The mechanisms by which thyroid hormones affect vascular physiology are mostly unknown. Nevertheless, few data are available regarding the effects of thyroid hormones on endothelial function. Experimental and clinical results give of conflicting information, but the influence of hyperthyroidism on endothelium-dependent relaxation is connected with the change of production of Nitric Oxide. The effects of subclinical thyrotoxicosis (ST) on endothelial function and possibilition its correction are unknown. ST characterized by low serum TSH and normal FT₄ and FT₃ levels. The present study includes 49 normotensive patients with ST without any CVD (the age of 20–60 years, 5 men and 44 women); mean serum TSH level – 0,09 ± 0,01 mU/l (normal range, 0.4–4.6 mU/l). The control group consisted of 19 healthy volunteers (16 women and 3 men) matched in age (the age of 21–62 years) and body mass index (BMI). Assessment of endothelial function was performed in the brachial artery using high-resolution vascular ultrasonography by standard method for the assessment of endothelium-dependent vasodilatation (EDV) Also level of von Willebrand factor (VWF) has been studied. Studies were performed before and after treatment of β-blockers during 3 weeks. After the first examination, patients were randomised in to 2 groups. 1st group was treated by nebivolol (2,5 mg/day), 2nd group was treated by metoprolol (25 mg/day). Before treatment level of VWF has been increase (97,7 ± 8,7%) in comparison with control group (46,8 ± 7,3%, p<0,01) and EDV has been decreased at 50% of patients. After treatment FW has not variated in group 1 (98,6 ± 6,9%) compared in control, but has increased in group 2 (119,8 ± 8,8%). EDV was normalized at 60% of patients in group 1, but has not variated in group 2. These results demonstrated, first, that the endothelial dysfunction developed at ST, in the second, that these changes can be removed by therapy of nebivolol.