---

Introduction: Hypothyroidism causes many hematologic abnormalities including anemia. Anemia can be due to iron deficiency, impaired hemoglobin synthesis, poor diet, structural abnormalities of the red blood cells (RBC) and reduced half-life of RBC. In this study RBC fragility in hypothyroid rats were compared to that of control animals. Materials and method: Hypothyroidism was induced by methimazole (500 mg/L of drinking water for 20 days) in the rats (body weight, 246±5gr). Blood samples were collected through carotid vein on EDTA for osmotic fragility test. RBC fragility of RBC was tested by a routine method in which the cells were placed in different concentrations of NaCl (0.09 gr/L) and hemolysis was determined by colorimetric method. The hemolysis precentage of the samples in each solution was calculated on the basis of the 100% hemolysis in distilled water. Results: RBCs from hypothyroid animals had significantly (p<0.05) higher hemolysis in NaCl concentrations of 2.5, 3, 4, 4.5, 5, 5.5, 6 and 7 gr/L as compared to control animals. Conclusion: Results of this study prove that anemia in hypothyroid rats could also be due to increased fragility of RBC.