objective To investigate the clinical value of cystatin C in 131I treatment of patients with hyperthyroidism. Methods The thyroid hormone level and serum level of Cys C and Cr in 43 patients with hyperthyroidism before and after 3 and 6 months of 131I treatment and in 40 healthy controls were determined. Results The serum Cys C levels in patients with hypothyroidism were higher than that of healthy controls. The thyroid hormone level in patients with hypothyroidism were back to normal and serum Cys C levels were decreased after 131I treatment (P<0.05)．Conclusion Cystatin C can be the auxiliary diagnostic examination item of hyperthyroidism and could be acted as an auxiliary diagnosis index of efficacy evaluation after 131I treatment.