Free Thyroxine Index (FTI) Calculator

Free Thyroxine Index (FTI) Calculator for Endocrinology. The output is a convention-specific index, not a direct free-T4 concentration. The tool does not label values hypothyroid, euthyroid or hyperthyroid. Compare only with a reference interval produced for the same calculation and assay combination.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

This tool implements the performing-laboratory T7 convention: total T4 in mcg/dL x T3 uptake percentage / 100. It uses the raw uptake fraction, not uptake divided by an assumed normal value of 30. FTI methods are not interchangeable, so the laboratory's exact reporting convention is essential.

When to use this calculator

Use only when the laboratory specifies this T7 calculation and provides a matching reference interval. Check the report for percentage uptake versus a normalized binding ratio before calculating. Interpret alongside TSH, relevant thyroid measurements and the clinical setting.

Inputs used

  • Total T4 in mcg/dL
  • T3 uptake reported as a percentage

Clinical interpretation

The output is a convention-specific index, not a direct free-T4 concentration. The tool does not label values hypothyroid, euthyroid or hyperthyroid. Compare only with a reference interval produced for the same calculation and assay combination.

Worked example

Total T4 15.5 mcg/dL with uptake 22% gives 15.5 x 22 / 100 = 3.41. Total T4 8 with uptake 30% gives 2.40, not 8.00. These examples demonstrate the selected scale and do not assign thyroid status.

Limitations and safety notes

Different laboratories use different binding measurements and normalization schemes; a numerical interval from another FTI convention can misclassify the result. Pregnancy, binding abnormalities, medication effects and assay interference require clinical consideration. The software cannot identify which method a laboratory used.

Frequently asked questions

Should uptake be entered as 0.30 or 30?

Enter 30 when the report says 30%. The equation divides by 100; a normalized ratio belongs to a different method.

Why is there no universal normal range?

The numerical scale depends on the laboratory convention. A range from a different FTI method is not safely transferable.

References

  • Performing laboratory primary T7 method: total T4 times T3 uptake percentage divided by 100; reference interval reported by the laboratory. https://dos.compunetlab.com/tests/1560
  • Primary terminology authority on noninterchangeable FTI calculation variants, not a universal FTI reference range. https://loinc.org/kb/users-guide/loinc-technical-briefs/free-thyroxine-index-variants

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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