BACKGROUND: The functioning of the thyroid gland is a multi-component process that in some conditions may undergo alterations. The thyroid gland is part of the endocrine system that produces the iodine-containing hormones thyroxine and triiodothyronine. Thyroid hormones, control metabolism and energy, growth processes, maturation of tissues and organs, regulation of blood flow, and, therefore, providing vital functions of the body. The role of thyroid hormones in the regulation of blood flow is determined by the intensity of their production and the quantity in the blood. Presumably, in case of oncological and non-oncological diseases of the thyroid gland, the fluidity of the blood, which depends on the rheological properties, will be different. OBJECTIVE: Our aim was investigating rheological characteristics for studying of changes of rheology in patients with thyrotoxicosis, with benign tumor pathology of the thyroid gland, with thyroid cancer and finding possible diagnostic markers for differentiating benign and malignant thyroid tumors METHODS: In this regard, we examined, using modern methods accepted in clinical practice, a standard list of recommended diagnostic tests in the group of patients (thyrotoxicosis: n = 25; benign tumor: n = 47), thyroid cancer: n = 35) and control group (n = 15), and with new original methods, parameters that describe the rheological properties of the blood, such as blood rheological index, volume, thickness, surface area of erythrocytes, erythrocyte aggregation index, deformation index, plasma viscosity, hematocrits. RESULTS: Against the background of relative changes in the studied values, it is necessary to pay attention to the fact that erythrocyte aggregation in patients with a benign form and control, as well as in patients with a malignant form and control, differ significantly from each other, in addition, there is a significant difference between aggregation in the group of patients with benign and control aggregation. malignant forms of the disease. It is significant that aggregability differs in patients with thyrotoxicosis and in controls. This indicates that erythrocyte aggregation is particularly informative. The blood rheological index most clearly demonstrated the difference between benign and malignant forms of the disease. Significantly changed compared to control in various forms of thyroid diseases. CONCLUSION: Additional diagnostic markers for differentiating benign and malignant thyroid tumors may be consideredeerythrocyte aggregation index and blood rheological index.