endocrine laboratory index that corrects for variations in thyroid hormone-binding proteins.
Specialty
EndocrinologyCategory
EndocrinologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Adult reference range is typically ~1.0–4.5 (assay-dependent; usually interpreted with the T4 Uptake)
| Classification | Range |
|---|---|
Low typical adult reference (varies by assay) | <1.0 |
Normal typical adult reference (varies by assay) | 1.0–4.5 |
High typical adult reference (varies by assay) | >4.5 |
Calculates the free thyroxine index (FTI or T7) from total T4 and the T3 resin uptake, an indirect estimate of free thyroid hormone used when direct free T4 assays are unavailable or binding-protein status is in question.
The typical adult reference range is approximately 1.0–4.5, though this is assay- and laboratory-specific. FTI below 1.0 is low (hypothyroidism pattern), and above 4.5 is high (hyperthyroidism pattern). The index corrects total T4 for changes in thyroid hormone-binding proteins but is largely superseded by direct free T4 assays where available.
The free thyroxine index was a long-standing standard for estimating free hormone when direct assays were unavailable, correcting total T4 for binding-protein variation in pregnancy, estrogen therapy, and systemic illness.
A 40-year-old woman on oral estrogen has a total T4 of 8 µg/dL and a T3 resin uptake of 30%.
Inputs:
FTI = 8 × 30/100 = 2.4. This value is within the typical adult reference range (1.0–4.5).
Free T4 is a direct (or calculated) measure of unbound T4. FTI is an indirect index derived by multiplying total T4 by the T3 resin uptake, used when direct free T4 measurement is not available or binding-protein effects are confounding.
Abnormalities in thyroid hormone-binding proteins (pregnancy, estrogens, androgens, liver disease, nephrotic syndrome) and drugs (salicylates, phenytoin, heparin) can alter total T4 and T3 uptake, changing the FTI even when free hormone is normal.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Use the performing laboratory's reference interval and interpret the FTI together with TSH and clinical status.