Endocrine Abstracts

Introduction: The definitive treatment of hyperthyroidism caused by toxic adenoma (TA) and toxic multinodular goiter (TMNG) is ablative therapy with radioactive iodine (RAI) or surgical resection of the hyperfunctioning thyroid nodules. Hypothyroidism after RAI therapy is a common complication.

Objectives: To evaluate the prevalence and risk factors of hypothyroidism in patients undergoing RAI for AT and BMNT.

Methods: A Multicentre non-randomized cohort study was performed. Patients diagnosed with AT or TMNG who underwent RAI from January 2018 to November 2021 were eligible. The following inclusion criteria were used: >18 years old, negative thyrotropin(TSH)-receptor antibodies and positive technetium-99m scintigraphy for hyperfunctioning adenomas. Patients with post-therapy resolution of clinical and biochemical hyperthyroidism (TSH>0.4µUI/mL) were considered cured. Biochemical hypothyroidism was defined as TSH>4.2 µUI/mL RAI activity was chosen based on thyroid volume and hyperthyroidism severity.

Results: One hundred-forty-eight patients (126 women) with AT (n =57) and TMNG (n =91) treated with RAI (activity between 5-20mCi), with a mean age of 65 ± 13 years, were admitted for analysis. Forty-five patients with TMNG presented with more than 3 hyperfunctioning nodules and 86.8% of TMNG patients had hyperfunction nodules in both thyroid lobules (bilateral disease). In the first year after RAI, 94,6% (n =140) patients had biochemical cure of hyperthyroidism and 35.8% of patients developed hypothyroidism requiring levothyroxine supplementation. In the majority (n =32; 60.4%) of patients who developed hypothyroidism, it manifested within the first 6 months after RAI therapy. The onset of hypothyroidism was more prevalent among women (Chi-square test: 36.5% vs 22.7%, respectively, P-value <0.001) and more common in patients with the diagnosis of AT when compared with those with TMNG (Chi-square test: 42.1% vs 29.7%, respectively, P-value <0.001). Moreover, patients with bilateral hyperfunctioning nodules presented with lower hypothyroidism rate than those with unilateral thyroid disease (Chi-square test: 30.3% vs 39.1%, respectively, P-value <0.001). No difference in post-RAI hypothyroidism’s prevalence was found regarding age, RAI activity, TSH or T4L levels at the diagnosis.

Conclusion: In this study, hypothyroidism after RAI in AT/TMNG presented in 35.8% of the patients. Hypothyroidism after RAI was more frequent in women, patients with a single hyperfunctioning adenoma and unilateral disease. This evidence may indict that ablative activity of RAI under normal-functioning thyroid tissue may have an impact on hypothyroidism incidence post-RAI therapy.