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

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

Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)

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

The determination of the total thyroxine (T4) concentration is of importance in laboratory diagnostics for differentiating between euthyroid, hyperthyroid, and hypothyroid conditions. As the major fraction of the total T4 is bound to transport proteins (thyroxine-binding globulin: TBG, prealbumin, and albumin), the determination of total T4 only provides correct information when the thyroxine-binding capacity (TBC) in serum is normal. The free thyroid hormones are in equilibrium with the hormones bound to the carrier proteins.

 

The TBC or T-uptake assay provides a measure of the available thyroxine-binding sites. Determination of the free thyroxine index (FTI) from the quotient of total T4 and thyroxine-binding index (ie, result of the T-uptake determination) takes into account changes in the thyroid hormone carrier proteins and the thyroxine level.

 

While total T4 is a relatively reliable indicator of T4 levels in the presence of normal binding proteins, it is not a reliable indicator when binding proteins are abnormal. For example, increases in thyroxine-binding proteins may cause increased total T4 levels despite normal free T4 levels and normal thyroid function.

 

Results are changed by drugs or physical conditions that alter the patient's TBG levels, or drugs that compete with endogenous T4 and T3 for protein-binding sites.

 

Direct measurement of free thyroxine (FRT4 / T4 [Thyroxine], Free, Serum by immunoassay) has replaced the FTI test in most clinical situations.

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.

THYROXINE BINDING CAPACITY (units are in Thyroxine Binding Index: TBI):

0-19 years: 0.8-1.2 TBI

> or =20 years: 0.8-1.3 TBI

 

T4 TOTAL (T4):

0-5 days: 5.0-18.5 mcg/dL

6 days-2 months: 5.4-17.0 mcg/dL

3-11 months: 5.7-16.0 mcg/dL

1-5 years: 6.0-14.7 mcg/dL

6-10 years: 6.0-13.8 mcg/dL

11-19 years: 5.9-13.2 mcg/dL

> or =20 years: 4.5-11.7 mcg/dL

 

FREE THYROXINE INDEX:

0-5 days: 5.1-20.8 mcg/dL

6 days-2 months: 5.5-18.0 mcg/dL

3-11 months: 5.7-16.8 mcg/dL

1-5 years: 5.9-15.0 mcg/dL

6-10 years: 6.0-13.9 mcg/dL

11-19 years: 5.9-13.2 mcg/dL

> or =20 years: 4.8-12.7 mcg/dL

 

For SI unit Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html

Interpretation Provides information to assist in interpretation of the test results

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

FTI = Thyroxine (T4)/Thyroid Binding Capacity

 

The FTI is a normalized determination that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins.

 

Hyperthyroidism causes increased FTI and hypothyroidism causes decreased values.

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

This test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4 to 6 weeks to allow the physiological state to become re-established.

 

Autoantibodies to thyroid hormones can interfere with the assay.

 

Binding protein anomalies seen with familial dysalbuminemic hyperthyroxinemia, for example, may cause values which, while characteristic of the condition, deviate from the expected results.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Whitley RJ, Meikle AW, Watts NB: Thyroid function. In: Burtis CA, Ashwood, ER, eds. Tietz Fundamentals of Clinical Chemistry. 4th ed. WB Saunders Company; 1996:pp 645-646

2. Wilson JD, Foster DW, Kronenburg MD, et al: Williams Textbook of Endocrinology. 9th ed. WB Saunders Company; 1998:407-477

3. Freedman DB, Halsall D, Marshall WJ, Ellervik C: Thyroid disorders. In: Rifai N, Horvath AR, Wittwer CT, eds.  Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018; 1572-1616

4. Ross DS, Burch HB, Cooper DS, et al: 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016 Oct 26(10):1343-1421

5. Persani L, Cangiano B, Bonomi M: The diagnosis and management of central hypothyroidism in 2018. Endocr Connect. 2019 Feb;8(2):R44–R54. doi: 10.1530/EC-18-0515