The effect of prior antithyroid drug use on delaying remission in high uptake Graves' disease following radioiodine ablation

Antithyroid drugs (ATDs) have been shown to attenuate the effectiveness of radioiodine (radioiodine ablation, RIA) therapy in Graves' disease. We undertook a study to look at the impact of iodine uptakes on the outcome of (131)I therapy. To determine the effect of prior ATD use on the duration of time to achieve cure in patients with high vs intermediate uptake Graves' disease who received a fixed dose (15 mCi) of (131)I radioiodine. In a retrospective study of patients with Graves' disease, 475 patients who underwent RIA were followed-up on a two-monthly basis with thyroid function tests. Of the 123 patients with a documented preablation RAIU and consistent follow-up it was observed that 40 patients had an intermediate RAIU (10-30%) and 83 subjects had a distinctly increased uptake (>30%). Successful cure was defined as the elimination of thyrotoxicosis in the form of low free thyroxin and rising TSH levels. When a standard dose of 15 mCi (131)I was administered, a cure rate of 93% was achieved. The median duration of time to cure (TC) was 129 days. Surprisingly, a direct proportional linear relationship (R(2)=0.92) was established between time to cure and radioiodine uptake (TC> 3 0%=172days, TC10 - 3 0%=105 days, P<0.001). Patients who used ATD medications took a proportionately longer duration to achieve remission (TCNO ATD=102days, TCATD=253days, P<0.001). The effect of prior ATD therapy in delaying remission was amplified in the subset of patients with higher uptakes (TC> 3 0% + ATD=310days, TC> 3 0% + NO ATD=102days, P<0.001) compared to those with the intermediate uptakes (TC10 - 3 0% + ATD=126 days, TC10 - 3 0% + NO ATD=99 days, P<0.001). RIA, using a dose of 15 mCi achieved a high cure rate. Higher uptakes predicted longer time to achieve remission, with prior ATD use amplifying this effect.