Free Thyroxine Index (FTI)

Determines the FTI based on the fraction between total thyroxine and thyroid binding capacity.

Refer to the text below the calculator for more information on the index and its component variables.

The free thyroxine index (FTI) is the quantity of circulating, unbonded thyroxine based on the laboratory determinations of total thyroxine and thyroid binding capacity (T-uptake).

The FTI takes into account changes in the thyroid hormone carrier proteins and the thyroxine level.

The free thyroxine index (FTI) is determined by the following calculation:

FTI = Thyroxine (T4) / Thyroid Binding Capacity

FTI increases could be caused by hyperthyroidism whilst decreased FTI could be attributed to hypothyroidism.

Normal values range:

6-10 years: 6.0-13.9 mcg/dL

11-19 years: 5.9-13.2 mcg/dL

≥20 years: 4.8-12.7 mcg/dL

Thyroxine (T4)
Thyroid Binding Capacity
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About the FTI

Total thyroxine T4 concentration is vital in the diagnosis of euthyroid, hyperthyroid, and hypothyroid conditions.

The major fraction of total thyroxine is bound with proteins (such as TBG, prealbumin or albumin), the remainder is free in serum. The free thyroid hormones are in equilibrium with the bound hormones.

The thyroid binding capacity or T-uptake assay provides a measure of the available thyroxine-binding sites.

The FTI is a normalized determination that is known to be relatively constant in healthy individuals and to compensate for abnormal levels of binding proteins.

FTI = Thyroxine (T4) / Thyroid Binding Capacity

Currently, direct measurement of serum free thyroxine is starting to replace the FTI formula in most clinical situations. Autoantibodies to thyroid hormones can, however, interfere with the assay.

Patients are sent for an FTI and other thyroid tests if they have symptoms that could indicate a thyroid disease, especially in the presence of other risk factors (female gender, age below 40, family history of thyroid disorders).

Low levels of free T4 can also be found in the autoimmune thyroid disease called Hashimoto's thyroiditis. High levels of FTI lead to testing for Graves' disease, thyroiditis, or goiter.


Reference values for FTI and its component variables

Patient age Free thyroxine index in mcg/dL Thyroxine (T4 total) in mcg/dL Thyroxine binding capacity in TBI
0 - 5 days 5.1 - 20.8 5.0 - 18.5 0.8 - 1.2
6 days - 2 months 5.5 - 18.0 5.4 - 17.0
3 - 11 months 5.7 - 16.8 5.7 - 16.0
1 - 5 years 5.9 - 15.0 6.0 - 14.7
6 - 10 years 6.0 - 13.9 6.0 - 13.8
11 - 19 years 5.9 - 13.2 5.9 - 13.2
≥20 years 4.8 - 12.7 4.5 - 11.7 0.8 - 1.3


Whitley RJ, Meikle AW, Watts NB: Thyroid function. In Tietz Fundamentals of Clinical Chemistry. Fourth edition. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Company, 1996, pp 645-646

Wilson JD, Foster DW, Kronenburg MD, et al: Williams Textbook of Endocrinology. Ninth edition, WB Saunders Company, 1998, pp 407-477

Pantalone KM et al. Measurement of Serum Free Thyroxine Index May Provide Additional Case Detection Compared to Free Thyroxine in the Diagnosis of Central Hypothyroidism Case Rep Endocrinol. 2015; 2015: 965191.

Specialty: Endocrinology

System: Endocrine

No. Of Variables: 2

Abbreviation: FTI

Article By: Denise Nedea

Published On: April 16, 2020

Last Checked: April 16, 2020

Next Review: April 16, 2025