Although the TSH-receptor antibody (TRAb) plays a central role in the pathogenesis of Graves' disease (GD), the association between TRAb at first diagnosis and clinical and laboratory parameters is not well known. On the other hand, a minority of patients with GD may be TRAb negative, and there is a lack of adequate evidence to demonstrate the clinical and laboratory characteristics of these patients. Therefore, we aimed to investigate the association of TRAb at the initial diagnosis of GD with the clinical and laboratory parameters in a large number of patients with GD and to compare the clinical and laboratory parameters between patients with high TRAb levels and TRAb-negative patients. This study included 440 patients [326 (74%) female, 114 (26%) male]. All patients were classified according to gender, age, smoking habit, and TRAb levels. TRAb levels were significantly higher in male compared to female patients and in smokers compared to non-smokers. Smoking male patients had the highest TRAb levels. In regression analysis, goiter size, male gender, cigarette smoking, Graves' orbitopathy, fT3, and anti-TPO antibody levels were independently associated with high TRAb levels, while age at diagnosis and fT4 levels were not independently associated with high TRAb levels. TRAb-negative GD was diagnosed in 80 (18%) patients. TRA-negative patients had markedly less severe clinical and laboratory hyperthyroidism compared to patients with high TRAb levels. Moreover, the smoking habit was significantly lower in patients with TRAb-negative GD. According to our study results, TRAb levels at the initial diagnosis of GD are differently associated with clinical and laboratory parameters. Male patients and smoking patients with GD tended to have markedly higher TRAb levels and more severe clinical hyperthyroidism. Therefore, besides other contributing factors, male gender and smoking may affect TRAb levels and consequently the severity of hyperthyroidism in patients with GD. Furthermore, male gender and smoking may have a synergistic effect on TRAb levels and consequently on the severity of hyperthyroidism in patients with GD.