Endocrine Abstracts

Introduction: Radioiodine (I-131) therapy is an effective treatment for thyroid autonomy but may induce Graves’ disease (GD) but in up to 5% of patients. GD is characterised by antibodies against TSH receptor (TSHRAb). We set out to evaluate the incidence and risk factors of I-131-induced GD in patients with thyroid autonomy treated with I-131.

Methods: We retrospectively reviewed patients with solitary toxic adenoma or toxic nodular goiter who were negative for TSHRAb and received I-131 between January 2013 and December 2018. Prior to treatment, antibodies against thyroid peroxidase (TPOAb) and thyroglobulin (TgAb) were measured, and the uptake of iodine-123 (I-123) at 20-hours or technetium-99m-pertechnetate (Tc-99m) was determined. Patients were treated with median activity of 747 MBq I-131 (range 478–1140 MBq) and followed-up for 12 months. We monitored for de novo occurrence of GD by measuring thyroid function and TSHRAb concentration. Patients’ characteristics influencing the occurrence of I-131-induced GD were analysed; p-value of <0.05 was considered statistically significant.

Results: A total of 1551 patients (277 males, 1274 females) with an average age of 68.6±14.0 years (range 14-95) were included. Prior to I-131 therapy, TPOAb and/or TgAb concentration were raised in 14.5% (225/1551) of patients. The median I-123 uptake, measured in 64.7% (1004/1551) patients, was 29.0%, and the median Tc-99m uptake, measured in 35.3% (547/1551) patients, was 0.82%. An increase in TSHRAb concentration was observed in 4.4% (68/1551) patients at 4.1±3.3 months following I-131 application; of those, 63.2% (43/68) presented with overt hyperthyroidism. Patients with de novo occurrence of GD were significantly younger (63.1±14.3 vs 68.7±13.8 years, P < 0.01) and more likely to be positive for TPOAb and/or TgAb before I-131 application than those with negative TSHRAb (47.1% vs 13.0%, P < 0.001). Furthermore, they had significantly higher median concentrations of TPOAb (41.6 KU/l vs 30.3 KU/l, P < 0.001) and TgAb (15.0 KU/l vs 15.0 KU/l, P < 0.001). Additionally, their median uptake of I-123 before treatment was significantly higher (32.5% vs 29.0%, P < 0.05), but their Tc-99m uptake did not differ (0.89% vs 0.82%, P = 0.72). There was no significant difference in applied activity of I-131 (median, 741 vs 747 MBq, P = 0.09), or gender (P = 0.47).

Conclusion: We show that I-131-induced GD occurs in 4.4% of patients treated for thyroid autonomy. Younger patients and those with increased TPOAb and/or TgAb levels prior to I-131 therapy are at a higher risk of developing GD post-treatment. We therefore recommend monitoring these patients closely following I-131 application.