Endocrine Abstracts

Aim: In Graves’ disease (GD) the goal of radioiodine (I-131) application is the elimination of hyperthyroidism. The reported success rate 1 year after treatment is up to 90%. The aim of this retrospective study was to assess the factors influencing the early outcome 3 months after I-131 treatment of GD patients in an iodine sufficient area.

Materials and Methods: We reviewed medical records of GD patients who received I-131 therapy between January 2013 and December 2017. In all patients antithyroid drugs were discontinued at least 3 days before I-131 application. Prior I-131 therapy, uptake of iodine-123 (I-123) at 20-hours or technetium-99m-pertechnetate (Tc-99m) was measured. During follow-up, thyroid function was evaluated up to 3 months after I-131 application and time to occurrence of hypothyroidism was established. Patients’ characteristics influencing the early outcome of I-131 treatment were estimated, p value of <0.05 was considered statistically significant.

Results: We evaluated 849 GD patients (660 females and 189 males) aged between 16 and 91 years (mean age, 50.4±17.0 years). The median I-123 uptake, measured in 37.8% (321/849) patients, was 69.0%, and the median Tc-99m uptake, measured in 62.2% (528/849) patients, was 3.18%. The median treatment I-131 activity was 623 MBq I-131 (range, 436-1123 MBq). Follow-up data were available for 91.6% (778/349) patients. At 3 months after I-131 application we confirmed hypothyroidism in 81.6% (635/778) patients, hyperthyroidism in 14.1% (110/778) and euthyroidism in 4.3% (33/778) patients. Hypothyroid patients were younger than hyperthyroid and euthyroid patients (49.1± 16.7 years, 55.7±17.4 years and 57.8±17.0 years, respectively, P < 0.001), with a higher proportion of females (chi-square=6.6, P = 0.04). Their median I-123 uptake was higher (72.2%, 62.5% and 33.0%, respectively, P = 0.02), but their Tc-99m uptake did not differ (3.20% and 2.78% and 2.98%, respectively, P = 0.38). In hypothyroid patients applied I-131 activity was lower (587 MBq, 737 MBq and 728 MBq, respectively, P < 0.001). Mean time to hypothyroidism was 2.3±0.9 months. There was a significant correlation between time to hypothyroidism and younger age (r =-0.087, P = 0.03), but no correlation was confirmed with I-123 uptake (r =-0.004, P = 0.95), Tc-99m uptake (r =-0.017, P = 0.73) or applied I-131 activity (r =-0.007, P = 0.86).

Conclusion: More than 80% of GD patients develop hypothyroidism already within 3 months after I-131 therapy. This outcome is more frequently associated with younger age, female sex and higher uptake of I-123. Careful monitoring of patients following I-131 application is necessary to identify and treat the early outcome in a timely manner.

References: none