Abstract: Method and compositions are provided for determining the thyroxine binding capacity of serum, the value finding use in determinations of free thyroxine index. A competitive assay is performed where antibody for T-3 (anti-triiodothyronine) and serum thyroxine binding sites compete for T-3(triiodothyronine) in the assay medium: the T-3 is present as free T-3 added in a predetermined amount and T-3 conjugated to an enzyme. By combining the materials in a buffered aqueous medium with enzyme substrates, and determining the rate of reaction by the changing concentration of a light absorptive substance over a predetermined period of time, the thyroxine binding capacity of the serum can be determined.

Abstract: Composition and method are provided for the pretreatment of serum prior to a thyroxine assay. Prior to carrying out of the assay, particularly an immunoassay or competitive protein-binding assay, the serum sample is combined with cyclodextrin preferably in combination with an hydroxylated aromatic carboxylic acid in a basic medium, in amounts sufficient to minimize interference with obtaining an accurate and reliable determination of polyiodothyronine in the serum. Normally, after the reagents have been added to the serum, a small incubation period is allowed, after which time the additional reagents may be added for carrying out the assay.