From Hyperthyroidism to Hypothyroidism: A Detailed Analysis of Radioactive Iodine Therapy Outcomes Based on Dosage

Background: Hyperthyroidism, a common thyroid disorder, is often managed 
with radioactive iodine (RAI) therapy. However, the efficacy and outcomes of 
different RAI doses over time remain incompletely understood. This study 
aimed to evaluate the impact of various RAI doses on thyroid function in 
hyperthyroidism patients, assessing changes over a 12-month period post-
therapy.
Patients and Methods: We conducted a retrospective analysis of 303 
hyperthyroidism patients who underwent RAI therapy at two medical centers. 
Patients were predominantly female (54.8%) with a median age of 38 years. We 
analyzed thyroid function at 6 and 12 months post-therapy, correlating outcomes 
with RAI doses ranging from 10-20 mCi. Symptoms, hormone levels, and the 
presence of anti-TPO antibodies were also assessed.
Results: At 6 months post-RAI, 53.5% of patients developed hypothyroidism, 
27.0% were euthyroid, and 19.5% remained hyperthyroid. By 12 months, 79.5% 
had hypothyroidism. The incidence of hypothyroidism varied with RAI dose, 
being highest in the 18-20 mCi group. Notably, a dose-dependent response was 
observed, with higher RAI doses more likely to result in hypothyroidism.
Conclusions: Most Graves’ disease patients developed hypothyroidism within 
one year following RAI131 treatment, with higher doses associated with earlier 
onset hypothyroidism. RAI131 dosage and other parameters may predict 
hypothyroidism timing and help guide therapy individualization.