THE INFLUENCE OF TRIIODOTHYRONINE (T3) TREATMENT ON THE RESIN UPTAKE OF 131I-TRIIODOTHYRONINE

Abstract

ABSTRACT The effects of T3 treatment on the resin uptake of 131I-T3 as well as on serum thyroxine, have been studied in a euthyroid normal control group, in a group of hypothyroid patients, and in a group of pregnant women. T3 therapy resulted in a decline in the resin uptake of 131I-T3 in the normal control group; this was considered due to a suppression of serum thyroxine (consequent to TSH suppression), with a consequent increase in the amount of unsaturated thyroxine binding globulin. In patients with hypothyroidism, however, the resin uptake of 131I-T3 actually increased as a result of T3 treatment. Since the serum thyroxine in this group could not decrease any further as a result of the T3 administration, the T3 was truly additive, saturating more TBG binding sites, thus increasing the resin uptake of 131I-T3. In the pregnant group, large doses of T3 also caused an increase in the resin uptake of 131I-T3. While the decline in serum thyroxine was only modest, it was still greater than the accompanying rise in serum T3, so that the net amount of circulating thyroid hormone was less in the post treatment phase. Thus, the rise in the resin uptake of 131I-T3 is not completely explicable in this group.