The Free Thyroxine Index (FTI) is the calculated product of T-3 resin uptake and serum total thyroxine (T-4) and is the best initial screening test for hyperthyroidism. It permits correction of misleading results in T-3 and T-4 determinations caused by conditions that alter the thyroxine-binding protein concentration (e.g., pregnancy, estrogens, birth control pills). The American Thyroid Association now recommends the term thyroid hormone-binding ratios (THBR).
free thyroxine index =
= ((percent T3-uptake in patient) / (percent T3-uptake in control)) * (T4)
where:
• The percent T3-uptake is normally 25-35%.
• The ratio of T3-uptake for patient to that of the control is called the thyroid hormone binding ratio, or THBR.
• Thyroxine is in µg/dL.
• The result of the index does not have units.
Interpretation
The FTI is a useful parameter regarding clinical thyroid status in the presence of thyroid binding globulin abnormalities, in which the T3 and T4 alone are abnormal.
Age
FTI
cord blood
6.0-13.2
1-3 days
9.9-17.5
1 week
7.5-15.1
1-12 months
5.0-13.0
1-3 years
5.4-12.5
3-10 years
5.7-12.8
> 10 years & adults
4.2-13.0
While the FTI generally correlates with free T4, it is not a substitute for the measurement of free T4 in patients with thyroid disease. With better tests for free T4 levels now available, the need for the FTI ha decreased in recent years.
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