Objective: To investigate the prevalence of triiodothyronine (T(3)) antibodies in patients with goiter, the influence of these antibodies on the results of thyroid hormone measurements, and the potential relationship between T(3) antibodies and thyroid autoantibodies.
Methods: We undertook a study of 100 healthy control subjects (80 female and 20 male subjects) who had no clinical or biochemical evidence of thyroid disease and 100 patients (81 female and 19 male patients) with goiter. Serum concentrations of total T(3), total thyroxine, free T(3), and free thyroxine were measured by radioimmunoassay kits with a coated tube method. An immunoprecipitation method was used to detect anti-triiodothyronine auto-antibodies.
Results: Patients with a T(3) antibody binding ratio +3 standard deviations (SD) above the mean for the normal control group were considered to have T(3) antibodies. Normal control group binding was 2.4 +/- 1.3%. On the basis of +3 SD, seven patients had T(3) antibodies. The mean serum total T(3) level was 1.61 +/- 0.1 nmol/L in patients with binding ratios above +3 SD, whereas the mean serum total T(3) level was 2.23 +/- 0.3 nmol/L in patients without T(3) antibodies. Thus, patients with a binding ratio above +3 SD had a significant decrement in total T(3) levels. No correlation was found between the presence of T(3) antibody and antimicrosomal and antithyroglobulin antibodies.
Conclusion: When serum thyroid hormone concentrations are measured with a coated tube-radioimmunoassay method, antibodies can result in low T(3) concentrations. T(3) antibodies should be suspected whenever a discordance is noted between the measured thyroid hormone concentrations and the clinical status.