Free Thyroxine Index (FTI) Calculator

Free Thyroxine Index (FTI) Calculator for Endocrinology. FTI is reported in the same arbitrary units as total T4 and read against the same reference interval (commonly ~4.5-12.0, laboratory-specific); values above range suggest true hyperthyroxinemia and below range true hypothyroxinemia, independent of binding-protein status. The whole purpose is discordance-detection: when total T4 is high or low but FTI normalizes, the abnormality is in binding proteins, not thyroid status; when total T4 and FTI move together, a genuine thyroid disturbance is likely. Always interpret the FTI alongside TSH, since the FTI is a peripheral hormone estimate and does not capture the pituitary-thyroid feedback loop.

How this calculator works

The FTI is a calculated surrogate for free thyroxine that corrects a measured total T4 for the patient's thyroid hormone binding capacity. In the classic form it multiplies total T4 by the T3-resin uptake (T3RU) expressed as a fraction (FTI = total T4 x T3RU%/100); on modern platforms the binding term is the thyroid hormone binding ratio (THBR), a T-uptake result normalized to a reference serum, giving FTI = total T4 x THBR. Because T3RU/THBR rises when binding proteins fall and falls when they rise, the product moves total T4 in the opposite direction to the binding abnormality, tracking the free hormone the patient actually sees.

When to use this calculator

Use the FTI to interpret a total T4 whenever thyroxine-binding globulin (TBG) is expected to be abnormal but a direct free T4 immunoassay is unavailable or suspected to be unreliable: pregnancy, estrogen or tamoxifen therapy, oral contraceptives, nephrotic syndrome, severe hepatic or protein-losing states, and inherited TBG excess or deficiency. It is a historical/backup index; when a validated free T4 assay (or equilibrium dialysis in complex cases) is available, that should be the primary test. Do not use FTI to override an unmistakably discordant TSH, and avoid relying on it in critically ill (nonthyroidal illness) patients, where both total T4 and binding shift unpredictably.

Inputs used

  • Total T4
  • T3 uptake or binding correction

Clinical interpretation

FTI is reported in the same arbitrary units as total T4 and read against the same reference interval (commonly ~4.5-12.0, laboratory-specific); values above range suggest true hyperthyroxinemia and below range true hypothyroxinemia, independent of binding-protein status. The whole purpose is discordance-detection: when total T4 is high or low but FTI normalizes, the abnormality is in binding proteins, not thyroid status; when total T4 and FTI move together, a genuine thyroid disturbance is likely. Always interpret the FTI alongside TSH, since the FTI is a peripheral hormone estimate and does not capture the pituitary-thyroid feedback loop.

Worked example

A pregnant woman at 24 weeks has total T4 of 15.5 ug/dL (nonpregnant reference ~4.5-12.0) and a low T3RU of 22% (reference ~25-35%), reflecting estrogen-driven TBG excess. FTI = 15.5 x (22/100) = 3.4. This lands within a typical FTI reference range (roughly 4.5-12.0 in the same arbitrary units as T4, i.e. FTI is scaled to the T4 range), showing the elevated total T4 is explained by high TBG rather than true hyperthyroxinemia. Paired with a normal TSH, the FTI correctly reclassifies her as euthyroid and averts unnecessary antithyroid treatment.

Limitations and safety notes

The FTI's binding correction is only approximate: it assumes the T3RU/THBR faithfully mirrors T4 binding, so it can fail at extremes such as marked familial TBG excess, where total T4 and FTI may both be spuriously raised and lead to inappropriate treatment. It does not correct for abnormal binding to albumin or transthyretin (familial dysalbuminemic hyperthyroxinemia) and remains unreliable in nonthyroidal illness and in the presence of certain drugs (heparin, high-dose salicylates, phenytoin, furosemide) that displace T4. In pregnancy the changing reference ranges and altered binding make trimester-specific interpretation essential, and a normal FTI never excludes central hypothyroidism.

Frequently asked questions

How is FTI different from a direct free T4 measurement?

FTI is a calculated estimate derived from total T4 and a binding-uptake test, whereas free T4 is measured directly by immunoassay or dialysis. FTI was the standard workaround before reliable free T4 assays existed; it corrects for TBG changes reasonably well but is less accurate than modern direct or dialysis-based free T4, especially at binding extremes.

Why does the T3-uptake go DOWN when binding proteins go UP?

T3RU/THBR measures unoccupied binding sites, not thyroid status. When TBG rises (e.g., pregnancy, estrogen), there are more empty sites to soak up the labeled tracer, so less tracer binds the resin and the uptake reads low. Multiplying the correspondingly elevated total T4 by this low uptake pulls the FTI back into the normal range.

Is a normal FTI enough to rule out thyroid disease?

No. FTI reflects circulating thyroxine only. It can be normal in central (pituitary/hypothalamic) hypothyroidism, in early or subclinical disease driven by TSH changes, and in T3-toxicosis. Always pair the FTI with TSH, and add free T3 or further testing when the clinical picture and TSH are discordant.

Can I use FTI in a critically ill patient?

Cautiously, if at all. In nonthyroidal illness syndrome both total T4 and binding are perturbed, and the FTI correction breaks down, sometimes giving low values in patients who are not truly hypothyroid. TSH plus a direct free T4 (ideally by dialysis) is preferred, and thyroid testing is often best deferred until recovery unless myxedema or thyroid storm is suspected.

References

  • Neild JE, Byfield PG, Lalloz MR, et al. Familial abnormalities of thyroxine binding proteins: some problems of recognition and interpretation. J Clin Pathol. 1985. PMID: 3919066.
  • Baloch Z, Carayon P, Conte-Devolx B, et al. Laboratory medicine practice guidelines. Laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid. 2003. PMID: 12625976.
  • Nusynowitz ML. Free-thyroxine index. JAMA. 1975. PMID: 805266.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: June 1, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • 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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