Free Thyroxine Index (FTI / T7)
Thyroid marker
Free T4 Index
Free Thyroxine Index (FTI / T7)
A calculated estimate of free thyroid hormone, historically called T7, derived by multiplying Total T4 by the T3 Uptake fraction. By combining a total hormone level with a binding-protein measure, it corrects for changes in binding proteins and approximates what a direct Free T4 assay measures. It is a legacy calculation from the era before reliable direct Free T4 tests.
PED Notes
The Free T4 Index exists to strip out binding-protein noise: if Total T4 is high only because oestrogen or AAS-related changes raised TBG, the accompanying low T3 Uptake pulls the index back into the normal range, correctly indicating normal thyroid status. This makes it more informative than Total T4 alone in athletes whose binding proteins are shifted by oral oestrogens or high-dose androgens. That said, the modern direct Free T4 assay already in this knowledge base is the preferred test; the index is mainly seen on older or budget panels. Reference ranges are strongly method-dependent, so compare against the reporting lab's own range.
Interpreting the Free T4 Index:
- A normal Free T4 Index with an abnormal Total T4 usually confirms that the Total T4 abnormality is a binding-protein effect, not a thyroid problem.
- A genuinely high index (with a suppressed TSH) suggests hyperthyroidism; a genuinely low index (with a raised TSH) suggests hypothyroidism. In both cases confirm with TSH and a direct Free T4 before treating.
- There is no treatment for the index itself: manage the underlying thyroid state. See the Free T4, Free T3, and TSH markers for supplement and pharmacological options.
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range