Free thyroxine index optimal range and reference range

Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026

Sera's bands for Free thyroxine index:

  • Optimal 1.2 – 4.9; out of range below 1.2 or above 4.9.

Evidence grade: Published reference interval. A band that primarily reflects a published adult laboratory or population reference interval. It is not necessarily applicable to every assay or laboratory.

All units

UnitOptimalSufficientOut of range
ratio1.2 – 4.9—below 1.2 or above 4.9

No separate Sufficient band is currently defined for this marker.

What the evidence says

The free thyroxine index is calculated by the laboratory from total T4 and T3 uptake. It estimates free T4 by correcting total T4 for binding proteins, and it is now largely replaced by free T4 measured directly. The band is 1.2 to 4.9, Labcorp's interval, where the index is total T4 times percent uptake. Laboratories that use a binding ratio instead print numbers on the T4 scale, about 4.4 to 11.4, so check which scale the report uses before reading a result. Read it with TSH. It is not reliable when binding proteins are very abnormal, in pregnancy or during acute illness.

Sources

  • Thyroid Panel sample report (test 000455) — Labcorp, 2026 · Laboratory / reference-interval sourceLabcorp's sample report for the thyroid panel prints total T4 4.5 to 12.0 μg/dL, T3 uptake 24 to 39 % and free thyroxine index 1.2 to 4.9. Matches the Labcorp-scale row.
  • Thyroid Profile (test 000455) — Labcorp test directory, 2026 · Laboratory / reference-interval sourceDirectory entry for the panel: describes the free thyroxine index as an index correlating with free T4. No numbers on the page.
  • T Uptake (Thyroxine Binding Capacity) (test 918) — Northwell Health Laboratories, 2026 · Laboratory / reference-interval sourcePrints both scales: FTI 4.4 to 11.4 (from the binding ratio) and percent FTI 1.4 to 4.8 (from percent uptake), and calculates the index when T4 is also ordered.
  • ABIM Laboratory Test Reference Ranges, January 2026 — American Board of Internal Medicine, 2026 · Laboratory / reference-interval sourceUS reference table: free thyroxine index (estimate) 5 to 12, on the ratio scale.

What Free Thyroxine Index (FTI) measures

Total T4 corrected for binding proteins using T3 uptake, calculated by the laboratory.

An older estimate of free T4, now largely replaced by measuring free T4 directly. Read it against free T4 where both are available.

Related thyroid health markers

See Free thyroxine index in the full list of biomarker ranges

About the author

Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture. She is not a clinician: these ranges are compiled from the published sources linked on each page, with every source named. Alina Radu on LinkedIn

This page is a reference for practitioners, not medical advice. Sera's bands are practitioner targets for structured review, not diagnostic thresholds, and Sera does not diagnose or recommend treatment. Read a result alongside the interval printed by the issuing laboratory and the client's history and symptoms. Range set: Sera Production, version 1.0, last reviewed November 9, 2026.