RESUMO
The incidence of low neonatal free thyroxine (T(4)) assay results is methodology-dependent. Nonanalog free T(4) assay results represent free T(4) concentrations when free T(4) is the only form of T(4). Similar analog-based free T(4) assay results are produced by an extraordinary range of free T(4) concentrations, when free T(4) is the only form of T(4). Adding albumin or transthyretin to free T(4) concentrations greatly decreased free T(4) concentrations, as expected, but increased analog-based free T(4) assay results. By contrast, adding thyroxine-binding globulin decreased free T(4) concentrations and free T(4) assay results; but these free T(4) concentrations were not represented by assay results. There was no specificity for the free form of T(4) versus bound forms of T(4) in some free T(4) assay results. The protein that binds T(4) can have a major influence on some of the total T(4) assay results that may be used in free T(4) index methods.