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Thyroid Function Testing: A Brief Update

Targets for Assessment of Thyroid Function: Biological Considerations

Slide 7

April 2009

The free hormone measurements can obviously address some of the shortcomings of the total hormone measurements. They also, at least for free T4, are reasonable reliable, give a good measure of the thyroid gland hormone output, change fast with thyroid gland activity and, most importantly, are independent of TBG concentrations and they represent the biologically active fraction of T4. However, for the free T4, the same goes as for the total T4, it is a pro-hormone—the active hormone is T3, and levels can occasionally fluctuate with nonthyroidal illness, when binding protein levels change very rapidly, and it takes awhile for new equilibrium to be established. In those situations, free T4 measurements can be misleading. Standard assays again have a very limited dynamic range and at some ranges of binding protein concentrations, they may be unreliable.

Biological Considerations


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