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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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Thyroxine (T4)

Slide 7

July 2010

Thyroxine, called T4, is the  main thyroid hormone. T4 circulates in 2 forms,  protein bound (which accounts for over 99% of T4) and free. Free T4 accounts for <1% of circulating T4, but is the biologically active form of thyroxine. Both bound and free forms of thyroxine are measured by  total T4 assays. While in most situations the total T4 test is a relatively reliable indicator of thyroxine status, when thyroid hormone-binding proteins are abnormal, total T4 results may be misleading. For example, increases in thyroxine-binding proteins can cause increased Total T4 levels despite normal free T4 levels and despite clinically normal thyroid function.

Thyroxine (T4)


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