Differences in the Interaction of Triiodothyronine-131I with Serum Proteinsin Vitro

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Abstract
The transport of triiodothyronine-I131 (T3-1I3I) by serum proteins was studied in vitro by zone electrophoresis on Whatman No. 3 filter paper (barbital buffer, pH 8.6), and the fraction of radioactivity found in association with thyroxine-binding globulin (TBG) was expressed as a percentage of the total radioactivity on the paper strip. Examination of sera collected from patients with thyroid disease revealed that the "uptake" of T3-I131 by thyroxine-binding globulin was reduced significantly in hyperthyroidism and increased in myxedema and in pregnancy when compared with values in normal controls. In hyperthyroidism, after treatment with radioiodine or antithyroid drugs, the low TBG-T3-I131 values gradually increased, while in myxedema therapy with desiccated thyroid caused the elevated TBG-T3-I131bindingvalues todecline. Enrichment of sera from normal or hypothyroid subjects with substantial amounts of stable thyroxine (>50 [mu]g/100 ml) resulted in values which approximated those seen in hyperthyroidism. When studies were carried out in which the technique of reverse-flow electrophoresis was used, triiodothyronine- I131 was found partitioned solely between albumin and thyroxine-binding globulin in a ratio that varied with the functional state of the thyroid gland.