Abstract: An immunoassay for the measurement of free thyroid hormone, i.e., thyroxine or 3,5,3'-triiodothyronine, in a liquid sample in which the thyroid hormone is present in both free and combined states, which immunoassay comprises the steps of:A. combining the sample with a labeled thyroid hormone-horseradish peroxidase conjugate which does not significantly interact with the thyroxine-binding globulin and thyroxine-binding prealbumin originally present in the sample and immobilized antibody which is specific for the thyroid hormone;B. incubating the resulting mixture;C. separating a solid phase from the liquid phase; andD. measuring the amount of labeled thyroid hormone-horseradish peroxidase conjugate present in either phase by means of the activity of the label.

Abstract: Thyroxine binding globulin (TBG) and thyroxine (T.sub.4) can be efficiently removed fron serum containing TBG and T.sub.4 by contacting the serum with finely-divided alumina particles to adsorb a major portion of the T.sub.4 and then reacting the serum at an acidic pH with an anion exchange resin to extract substantially all TBG and the remaining T.sub.4.

Abstract: The concentration of thyroxine binding globulin (TBG) in a fluid sample can be determined via radioassay technique with an immunochemical composite comprising anti-TBG antibodies immobilized on a water-insoluble carrier. The composite is incubated with the sample in a quantity sufficient to assure complexation of substantially all TBG in the sample to the antibodies of the composite. The quantity of TBG complexed is determined with radiolabeled thyroxine (T.sub.4) which complexes with at least a portion of the complexed TBG. By separating the labeled composite from the remaining labeled T.sub.4, and counting the bound (or unbound) T.sub.4, the concentration of TBG can be determined from a standard curve.