Thyroxine Index, Free

Alternate Names

  • FT4 Index
  • FT4I

T4 (thyroxine) is a component of thyroid hormone, making up 90 percent of that hormone. T4 is bound to proteins in the blood, with a small amount being unbound. This unbound T4 is called Free T4; this is the T4 that is available to be used by the body. FT4I (Free T4 Index) is a measurement of the amount of Free T4, and is more accurate than TBG, Total T4, or T3 blood levels. The index is a number that is derived by the use of a mathematical formula, and this number is used to diagnose who well your thyroid is working. Low FT4I blood levels are seen in people with low levels of iodine in the diet, a non-functioning hypothalamus, an under-active pituitary gland, surgical removal of the thyroid gland, or an under-active thyroid gland. High FT4I levels are seen in people with thyroid infection, thyroid cancer, or Grave's disease.


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