A convenient, specific, precise, and reproducible radioimmunoassay system for measurement of triiodothyronine (T3) in human serum has been developed. The procedure compares the ability of standards and unknowns to compete with radioactive T3 for binding sites on a T3-binding antiserum produced in rabbits by immunization with human thyroglobulin. The assay is set up in the presence of 250 ng thyroxine (T4) in all tubes, to mobilize T3 from its binding with the thyronine-binding globulin (TBG), and athyreotic sheep serum in standards to correct for the TBG in the unknowns. The method regularly detected 0.4 ng T3, which would correspond to a T3 concentration of 100 ng/100 ml when 400 μl of serum is analyzed. The mean recovery of unlabeled T3 added to normal serum pools was 106%. Serial dilution of hyperthyroid sera containing high concentrations of T3 with athyreotic sheep serum yielded expected values.
Inder J. Chopra, David H. Solomon, Gildon N. Beall