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Test ID: FRTIX    
Free Thyroxine Index (FTI), Serum

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Useful For Suggests clinical disorders or settings where the test may be helpful

T-Uptake (TU) and total thyroxine (T4) are used to estimate the amount of circulating free T4. The estimate, or the free thyroxine index (FTI), is a normalized measurement that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins. Hyperthyroidism causes increased FTI and hypothyroidism causes decreased values. TU results are changed by drugs or physical conditions that alter the patient's thyroxine-binding globulin levels, or drugs that compete with endogenous T4 and triiodothyronine (T3) for protein-binding sites. When serum contains high levels of T3 and T4 as in hyperthyroidism, fewer unoccupied binding sites are available. Conversely, in hypothyroidism, more binding sites are available.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

The T-Uptake is an assessment of unsaturated thyroid binding proteins in serum. The free thyroxine index is an estimate of the amount of circulating free thyroxine.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.


Males: 27-37%

Females: 20-37%




<1.5 suggest hypothyroidism

1.5-3.0 suggest euthyroidism

>3.0 suggest hyperthyroidism


<1.3 suggest hypothyroidism

1.3-3.0 suggest euthyroidism

>3.0 suggest hyperthyroidism




0-11 months: not established

1-9 years: 6.0-12.5 mcg/dL

10-23 years: 5.0-11.0 mcg/dL

> or =24 years: 5.0-12.5 mcg/dL


0-11 months: not established

1-9 years: 6.0-12.5 mcg/dL

10-17 years: 5.0-11.0 mcg/dL

> or =18 years: 5.0-12.5 mcg/dL

Interpretation Provides information to assist in interpretation of the test results

The T-Uptake (TU) values used are interpreted in conjunction with total thyroxine (T4) measurements. The free thyroxine index is calculated as:

-FTI=(T4 concentration) x (% TU)/100

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Direct measurement of free thyroxine (T4) has replaced the T-Uptake (TU) test in most clinical situations.


TU values are a function of thyroid hormone binding capacity of serum. Some conditions may result in abnormal TU values even when no thyroid malfunction is present. The conditions listed below may alter TU values.


Conditions that decrease TU values:

-Pregnancy (especially in the last trimester)

-Elevated estrogen levels

-Acute hepatitis

-Drugs which prevent ovulation


Conditions that elevate T-Uptake values:

-Protein malnutrition

-Chronic liver disease

-Nephrotic syndrome

-Phenytoin therapy

-Heparin therapy


-Large doses of salicylates



Thyroid preparations that maintain normal triiodothyronine (T3) and T4 concentrations can elevate T-Uptake values when the dose hormone is excessive and decrease TU values when the dose is inadequate. Replacement therapy with T3 results in decreased TU values.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Whitley RJ, Meikle AW, Watts NB: Thyroid Function. In Tietz Fundamentals of Clinical Chemistry. Fourth Edition. Edited by CA Burtis, ER Ashwood. Philadelphia, EB Saunders Company 645-646, 1996

2. Klee GG, Hinz VS: The Ciba Corning ACS:180 Plus. In Immunoassay Automation: An Updated Guide to Systems. Edited by DW Chan, Associated Press, New York 63-102, 1996