Free thyroxine index is considered to be a reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding. The free thyroxine index has generally been replaced by Free Thyroxine in the assessment of thyroid function, but is occasionally useful when a free T4 result is suspected of being anomalous.
Reference ranges for the 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
How to calculate the free thyroxine index?
The free thyroxine index (FTI) is determined by the following calculation:
High levels are caused by hyperthyroidism.
Low levels are caused by hypothyroidism. Low results may also occur in hospitalised patients with non-thyroidal illness.