Page "Thyroxine-binding globulin" Paragraph 2

The patient's serum is mixed with the labeled thyroid hormone ; then, the resin is added to the whole mixture to measure the amount of free labeled thyroid hormone.

So, for instance, if the patient is truly hypothyroid, and TBG levels are normal, then there are many sites open for binding on the TBG, since the total thyroid hormone level is low.

Therefore, when the labeled hormone is added, it will bind mostly to the TBG, leaving little of it left for binding to the resin.

In contrast, however, if the patient is truly hyperthyroid, and TBG levels are normal, the patient's endogenous hormone will saturate the TBG binding sites more, leaving less room for the labeled hormone, which allows greater binding to the resin.