Efficacy of radioactive iodine treatment of graves' hyperthyroidism using a single calculated I dose.

Objective: To evaluate the success rate of therapeutic administration of a single calculated I activity for eliminating hyperthyroidism due to Graves' disease.

Methods And Materials: Patients with Graves' hyperthyroidism underwent pinhole thyroid imaging, 24-h radioactive iodine uptake (RAIU) measurements and clinical examination and received a calculated I activity of 0.2 mCi per estimated gram of thyroid tissue, adjusted for the 24-h RAIU. The goal of RAI treatment was to achieve hypothyroidism within 3-6 months of I administration. Response to RAI therapy was assessed at 7 weeks and 3 months by clinical and biochemical follow-up.

Results: The study included 316 hyperthyroid patients with Graves' disease (F238:M78, mean age 42.1 ± 16 y, 4-94). 179 patients (56.6%) had no prior therapeutic intervention (treatment-naive patients), whereas 6 patients had prior thyroid surgery, and 131 (41.5%) had been treated with anti-thyroid medications.The mean estimated thyroid gland size was 50.2 g ± 18, range 15-100. Mean RAIU was 0.57 ± 0.17 (normal 0.07-0.30). RAI doses ranged from 5 to 70 mCi (mean dose = 18.1 mCi). Successful treatment of hyperthyroidism at our institution was obtained after a single therapeutic 131-I activity administration in 295 of 316 (93.3%) patients. Multivariate logistic regression analysis demonstrated that failure of I therapy was associated with previous PTU therapy ( <  0.001).The mean response time after successful RAI therapy was 110.2 days, with cumulative response of 25% at 61 days, 50% by 84 days and 75% by 118 days after radioiodine administration. The mean time to respond for those on prior PTU medications was 297 days compared to 116 days for those on MMI and 109 days for those not previously treated with antithyroid medications. In patients with persistent hyperthyroidism, failure of RAI therapy was documented in 16 patients (76.2%) within (less than) one year after I administration and in 5 patients (23.8%) more than one year after initial therapy, considered late failure.

Conclusion: Successful I therapy for Graves' hyperthyroidism with a single calculated dose can be achieved in the majority (> 90%) of patients, adjusting for the thyroid size and 24 h uptake measurement.