The effects of experimental hypothyroidism on hemorheology and plasma fibrinogen concentration Thyroid hormones are responsible for arrangement of general body metabolism. In their deficiency pathological findings arise related to various physiological systems. In hypothroid cases it has been observed that blood rheology alters. Hematological changes, especially anemia, are common and well characterized in hypothyroidism. This study has been planned with the purpose to determine the effects of experimental hypothyroidsm on hemorheological parameters and fibrinogen concentration. In this study, twenty Spraque-Dawley type albino female rats weighing 160-200g were used in both control and experimental group animals. Both control (n=10) and experimental group (n=10) animals were kept in same physical conditions during experimental period and were fed with standard fodder and tap water. To induce hypothyroidism, methimazole (75-mg/100g fodder) was given to experimental group rats for 20 days. At the end of the experimental period, blood samples were drawn from the abdominal aorta of the rats under the light ether anesthesia. Thyroid stimulant hormone (TSH), triiodothyronine (T3), thyroxine (T4) hormone levels, hematocrit (% Hct), hemoglobin (Hb), blood viscosity, erythrocyte rigidity index, plasma viscosity and plasma fibrinogen concentration values of both control and experimental group animals were determined and evaluated. Statistical analysis was done by the Mann-Whitney U test. Data were expressed as means ± SD. Differences between groups were considered significant at the p<0.05 level. According to results of our study when values of control group and experimental groups were compared: While T3 and T4 levels of experimental group were found lower (p<0.001) but TSH level higher (p<0.001) than that of the control group. Statistically significant decreased T3, T4 and increased TSH values in experimental group were taken as the indicator of the primary hypothyroidism. Plasma viscosity and fibrinogen concentration of hypothyroid group were found significantly higher than controls (p<0.01). Hematocrit and hemoglobin values were also found lower in experimental group than the control group animals (p<0.01). However, there was no significant difference found in blood viscosity at the original Hct value but there was a significant increase at standart Hct value (p<0.01). There was also no change in erythrocyte rigidity index between control and experimental groups. According to results of our study it may be concluded that in the early phase of hypothyroidism, changes in hemorheology are only seen as an increase in plasma viscosity. Such an increase may be dependent on increased fibrinogen concentration.