Radioimmunoassay of triiodothyronine in human serum with special reference to methodological problems.

A radioimmunoassay for measuring the level of triiodothyronine in human serum has been developed and studied with respect to interference from hormone-binding substances present in human serum. Dextran-coated charcoal was used for the separation of free and antibody-bound hormone. ANS (8-anilino-1-naphthalenesulfonic acid) was used to reduce the interference from hormone-binding substances in serum. Extraction of the hormone by ethanol before the radioimmunoassay did not solve the problem of interference, even in the presence of an internal standard. The protein concentration in the assay system strongly influenced the charcoal adsorption of free and antibody-bound hormone, and optimal concentrations for both proteins and ANS had to be worked out to get a reliable assay for unextracted serum.