Ultrasonographic thyroid volume as a reliable prognostic index of radioiodine-131 treatment outcome in Graves' disease hyperthyroidism.

Objective: We studied the relationship between thyroid volume, thyroid function and immunological markers of Graves' disease (GD) to determine prognostic factors of treatment response to low-dose radioiodine-131 (131I).
Material and Methods: A prospective study of 40 patients with GD hyperthyroidism treated with 131I (141 +/- 85MBq) and 10 GD patients who went spontaneously into remission (controls). Free T4, total T3 and basal TSH levels, TSH-receptor antibodies (TRAb) and anti-thyroid peroxidase antibodies (TPOAb) were studied. Thyroid volume was determined by ultrasonography. Logistic regression models were used to predict the probability of final thyroid status. Receiver-operating characteristics (ROC) curves and Hosmer Lemeshow tests were used to evaluate the final statistical models.
Results: Of 40 patients treated with 131I, 16 became euthyroid, 12 hyperthyroid and 12 hypothyroid at 12 months. Median thyroid volume was reduced from 24.8 ml before to 8.5 ml at 12 months (p<0.001). In 10 control patients, the median reduction was from 16.6 ml to 11.3 ml (p=0.029). Thyroid volume reduction was lower in the hyperthyroid than in the euthyroid group, but higher in the hypothyroid group. Thyroid volume at baseline and at 3 months predicted hyperthyroidism outcome with a cut-off of 45 ml and 24.4 ml, respectively (odds ratio 1.074, p=0.003, ROC curve 0.78 and odds ratio 1.182, p=0.012, ROC curve 0.86 respectively). Thyroid volume at 6 months differentiated the hyperthyroid group with a cut-off of 17 ml. Thyroid volume at 3 and 6 months with a cut-off of 8.5 ml and 9.3 ml respectively, predicts permanent hypothyroidism outcome (odds ratio 0.768 and 0.685, p=0.012 and p=0.008, ROC curve 0.89 and 0.88, respectively). Changes in thyroid echogenicity and TRAb and TPOAb levels did not show any predictive value in the follow-up after 131I therapeutic outcome.
Conclusion: The study shows that the ultrasonographic thyroid volume at 3 and 6 months after low-dose 131I treatment for GD hyperthyroidism could be a reliable prognostic factor of thyroid function outcome in the first year after treatment, and also reveals that the changes in the thyroid echogenicity and in the immunological markers of GD have no prognostic value.