Free thyroxine index (FTI) calculator
A calculated estimate of how much active thyroxine (T4) is available, worked out from total T4 and T3 uptake. It is an older alternative to measuring free T4. Type your results in the units on your report and it works out Free thyroxine index as you type, in your browser.
AI-drafted. Awaiting verificationLast reviewed 2026-10-03Thyroid
Type your results above, or fill in the example, to see Free thyroxine index.
Nothing you type here leaves your browser.
Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.
How it’s worked out
The formula
FTI = total T4 (µg/dL) × T3 uptake (%) ÷ 100
A worked example
Example numbers, not anyone’s real results, run through the same code as the calculator above.
| Total T4 | 8 µg/dL |
|---|---|
| T3 uptake | 30 % |
| Free thyroxine index | 2.4 index |
For an adult, this result is in the usual range.
What Free thyroxine index tells you
The free thyroxine index (FTI, also called T7) is not measured directly. A lab calculates it from two blood tests, total T4 and T3 uptake. Most T4 in your blood rides on carrier proteins, and the amount of carrier protein changes in pregnancy and with some medicines. Total T4 alone can then look high or low even when the active, unattached part is normal. The index adjusts for that. Many labs now measure free T4 directly instead, so FTI is less common than it used to be.
FTI can give a clearer picture of thyroid hormone than total T4 alone when carrier proteins are higher or lower than usual. It is read together with TSH. Because it is an older, indirect estimate, your clinician may also want a direct free T4 test.
Read the full page on Free thyroxine index, with what a high or low result can mean.
Usual ranges
The ranges the result above is placed on. Your own lab’s printed range comes first if it gives one.
- Adults1.4–3.8 index
Sources: Cureus, Case Reports in Endocrinology
Sources
The equation
- Thyroid Stimulating Hormone and Thyroid Hormones (Triiodothyronine and Thyroxine): An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status, Thyroid (American Thyroid Association) (2023)
- Post-COVID Myoclonus-Ataxia Syndrome: A Case Report of Successful Recovery With Intravenous Immunoglobulin (IVIG) Treatment (Table 3: laboratory thyroid panel with reference ranges), Cureus (2025)
3 more sources for the ranges and the explanation
- New-Onset Paroxysmal Atrial Fibrillation in a Healthy Male With Post-COVID-19 Painless Thyroiditis: A Case Report (Tables 2 and 3: thyroid tests with reference ranges), Cureus (2024)
- A Rare Case of Primary Thyroid Lymphoma (Table 1: laboratory results with reference ranges), Cureus (2026)
- Measurement of Serum Free Thyroxine Index May Provide Additional Case Detection Compared to Free Thyroxine in the Diagnosis of Central Hypothyroidism, Case Reports in Endocrinology (2015)
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. The calculator is version 1.0.0 of our code for this equation, tested against published examples. How we build and check these pages.