Insulin resistance has been proposed to play a pivotal role in vasoconstriction due to increased oxidative stress. Hyperthyroidism would amplify cardiovascular disease risk in diabetic patients, though thyroid hormone advance vascular relaxation and reduce vascular contraction by virtue of NO production in vascular smooth muscle cells (VSMCs). Thus, we aimed to investigate the vascular tone and its underlying mechanism in insulin resistance accompanied by hyperthyroidism. Vascular reactivity studies showed that endothelium-denuded thoracic aortic rings from rats fed with high-fat high-sucrose (HFHS) diet and L-T4 (HFHS+L-T4) exhibited a stronger contractile response to noradrenaline than HFHS rats, which was reversed by L-NAME and GSH. Furthermore, rat thoracic aortic smooth muscle cells (A10) simultaneously stimulated with high glucose insulin (high Glc/Ins) and T3 demonstrated lower NO, superoxide anion (Symbol) levels, and higher iNOS, nitrite (Symbol), peroxynitrite (ONOO−) levels than that treated with T3 only. Excessive ONOO− markedly aggravated oxidative stress. Thus, we hypothesized that the elevated concentration of ONOO− which is generated by NO and Symbol could be critically instrumental in the progression of vasoconstriction by increasing oxidative stress.