Free Thyroxine Index (FTI / T7)
Free Thyroxine Index (FTI / T7): total T4 × (T3-resin uptake % ÷ reference-mean T3RU %), the binding-corrected free-hormone estimate used where a direct free-T4 assay is unavailable.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Total T4
- 12 µg/dL
- T3-resin uptake
- 25 %
Result: FTI 10 — total T4 12 × (T3RU 25% ÷ 30% reference).
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Use
- Corrects total T4 for binding-protein changes (pregnancy/estrogen raise TBG and total T4 but FTI stays normal). Interpret against the local reference range.
What you enter
- Total T4
- T3-resin uptake
- Reference-mean T3RU % (default 30)
How this is calculated
- Formula
- FTI = total T4 × (T3-resin uptake % ÷ reference-mean T3RU%, default 30%).
Clark F, Horn DB. Assessment of thyroid function by the combined use of the serum protein-bound iodine and resin uptake of 131-I-triiodothyronine. J Clin Endocrinol Metab. 1965;25(1):39-45. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.