<p>Protein carbonylation (PC), a marker of oxidative stress, was shown to be elevated in both hyperthyroid and hypothyroid disorders. These conditions are associated with unfavorable fibrin clot properties. We sought to investigate whether elevated PC is associated with prothrombotic markers in hyperthyroid and hypothyroid individuals before and following effective therapy. We studied 31 hyperthyroid, 29 hypothyroid patients, and 29 sex- and age-matched controls. Along with plasma total PC content, we measured fibrin clot properties (fibrin clot permeability, K_s; clot lysis time, CLT), fibrinolysis proteins, and thrombin generation before and after 3-month successful therapy. Hyperthyroid patients had a tendency to higher PC (+ 9.1%; <i>p</i> = 0.05), while hypothyroid individuals had 17.2% higher PC (<i>p</i> = 0.01) compared with controls, without any difference between the patient groups. Pre-treatment PC inversely correlated with K_s in both hyper- (<i>R</i>=-0.425, <i>p</i> = 0.017) and hypothyroid (<i>R</i>=-0.510, <i>p</i> = 0.005) individuals, while solely in hyperthyroid patients PC was associated with CLT (<i>R</i> = 0.556, <i>p</i> = 0.001), but not with fibrinolysis inhibitors, or other hemostatic markers. On-treatment PC, which decreased by 19.6% (<i>p</i> &lt; 0.001) in hyperthyroid and by 23.4% (<i>p</i> &lt; 0.001) in hypothyroid patients reaching the control levels, was associated with K_s (<i>R</i>=-0.401, <i>p</i> = 0.031) and CLT (<i>R</i> = 0.537, <i>p</i> = 0.003) only in the hypothyroid group. In hyper- and hypothyroid patients elevated PC may contribute to formation of more compact fibrin clot networks with impaired fibrinolysis in the former group. Reduced PC following thyroid hormone normalization maintained its impact on fibrin clot properties solely in hypothyroid patients, which indicates complex effects of oxidative stress on blood coagulation.</p> Graphical abstract <p></p>