Endocrine Abstracts

Objectives: Correction of 131I dose for thyroid gland size is one of the methods used to determine radioactive iodine (RAI) activity for patients with Graves’ disease (GD), but the predictive factors for its clinical outcome are still unclear. This study aimed to investigate which factors influence its success in a real-world cohort.

Methods: Observational and retrospective study of RAI treatments in a terciary referral center in Almada, Portugal, carried out between 2014 and 2022. Therapeutic success was defined as achieving eu- or hypothyroidism within 12 months of 131I administration. The data was analysed using SPSS® and Excel®.

Results: A total of 119 patients (103 females; mean age 48 years), with a mean estimated thyroid gland size of 58g, underwent 124 RAI treatments with a mean RAI dose of 11mCi. Four patients needed more than one 131I administration. A year later, success was reported in 89 (72%) treatments (group 1) with a 28% (n = 35) rate of therapeutic failure (group 2), with no difference in age and sex between groups. In group 1, 84% of patients were hypothyroid and 16% were euthyroid. In the pre-treatment analyses, group 1 had significantly lower TRAbs titers than group 2 (9 vs 15 IU/l; P = 0.01), as well as free T3 (7 vs 9 pg/ml; reference range 2-4.4; P = 0.039). Despite group 1 having shorter disease (57 vs 79 weeks) and antithyroid treatment duration (32 vs 40 weeks), as well as higher TSH levels (0.19 vs 0.03mU/l) and lower thyroid gland size (55 vs 64g) and mean administrated RAI dose (11 vs 12mCi), no association between these variables and treatment success was found. Smoking habits, presence of orbitopathy, corticosteroid therapy, 24h RAI uptake and prior antithyroid therapy were also not identified as risk factors for treatment failure.

Conclusion: With a success rate similar to previous studies, the present study found that lower titers of free T3 and TRAbs are strongly associated with treatment efficacy.