Background: Thyroid disorders (hyperthyroidism and hypothyroidism) have been known to affect renal function although studies regarding this are scanty. The goal of this study is to evaluate renal dysfunction in newly diagnosed untreated patients with hyperthyroidism and hypothyroidism using serum cystatin C. Methods: A total of 115 patients, 70 females and 45 males age 20 – 55 years with 60 cases of newly diagnosed hyperthyroidism and 55 cases of newly diagnosed hypothyroidism as well as 80 healthy controls (50 females and 30 males) matched for sex and age. Serum levels of thyrotropin (TSH), free tri-iodothyronine (fT3), free tetraiodothyronine (fT4) and cystatin C were measured in all the cases and controls. Serum fT3, fT4 and TSH was analyzed by enzyme linked immunosorbent assay (ELISA) technique using stat – fax- 2100. The serum cystatin C was evaluated using immunoturbidimetric method on TOSOH AIA 360 immuno assay machine. Results: In this study, serum cystatin C levels were significantly higher in both hyperthyroid and hypothyroid patients compared to controls (1.06 + 0.12 vs 1.26 + 0.27 vs 0.7 + 0.14; P - 0.01). Conclusion: Renal dysfunction has been observed in patients with untreated thyroid disorders and therefore, there is need to screen all such patients for renal function.