Introduction: Probiotic yoghurt is considered a functional food, containing beneficial probiotic microorganisms such as lactic acid bacteria. Both hypo and hyperthyroidism are associated with weight change, and since much attention is currently being given to the effects of probiotics on weight, the aim of this study was to investigate the effect of consuming probiotic yoghurt on weight change and thyroid hormones in male Wistar rats. Materials and Methods: In this case-control study, 10 male Wistar rats were randomly divided into two groups, fed with a probiotic yoghurt diet and a normal food one. At end of each week, after gavage, blood samples were collected for 32 days. Body weight was measured and thyroid hormones were detected using the Radio Immuno Assay method. Results: Mean weight and T4 did not differ significantly between two groups throughout the 32 day study. In the probiotic yoghurt group, mean T3 (ng/dl) increased significantly on day 24 compared day 16 (P: 0.04) but no significant difference in mean weight and T4 (mg/dl) was observed between days of sampling. Conclusions: Results showed that consuming probiotic yoghurt was only associated with the active form of thyroid hormones (T3), whereas weight changes were not related to probiotic compounds.