[Unusual concentrations of triiodothyronine in the serum due to anti-T3 autoantibodies].

Abnormally high T3 serum concentrations incompatible with the patients' clinical thyroid status were observed in a case of Graves' disease and in a euthyroid patient with hyperlipaemia. T3 was in the form of immune complexes precipitable by polyethyleneglycol. Specific anti-T3 autoantibodies were detected in the serum gammaglobulin fraction. The T3 affinity constants of these autoantibodies were found to be as high as 1.10(10) and 7.2.10(10)l/mol respectively. The antibodies interfered with radioimmunological T3 determination, resulting in an apparent increase of T3. However, after extraction, true T3 serum levels remained higher than expected. These findings illustrate the importance of hormone concentrations, affinity constants of binding proteins and equilibrium between free and bound forms in the resulting hormonal effect.