<i>Objective:</i> The aim of this study was to determine whether or not the titre of thyroid-stimulating hormone receptor antibody with stimulating (TRAb-S) activity changes in patients with Graves’ disease (GD) or toxic multinodular goitres (TMNG) 3 months after treatment with sodium iodide (¹³¹I). <i>Subjects and Methods:</i> Serum specimens were obtained from 21 hyperthyroid patients (15 with GD and 6 with TMNG) at a median 0.5 months before and 3 months after ¹³¹I treatment using a standard ablative dose of 555 MBq. TRAb-S activity was measured in a sensitive and specific luminescent bioassay employing the lulu cell line and expressed as a stimulation index (SI; normal ≤1.5). <i>Results:</i> The mean TRAb-S in the GD patients was 2.72 SI (95% CI: 1.51–4.03) 0.5 months before administration of ¹³¹I and 3.98 SI (95% CI: 1.20–6.76) 3 months after administration of ¹³¹I. The difference was not statistically significant at p<i> <</i> 0.8. It was not elevated in the TMNG patients before (0.57 SI; 95% CI: 0.41– 0.73) and after (1.00 SI; 95% CI: 0.74–1.26) treatment either. <i>Conclusions:</i> Radioiodine therapy for GD or TMNG did not induce a significant change in TRAb-S activity at 3 months after treatment with ¹³¹I, probably due to effective antithyroid therapy or the timing of samples.