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Thyroid Function Testing: A Brief Update



Targets for Assessment of Thyroid Function, cont.

Slide 6

April 2009

Total T3 has also got reliable assays, and it represents the active thyroid hormone. Again, changes occur fast with changes in thyroid gland activity and, over the other analytes mentioned, it can have occasionally the advantage of being selectively overproduced in thyrotoxicosis, particularly early on. However, similar to total T4, the concentration is linked to the highly variable thyroid hormone-binding globulin concentrations, and only a small free fraction of T3, again, is biologically active. Furthermore, the circulating levels of T3 may not be representative of the levels found at the actual tissue sites, as the majority of T3 is actually not secreted from the thyroid glands, but manufactured in peripheral tissues on demand. Finally, all T3 assays are competitive assays similar to T4 assays and, hence, suffer from the same problems of a limited dynamic range.

Targets for Assessment, cont.

 


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