Radioimmunoassay for measurement of triiodothyronine in human serum.

A convenient, specific, precise, and reproducible radioimmunoassay system for measurement of triiodothyronine (T(3)) in human serum has been developed. The procedure compares the ability of standards and unknowns to compete with radioactive T(3) for binding sites on a T(3)-binding antiserum produced in rabbits by immunization with human thyroglobulin. The assay is set up in the presence of 250 ng thyroxine (T(4)) in all tubes, to mobilize T(3) 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 T(3), which would correspond to a T(3) concentration of 100 ng/100 ml when 400 mul of serum is analyzed. The mean recovery of unlabeled T(3) added to normal serum pools was 106%. Serial dilution of hyperthyroid sera containing high concentrations of T(3) with athyreotic sheep serum yielded expected values. The serum T(3) concentration in 80% of 31 euthyroid normal subjects was less than 100 ng/100 ml (range < 100-170 ng/100 ml); it was greater than 170 ng/100 ml in 89% of 27 sera of hyperthyroid patients with untreated Graves' disease (range < 100-1300, mean 519 in 25 sera with detectable T(3)). The concentration of serum T(3) fell, frequently to undetectable levels, during treatment of hyperthyroid patients with antithyroid drugs. The serum T(3) concentration in four hypothyroid patients was less than 100 ng/100 ml.