Early Transient Hypothyroidism Is Associated With Long‐Term Outcomes of 131I Treatment in Graves' Disease: A Retrospective Cohort Study

Background Radioactive iodine ( 131 I) is a cornerstone therapy for Graves' disease (GD), yet predicting long‐term thyroid functional outcomes remains challenging. Early transient hypothyroidism (ETH) is a common phenomenon post‐therapy, but its prognostic value for differentiating between permanent hypothyroidism and persistent hyperthyroidism is not fully elucidated. Methods This retrospective cohort study included 222 GD patients treated with 131 I. Patients were categorized based on thyroid function within the first 6 months: the ETH group (occurrence of transient hypothyroidism, n = 59) and the Non‐ETH group ( n = 163). ETH was defined as TSH > 4.2 mIU/L with or without low FT4 on at least one measurement, with temporary levothyroxine support provided if symptomatic. Detailed baseline characteristics, including calculated dose per gram of thyroid tissue (mCi/g), were analyzed. The primary endpoints were permanent hypothyroidism, euthyroidism, or persistent/recurrent hyperthyroidism. Kaplan‐Meier survival analysis and multinomial logistic regression models were employed to identify independent predictors of long‐term outcomes. Results Baseline analysis revealed that the ETH group had significantly higher pretreatment TRAb levels (median 19.30 vs. 14.35 IU/L, p = 0.021) and a higher prevalence of palpitations (84.7% vs. 64.4%, p = 0.004) compared to the Non‐ETH group. Importantly, the ETH group had a significantly higher rate of developing permanent hypothyroidism (52.5% vs. 20.9%, p < 0.001), whereas the Non‐ETH group was prone to persistent hyperthyroidism (55.8% vs. 28.8%). In multinomial logistic regression adjusted for dose intensity (mCi/g), Non‐ETH status was a strong independent predictor for persistent hyperthyroidism (OR = 3.888, 95% CI: 1.435–10.535, p = 0.008) relative to permanent hypothyroidism. Furthermore, higher dose intensity (mCi/g) was universally associated with permanent hypothyroidism, while older age favored euthyroidism outcomes (OR = 1.055, p = 0.002). Conclusion Early transient hypothyroidism following 131 I therapy serves as a clinical indicator associated with effective glandular destruction and a high trajectory toward permanent hypothyroidism. Conversely, the absence of ETH signals a high risk of treatment failure. These findings suggest that patients with ETH require careful long‐term monitoring for hypothyroidism rather than aggressive anti‐thyroid drug re‐initiation, and the dose per gram of tissue is a definitive determinant of functional ablation.