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For 12 hours before specimen collection do not take multivitamins or dietary supplements conaining biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.
If patient is receiving treatment with lipid-lowering agents containing D-T4, discontinue for 4 to 6 weeks prior to specimen collection.
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.
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.
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/dL6 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
The Free Thyroxine index (FTI) is determined by the following calculation: FTI = 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.