Background: Chromosomal damage, as assessed by clastogenic factors (CFs) and micronuclei (MN) appearance, following radioiodine therapy of Graves' disease has been reported.

Objective and Methods: To evaluate the effect of Ginkgo biloba extracts (EGb 761) supplementation on time-course (up to 120 days) of CFs and MN appearance in lymphocytes from patients with Graves' disease after ¹³¹I therapy. Patients were randomly assigned to EGb 761 or placebo, in a blinded manner.

Results: In the placebo group, MN increased early (P < 0.001) after ¹³¹I, peaking at 21^th day (P = 0.0003) and declining thereafter. In EGb 761-treated patients MN increased early (P < 0.05), while returning toward baseline value thereafter. Therefore, mean MN increment was significantly higher in the placebo group as compared to EGb 761-treated patients (P < 0.01). Moreover, an early (P < 0.0001) and sustained (up to 35 days, P < 0.001) MN increase induced by CFs was observed in the placebo group. Conversely, in EGb 761-treated patients MN increase induced by CFs never reached the statistical significance; therefore, the mean of the MN increments was significantly lower than in placebo (P < 0.05). A significant positive correlation between MN maximum increment and the bone marrow dose was observed in the placebo group only (P = 0.03). No significant difference was observed in clinical outcome between the two groups.

Conclusions: EGb 761 supplementation neutralized genotoxic damage induced by radioiodine treatment, without affecting the clinical outcome. Although ¹³¹I therapy is generally safe, our data suggest that Gingko biloba extracts may prevent genetic effects of radioiodine therapy for hyperthyroid Graves' disease.