AbstractBackgroundFollowing thyroidectomy, radioiodine therapy is the standard management of differentiated thyroid cancer. The effects of such treatment on testicular function remained a concern for cases and clinicians.ObjectiveWe aimed to observe changes in fertility indicators in men treated with ablation.Materials and MethodsIn this prospective cohort study, 18 men with differentiated thyroid cancer from June to December 2020 underwent thyroidectomy plus radioiodine therapy. Participants were grouped based on iodine dose (8 men with 30 mCi vs. 10 men with≥150 mCi). Baseline values (VB) of the follicular stimulating hormone, luteinizing hormone, testosterone, and sperm analyses were measured 3 wk before iodine ablation and repeated 3 (V3) and 12 (V12) months later. They were analyzed once as a whole and once based on their groups via ANOVA and Friedman's tests where appropriate.ResultsThe mean age of participants was 35.61±9.74 yr. Follicular stimulating hormone levels showed a significant trend among all participants (VB: 12.51±1.72, V3: 13.54±1.41, and V12: 13.10±1.67 IU/mL; p<0.001). Luteinizing hormone showed a similar pattern (VB: 4.98±1.27, V3: 5.65±1.29, and V12: 5.21±0.95 IU/mL; p<0.001). Testosterone levels did not differ significantly from baseline. Sperm count decreased at the first checkpoint and returned to normal after 12 months (VB: 38.22±19.40, V3: 32.05±17.96, and V12: 36.66±18.81 million/mL; p<0.001). Sperm motility and morphology did not change significantly.ConclusionOur research showed that even less than 5 GBq irradiation could induce a transient testicular dysfunction in the first 3 months of therapy, but it was mostly reversible after 12 months.