Radioiodine therapy in hyperthyroidism: inverse correlation of pretherapeutic iodine uptake level and post-therapeutic outcome

BACKGROUND: High iodine uptake levels are widely accepted as a condition for successful treatment with radioiodine (RAI). However, the existing data are controversial and the correlation of pretherapeutic RAI uptake level and outcome of RAI therapy has not yet been quantified. The aim of this study was to analyze the influence of RAI uptake on the outcome after RAI treatment and to estimate uptake-dependent success rates. MATERIALS AND METHODS: We retrospectively analyzed 229 patients (m = 53, f = 176; age 64 +/- 14 years) suffering from toxic adenoma, multinodular goitre or Graves' disease, respectively. Clinical status and T3, fT4 and TSH levels were assessed 3, 6, 12 and 18 months after treatment. Successful treatment was defined as loss of hyperthyroidism 18 months after radioiodine therapy. Logistic regression was used to assess the relation between the maximum iodine uptake and the rate of success and hypothyroidism, respectively, after RAI treatment. RESULTS: Overall, patients presented with pretherapeutic RAI uptake values between 17% and 100%. Eighteen months after RAI treatment, an euthyroid state was achieved in 136 patients (60%), hypothyroidism occurred in 47 patients (20%) and 46 patients (20%) remained hyperthyroid. The patients with the lowest pretherapeutic RAI uptakes showed the highest success rates. The overall success rate significantly decreased from 92% at low RAI uptakes to 57% at high uptakes (P = 0.002). This effect was found in the patients suffering from multinodular goitre as well as in the patients with Graves' disease. CONCLUSION: In contrast to the current opinion, our results provide evidence that the pretherapeutic iodine uptake level and post-therapeutic outcome are inversely correlated.