Endocrine Abstracts

Introduction

The toxic solitary thyroid nodule is a major cause of hyperthyroidism, especially in iodine-deprived regions. The most widely used therapy is iodine-131, which is effective in achieving normal thyroid function and reducing nodule dimensions, however, it may induce hypothyroidism. The aim of this study was to evaluate the outcomes of radioiodine therapy in patients with toxic solitary nodules and to determine predictive factors for the development of hypothyroidism.

Methods

A retrospective analysis of medical records of patients with toxic thyroid nodule submitted to radioiodine therapy between 2008 and 2018 in our center was conducted.

Results

During the study period, 100 patients received radioiodine therapy for toxic solitary nodule, with a minimum follow-up of one year (77% female, age at diagnosis 63±14 years). The most used doses were 20 mCi (61%) and 15 mCi (35%). The therapy was effective in reversing hyperthyroidism in 99% of patients. However, 56% developed hypothyroidism, 79% of them being diagnosed within 6 months after therapy. The evolution to hypothyroidism was greater in women and the hypothyroidism group were younger, had smaller nodule size and thyroid volume and higher doses of iodine per thyroid volume. Logistic regression confirmed female sex (OR 2.90, P = 0.034), nodule size [OR 0.95 (mm), P = 0.026], thyroid volume [OR 0.94 (ml), P = 0.013] and radioiodine dose per volume [OR 4.47 (mCi/ml), P = 0.039] as predictive factors for hypothyroidism. There were no differences regarding the previous TSH value, use of antithyroid drugs, total iodine dose or iodine dose per kilogram of weight. In ultrasound reevaluation, within four years after treatment, there was a significant absolute and relative nodule reduction of 10 mm (5–14) and 33% (24–49), respectively (P<0.001). Higher absolute and lower relative reductions were found in larger nodules (R = 0.26, P = 0.014 | R = -0.29, P = 0.006). There was an absolute and relative reduction in thyroid volume of 10 ml (6–17) and 53% (32–73) (P<0.001). The patients who developed hypothyroidism showed higher reductions in thyroid volume (72% vs 37%, P<0.001). There were no differences in nodule or thyroid volume reduction regarding sex, use of antithyroid drugs or total dose of radioiodine.

Conclusion

Radioiodine therapy is effective in the treatment of hyperthyroidism caused by toxic thyroid nodules, and induces significant reductions in nodule and thyroid dimensions. However, this treatment may induce hypothyroidism, found in more than half of the cases in the present study. Female sex, lower nodule size and thyroid volume and higher radioiodine dose per volume are predictive factors of its development.