Metabolic Health

Free Thyroxine Index

A healthy result should fall into the range 4.8 - 12.7 Units.

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 The free thyroxine index has generally been replaced by Free Thyroxine in the assessment of thyroid function, but is occasionally useful when a free T4 result is suspected of being anomalous.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:



Free Thyroxine Index result calculator

insert the value from you Free Thyroxine Index test result.

What does it mean if your Free Thyroxine Index result is too low?

Low levels are caused by hypothyroidism. Low results may also occur in hospitalised patients with non-thyroidal illness.

What does it mean if your Free Thyroxine Index result is too high?

High levels are caused by hyperthyroidism.

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