Free thyroxine index

Last updated: Tuesday, 30, March, 2004
Item Process

5 mL blood in plain tube.


Total T4: immunoassay.
T3 uptake: radiometric assay.

Calculation of Free Thyroxine Index = Total T4 x T3 uptake.

Reference Interval

Depends on method. Refer to laboratory.


The free thyroxine index has generally been replaced by Thyroxine (free) in the assessment of thyroid function, but is occasionally useful when a free T4 result is suspected of being anomalous.
See Thyroid stimulating hormone, which is the preferred initial test for assessment of thyroid status.


The calculated free thyroxine index attempts to correct for changes in total thyroxine caused by increases (eg, pregnancy) or decreases in thyroxine binding proteins, which influence the protein-bound fraction.
This gives an indication of free thyroxine levels, being high in hyperthyroidism and low in hypothyroidism.
The test lacks specificity; low results may occur in hospitalised patients with non-thyroidal illness.


Ladenson PW et al. Arch Intern Med 2000; 160(11): 1573-1575.