Erythrocyte osmotic fragility and lipid peroxidation in experimental hyperthyroidism Thyroid hormones have multifaceted effects on humans. Among others, they influence the basal metabolic rate; the overproduction of thyroid gland hormones results in an increased rate of metabolism and oxygen consumption by cells. Increased thyroid hormone concentrations affect the oxidant/antioxidant equilibrium and may cause harm to the cell. Thyroid hormones, while leading to increase in free radicals also activate the antioxidant enzymes. The purpose of this study is to determine the structural integrity of red blood cells in experimental hyperthyroidism by assessing the osmotic fragility of erythrocytes in primary hyperthyroidism and its relation between lipid peroxidation. In this study, twenty-two Spraque-Dawley-type female rats weighing between 160 and 200 g were divided into two, as control (n=10) and experimental (n=12), groups. The experimental group animals have received tap water and L-Tiroksin (0.4 mg/100 g fodder) added standard fodder for 30 days to induce hyperthyroidism. Control group animals were fed tap water and standard fodder for the same period. At the end of the experimental period, blood samples were drawn from the abdominal aorta of the rats under light ether anesthesia. Triiodothyronine (T3), thyroxine (T4), and thyroid stimulant hormone (TSH) levels, osmotic fragility, malondialdehyde (MDA), glutathione (GSH) levels and superoxide dismutase (SOD) activity were measured. Statistical analysis was done by Mann-Whitney U test. Data were expressed as means ± SD. Differences between groups were considered significant at the (p<0.05) level. The statistically significant increase in T3, T4 and the significant decrease in TSH) of experimental group is the evidence of induced primary hyperthyroidism. There was a statistically significant increase found in erythrocyte MDA(p<0.001) levels, and SOD activity (p<0.001), but statistically a significant decrease was detected in GSH (p<0.05) levels in hyperthyroid group when compared to controls. In the controls, the osmotic fragility levels were found to be maximum at 0.32 % NaCl and minimum at 0.56 % NaCl, whereas in experimental group reached a maximum at 0.40 % NaCl and minimum at 0.64 %. When statistical evaluations were compared, statistically important increases were detected in maximum and minimum osmotic fragility limits between both the groups (p<0.001). This finding, which shows maximum hemolysis ratio, is the evidence of increased osmotic fragility of the erythrocytes in hyperthyroidism. As a result of our study, it may be concluded that hyperthyroidism may led to an increase in osmotic fragility of erythrocytes and this situation may possibly originate from the increased lipid peroxidation in hyperthyroidism.