Analysis of iodine-131-induced early thyroid hormone variations in Graves' disease

This prospective study aimed to assess iodine-131 (I)-induced early thyroid hormone variations in Graves' disease (GD) and determine the associated factors.

One hundred and seventy-one GD patients treated with I were evaluated (47 men, 124 women). I was administered at 9.0±4.9 mCi on average. Serum free triiodothyronine and free thyroxin were measured within 24 h before treatment and 8 (3-14) days after treatment. Patients were divided into increase, no change, and decrease groups, respectively, on the basis of hormone variations after treatment. χ-Test, analysis of variance, and the Kruskal-Wallis test were used to compare groups in terms of sex, age, course of disease, thyroid stimulating hormone receptor antibodies, antithyroid drug (ATD) pretreatment time, time of ATD discontinuation before I treatment, 24 h thyroid I uptake, thyroid weight, I activity, and I activity/thyroid weight (μCi/g). The Spearman method was used for correlation analyses.

Twenty-seven, 20, and 124 cases were assigned to increase, no change, and decrease groups, respectively. Significant differences were found among groups in the time of ATD discontinuation before I treatment [the median duration for methimazole was 11 (5-26), 16 (10-30), and 21 (1-30) days, P=0.000, the median duration for propylthiouracil was 12.5 (5-24), 22 (11-26), and 26 (21-30) days, P=0.000], thyroid weight (93.5±33.6, 90.3±48.8, and 74.1±26.0 g, P=0.003), and μCi/g (84.8±11.8, 100.4±24.9, and 121.1±44.0 μCi/g, P=0.000). Interestingly, μCi/g was negatively and positively correlated to the possibility of hormone increase and decrease, respectively. No significant differences were found in the other parameters assessed.

At the early stage of I treatment for GD, few patients showed increased thyroid hormone levels. Key factors may include time of ATD discontinuation before I treatment and μCi/g. High μCi/g might decrease thyroid hormone levels in early treatment, making it safe.