Thyroid function tests the effects of alteration to binding proteins.
The distribution of isotope between binding agents (resin or Sephadex) and free sites or THBPs is shown. In the euthyroid state, free T4 and TSH levels are normal, but alterations in TBG concentration or capacity will affect total T4. levels. (a) Increased TBG concentration increases total T4 levels. The number of free binding sites is also increased, and these avidly take up 125I-T3 (as in hypothyroidism). (b) Decreased TBG concentration decreases total T4 levels. There are also fewer free binding sites, so most 125I-T3 remains attached to the binding agent (as in hyperthyroidism). (c) Drugs (e.g. phenytoin) compete for binding sites on THBP, thus levels of total T4 are low. The residual binding sites are relatively normal, so 125I-T3 is distributed normally between BA and proteins, resulting in no ΓÇÿcorrectionΓÇÖ of the T4 and thus a falsely low FTI.
