Retrospective analysis of efficacy of radioactive iodine treatment of hyperthyroidism using a single calculated 131-I dose

Objectives To evaluate the success rate of therapeutic administration of a single calculated 131-I activity for eliminating hyperthyroidism.

Methods After Internal Review Board approval with waiver of consent we retrospectively reviewed the clinical records of consecutive patients with hyperthyroidism treated with 131-I over an eight year period at the University of Michigan Nuclear Medicine Therapy Clinic. In preparation for radioactive iodine (RAI) treatment, patients underwent pinhole thyroid imaging, 24-hour radioactive iodine uptake (RAIU) measurements and clinical examination. All patients received a calculated 131-I activity of 0.2 mCi per estimated gram of thyroid tissue. The goal of RAI treatment was to eliminate hyperthyroidism and achieve hypothyroidism within 6 months of 131-I administration. Success of RAI therapy was assessed at 7 weeks and 3 months by clinical and biochemical follow-up with measurements of thyroid stimulating hormone (TSH), free tri-iodothyronine (FT3) and free thyroxine (FT4) levels. Successful treatment was defined as initiation of levothyroxine replacement after development of hypothyroidism.

Results Data was obtained on 340 hyperthyroid patients (F258:M82, mean age 43.3 ± 17 y, 4-94); including 20 pediatric patients (5.9%) < 18 years), as follows: 323 patients (95%) with Graves’ disease, 14 patients (4%) with toxic multinodular goiter and 3 patients (1%) with an autonomously functioning thyroid nodule. Treatment history for hyperthyroidism prior to 131-I administration revealed: 190 patients (55.9%) had no prior therapeutic intervention (treatment-naïve patients), 7 patients had prior RAI treatment at another medical center, 8 patients had prior thyroidectomy, and 135 (39.7%) had been treated with anti-thyroid medications, either propylthiouracil (PTU) or methimazole (MTZ) or both. The mean estimated thyroid gland size was 49.8 grams ± 18 range 15 - 100. Mean RAIU was 0.55 ± 18.1 (normal 0.7-0.30). RAI doses ranged from 5 to 113 mCi (mean dose = 19.3 mCi). Successful treatment of hyperthyroidism at our institution was obtained after a single therapeutic 131-I activity administration in 311 of 333 (93.4%) patients. Persistent hyperthyroidism was recorded in 22 patients (6.6%) who were subsequently managed as follows: 18 patients required two RAI treatments, 2 patients required three RAI treatments and 2 patients were referred for thyroidectomy with subsequent cure of their hyperthyroidism, due to patient decision after considering their treatment options. Multivariate logistic regression analysis demonstrated that failure of 131-I therapy was associated with age (p = 0.011), previous PTU therapy (p < 0.001) and size of the thyroid (p = 0.014), but not with MMZ therapy, interval between diagnosis and RAI treatment, dose of 131-I therapy or RAIU values before therapy. The mean response time after successful RAI therapy was 112.7 days, with 25% responding at 62 days, 50% by 84 days and 75% by 120 days post radioiodine administration. The mean response time for treatment naïve patients (no prior antithyroid medications) was 137 days vs.181 days for patients managed with antithyroid medications (p=0.039). The mean time to respond for those on prior PTU was 240 days compared to 116 days for those on MMZ and 109 days for those not previously treated with antithyroid medications.

Conclusions Successful single calculated 131-I activity therapy for hyperthyroidism can be achieved using 0.2 mCi of 131-I per estimated gram of thyroid tissue corrected for thyroidal 131-I uptake at 24 hours. Treatment failure occurs in 7% of patients and is mainly associated with and prior PTU medication treatment and larger thyroid glands. $$graphic_9E74D22B-CFD7-48DB-96C1-C19BE809F139$$