Pre‐Treatment With Potassium Iodide and Higher Radioiodine Dose Are the Risk Factors of Mid‐to‐Late‐Phase Transient Thyrotoxicosis Following Radioiodine Therapy for Graves' Disease in Japanese Patients: A Retrospective Cohort Study

BACKGROUND: Radioiodine therapy (RIT) is an effective treatment for Graves' disease (GD). However, some patients experience transient exacerbation of thyrotoxicosis in the mid-to-late-phase period (2 weeks to several months post-RIT), and its risk factors are poorly understood. This study aimed to investigate the clinical predictors of mid-to-late-phase transient exacerbation of thyrotoxicosis following RIT. METHODS: We conducted a retrospective cohort study involving 200 Japanese patients with GD who received RIT at a single university hospital. Patients were categorized according to the clinical course as follows: no exacerbation, transient exacerbation or persistent exacerbation, likely due to residual or worsening GD. An exacerbation was defined as an increase of ≥ 1.0 pg/mL in free triiodothyronine (FT3) or ≥ 1.0 ng/dL in free thyroxine. Multivariable logistic regression, guided by the Akaike Information Criterion, was used to identify risk factors. RESULTS: I dose as a significant independent risk factor (odds ratio [OR] = 1.338, 95% confidence interval [CI] = 1.048-1.710, p = 0.020). Although not statistically significant, high pre-RIT FT3 levels and potassium iodide (KI) administration were identified as relevant predictors in the model. CONCLUSIONS: I dose was an independent risk factor for mid-to-late-phase transient thyrotoxicosis after RIT. Higher pre-RIT FT3 levels and KI use may contribute to this risk. Identifying patients with these factors may be helpful for closer post-RIT monitoring to prevent complications and avoid misdiagnosing this transient event as a treatment failure.