BACKGROUND It has been reported that overt and mild thyroid dysfunctions are associated with insulin resistance (IR). We performed this retrospective study to evaluate the relationships between thyroid-stimulating hormone (TSH) levels within the reference range and IR. MATERIAL AND METHODS A total of 447 outpatients were enrolled in this study: 298 with type 2 diabetes mellitus and 149 nondiabetic individuals. Based on a cutoff HbA1c value of 7%, diabetic patients were additionally divided into 2 groups: a high-HbA1c group (n=240) and a low-HbA1c group (n=58). The relationships of TSH levels and HOMA-IR were computed using linear regression models. RESULTS TSH levels were positively and linearly associated with HOMA-IR in both the nondiabetic and diabetic groups (r=0.210, p=0.011 and r=0.451, p<0.001), as well as in the high- and low-HbA1c groups (r=0.507, p<0.001 and r=0.259, p=0.048). A better correlation between TSH levels and HOMA-IR was found in the diabetic group and in the high-HbA1c group when compared with the nondiabetic group and the low-HbA1c group, respectively. Linear regression analysis showed that TSH levels were independently associated with HOMA-IR (p=0.034, =0.049 and <0.001 in nondiabetic, low-, and high-HbA1c groups, respectively). CONCLUSIONS Our data suggest that TSH is independently associated with insulin resistance.