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Common Mayo Medical Laboratories Test-Ordering Errors
Part 4: Overordered Tests

JAK2 on Blood and Bone Marrow
Free Thyroxine index (FTI) and T3 Uptake



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Free Thyroxine Index (FTI)

Slide 9

July 2010

The free thyroxine index is calculated from 2 measurements, total T4 and the T3 uptake (some of you may recall that the free thyroxine index was often referred to as a T7, referring to the T4 and T3 terms used in the calculation).  I won’t belabor the details of the free thyroxine index test, except to say that the T3 uptake reflected the amount of thyroid-binding globulin (abbreviated TBG) that was bound by T4 and, thus, not available to bind radiolabeled T3 that was added to the test system. T3 uptake was, therefore, a way of adjusting total T4 concentrations for variations in thyroid-binding globulin.  However, we now know that the free thyroxine index is unreliable in many situations. Many drugs and disease states, as well thyroid-binding globulin abnormalities, can alter the free thyroxine index, producing misleading results.   The free thyroxine index and T3 uptake test are now considered outdated, replaced by better, direct measures of free T4.

Free Thyroxine Index (FTI)

 


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