Free thyroxine index
Last updated: Tuesday, 30, March, 2004
5 mL blood in plain tube.
Total T4: immunoassay.
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The free thyroxine index has generally been replaced by Thyroxine (free) in the assessment of thyroid function, but is occasionally useful when a free T4 result is suspected of being anomalous.
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.
Ladenson PW et al. Arch Intern Med 2000; 160(11): 1573-1575.