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

Secondary ID A test code used for billing and in test definitions created prior to November 2011


NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.


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.

Method Name A short description of the method used to perform the test

Chemiluminometric Immunoassay

Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name

Free Thyroxine Index (FTI), S

Aliases Lists additional common names for a test, as an aid in searching

Thyroxine Index