The free thyroxine index estimates the amount of free thyroxine circulating, using the measurement of total thyroxine and the binding capacity of thyroxine serum proteins.
Determination of total thyroxine (T4) concentration is important in laboratory diagnosis to differentiate between conditions of euthyroidism, hyperthyroidism, and hypothyroidism. As the major fraction of total thyroxine binds to plasma transport proteins (thyroxine-binding globulin - TBG, prealbumin, and albumin), the determination of total T4 provides correct information only when the serum thyroxine binding capacity is normal. When the presence of thyroxine-binding proteins is not normal, measurement of total T4 is not a reliable indicator of thyroid status. For example, elevated levels of thyroxine-binding proteins may cause elevated levels of total T4 despite normal free T4 levels and normal thyroid function.
Thyroid hormone binding capacity or T uptake assay offers a measure of available thyroxine binding sites. Determination of free thyroxine index (FTI) takes into account changes in thyroid hormone protein levels and thyroxine levels. Measurement of the free thyroxine index is influenced by drugs or conditions that alter TBG levels and by drugs competing with T4 and T3 for the same protein binding sites.
Immediate measurement of free thyroxine (Free T4) tends to replace FTI in most clinical situations.
Important Note
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. The correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings, and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
At Diagnostiki Athinon we answer any questions you may have about the test you perform in our laboratory and we contact your doctor to get the best possible medical care.
