OBJECTIVE: Hyperthyroidism's impact on cardiovascular, hematopoietic systems and metabolism might lead to hemorheological changes. This study aimed to investigate the changes in hemorheological properties via erythrocyte deformability and aggregation, whole blood viscosity (WBV) and plasma viscosity (PV) in hyperthyroid patients. The effect of anti-thyroid treatment on hemorheology was also studied. MATERIAL METHODS: Thirty-six patients with overt hyperthyroidism, 19 patients with subclinical hyperthyroidism and 66 controls were included in the study. Hematocrit, erythrocyte deformability and aggregation, plasma and whole blood viscosity values were measured before treatment. Hemorheological parameters of the patients were compared with the control. Before and after treatment results of overt hyperthyroidism were analyzed. Methimazole was given as anti-thyroid treatment. Deformability and aggregation measurements were conducted using a laser ektacytometer (LORRCA) while viscosity measurements were performed with a cone-plate viscometer (Brookfield DV-III). RESULTS: The maximum elongation index (EImax) decreased significantly from 0.664 (0.01) pre-treatment to 0.657 (0.01) post-treatment (p = 0.04). The aggregation index was significantly higher in both the subclinical hyperthyroidism group [68.05 (7.66), p = 0.001] and the overt hyperthyroidism group [66.78 (8.815), p = 0.001] compared to the control group. Additionally, the aggregation half-time was significantly shorter in the subclinical hyperthyroidism group [1.9 (1.21-2.27), p = 0.001] and the overt hyperthyroidism group [1.91 (1.43-2.46), p = 0.001] relative to the control group. CONCLUSION: The hemorheological status of patients was influenced by excessive thyroid hormones in both subclinical and overt hyperthyroidism groups. Additionally, anti-thyroid therapy with methimazole may play a role in the observed decrease in the maximum elongation index following treatment.