Early change in serum thyroid hormone concentrations after administration of small therapeutic dose of 131I in untreated Graves' patients

Early change in thyroid function, including the serum concentrations of FT4, TBG and Tg, after administration of relatively small therapeutic doses of 131I in patients with Graves' disease was investigated. The subjects were 4 male and 16 female untreated Graves' patients with no complications. Ages were 32 to 62 years. Blood samples were withdrawn immediately before, and 1, 3, 7 and 10 days after 131I administration. The serum concentrations of T4, T3, FT4, rT3, TBG and Tg were significantly decreased in 7 cases (Group A) and increased in 3 (Group B). The serum concentration of Tg increased in all cases, but no significant change was observed in serum TBG concentration. The initial goiter size, serum concentration of Tg, and dose of 131I administered tended to be greater in Group B than in Group A. These results indicate that the serum concentrations of thyroid hormones decrease after treatment with a relatively small dose of 131I in most Graves' patients with small goiters, but in cases with large goiters they may increase after 131I application. There is thus some possibility of thyroid crisis in those patients. (author)