Enhanced thyroid iodine metabolism in patients with triiodothyronine-predominant Graves' disease.

Some patients with hyperthyroid Graves' disease have increased serum T3 and normal or even low serum T4 levels during treatment with antithyroid drugs. These patients with elevated serum T3 to T4 ratios rarely have a remission of their hyperthyroidism. The aim of this study was to investigate thyroid iodine metabolism in such patients, whom we termed T3-predominant Graves' disease. Mean thyroid radioactive iodine uptake was 51.0 +/- 18.1% ( +/- SD) at 3 h, and it decreased to 38.9 +/- 20.1% at 24 h in 31 patients with T3-predominant Graves' disease during treatment. It was 20.0 +/- 11.4% at 3 h and increased to 31.9 +/- 16.0% at 24 h in 17 other patients with hyperthyroid Graves' disease who had normal serum T3 and T4 levels and a normal serum T3 to T4 ratio during treatment (control Graves' disease). The activity of serum TSH receptor antibodies was significantly higher in the patients with T3-predominant Graves' disease than in control Graves' disease patients (60.5 +/- 19.2% vs. 20.4 +/- 18.2%; P less than 0.001). From in vitro studies of thyroid tissue obtained at surgery, both thyroglobulin content and iodine content in thyroglobulin were significantly lower in patients with T3-predominant Graves' disease than in the control Graves' disease patients. Thyroid peroxidase (TPO) activity determined by a guaiacol assay was 0.411 +/- 0.212 g.u./mg protein in the T3-predominant Graves' disease patients, significantly higher than that in the control Graves' disease patients (0.129 +/- 0.112 g.u./mg protein; P less than 0.01). Serum TPO autoantibody levels determined by immunoprecipitation also were greater in T3-predominant Graves' disease patients than in control Graves' disease patients (52.6 +/- 27.7% vs. 32.4 +/- 11.4%; P less than 0.05). Binding of this antibody to TPO slightly inhibited the enzyme activity of TPO, but this effect of the antibody was similar in the two groups of patients. The data suggest enhanced iodine metabolism in the thyroid gland of patients with T3-predominant Graves' disease, which may relate to the discordant T3 overproduction in patients with this type of Graves' disease.