Free Thyroxine Index (FTI) Calculator
Free Thyroxine Index (FTI) Calculator
Correct total T4 for binding-protein changes using T3 uptake — the historical workaround for a direct free T4 assay.
Free Thyroxine Index (FTI)
Total T4 × T3 uptakeTotal T4 8.5 µg/dL, T3 uptake 32%
Formula
- total T4
- µg/dL — measures both protein-bound and free hormone
- T3 uptake
- also called THBR, the thyroid hormone binding ratio — moves inversely with unoccupied binding capacity
- why multiply
- total T4 rises and falls with thyroxine-binding globulin (TBG); T3 uptake moves the opposite way, so the product largely cancels the TBG effect
Worked example
Total T4 8.5 µg/dL, T3 uptake 32%
8.5 × 32 = 272
272 ÷ 100 = 2.72
Between 1.5 and 4.5 → low-normal
What moves total T4 without thyroid disease
| Direction | Cause |
|---|---|
| Raises TBG and total T4 | Pregnancy, oestrogen therapy, oral contraceptives, acute hepatitis |
| Lowers TBG and total T4 | Nephrotic syndrome, androgens, glucocorticoids, severe systemic illness |
| Congenital | TBG excess or TBG deficiency, both benign and inherited |
Why a correction factor was needed at all
Most circulating thyroxine is bound to thyroxine-binding globulin (TBG), transthyretin and albumin, and only a small free fraction is biologically active. Early total T4 assays measured bound and free hormone together, so a total T4 result moved whenever TBG changed — pregnancy and oestrogen raise TBG and therefore total T4 with no thyroid disease at all, while nephrotic syndrome, androgens and severe non-thyroidal illness lower both TBG and total T4 in a person whose thyroid is entirely normal.
The T3 uptake test was devised to correct for this. It does not measure T3 in the patient’s serum; it measures how much labelled T3 added to the sample binds to the patient’s own unoccupied binding sites, which moves inversely with binding capacity. Multiplying total T4 by the T3 uptake therefore cancels most of the TBG effect, giving an estimate that tracks the free hormone concentration far better than total T4 alone.
FTI is largely historical now that direct free T4 immunoassays are standard in most laboratories — they measure the free fraction directly and do not need this arithmetic correction. FTI still earns its place when a free T4 result looks inconsistent with the TSH, since immunoassay interference and unusual binding-protein states can distort free T4 in ways FTI is not susceptible to, and in interpreting older records that report total T4 and T3 uptake rather than a modern free T4.
The reference range for FTI is not universal: it depends on the local assay’s T3 uptake reference interval, which varies between laboratories and methods. Quote the range from the laboratory that produced the underlying results rather than assuming a generic cut-off applies.
Frequently asked questions
What does the Free Thyroxine Index correct for?
It corrects total T4 for changes in thyroxine-binding globulin, using the T3 uptake test as an inverse marker of binding capacity, so the result tracks free hormone rather than bound-plus-free hormone.
Is FTI still used?
Rarely as a first-line test, now that direct free T4 immunoassays are standard. It remains useful when a free T4 result seems inconsistent with TSH, and when interpreting older laboratory records.
Why does pregnancy affect total T4 but not FTI?
Pregnancy and oestrogen raise TBG, which raises total T4 without any change in thyroid function. T3 uptake falls in the same state, so the product — the FTI — stays largely unchanged.
What reference range should I use for FTI?
There is no universal range. It depends on the T3 uptake reference interval of the laboratory that measured it, so use the range quoted alongside your own results rather than a generic figure.
Related calculators
References
- Larsen PR et al. Williams Textbook of Endocrinology, chapter on thyroid function testing.
- Demers LM, Spencer CA. Laboratory Medicine Practice Guidelines: laboratory support for the diagnosis and monitoring of thyroid disease. National Academy of Clinical Biochemistry, 2002.