Object: Multidrug resistance 1 gene is responsible for the resistance of a large variety of drugs in human cells. We tried to evaluate this in the present study in thyroid stimulant hormone receptor antibody positive subjects. Method: In study enrolled 23 female and 10 male subjects. Hyperthyroid subjects were treated with PTU and remission was assured between 6-12 months. Blood samples were collected before the start of this treatment. Permission for this study was taken from the patients and the local ethical committee. Results: Serum F-T3, F-T4 levels in Graves subjects were markedly high, whereas TSH levels were markedly low than normal range. We also found that with increased age of the Graves' subjects, MDR-1 gene expression decreased. There was also a direct correlation between blood MDR-1 gene expression and TSH-R Ab levels in patients with Graves's disease. We observed that the duration of being euthyroid was lengthened with the elevation of MDR-1 gene expression. There was a direct correlation between blood MDR-1 gene expression levels and ultra-sonografi c size of thyroid gland. Conclusion: As a result, raised blood MDR-1 gene expression levels in patients with Graves-Basedow disease may be associated with the activity of the disease and the resistance to its treatment. The more blood MDR-1 expression increases the more the duration of being euthyroid increases.