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Thyroid Function Testing: A Brief Update

Targets for Assessment of Thyroid Function, cont.

Slide 5

April 2009

Total thyroxin has several advantages of its own. First of all, the assays, like those for TSH, are pretty reliable and they are a good measure of thyroid gland output. Moreover, the changes occur fast when thyroid gland activity changes, which makes it very suitable for investigating acute changes in thyroid function. However, the concentration of total thyroxin is highly variable, and highly dependent on the variable thyroid hormone-binding globulin concentrations. TBG, which is short for thyroid hormone-binding globulin, is the main binding protein for thyroid hormones and its concentrations can vary widely depending on a number of exogenous factors such as sex-hormone concentrations or liver function. Only a small fraction of the total thyroxin is free and that’s actually the biologically active fraction. This small fraction, in addition, is a pro-hormone, the active hormone being actually T3. And, finally, the assays themselves, while reliable, have limitation of being all competitive assays and suffering from a limited dynamic range.

Targets for Assessment, cont.


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