Objective To investigate the changes of serum thyroid hormone in the patients with type 2 diabetes mellitus and its clinical significance. Methods A total of 120 patients with type 2 diabetes mellitus and 120 cases of healthy control were collected. The serum thyroid hormone including free triio-dothyronine (FT3), free thyroxine (FT4), and thyrotrophin (TSH) were measured by radioimmunoassay method. The levels of serum thyroid hormone were compared between the two groups. Moreover, 120 patients with diabetes mellitus were divided into four groups based on the levels of glycated hemoglobin as following: <5.5%, 5.5%-6.5%, 6.6%-9.5%, >9.5%, and thyroid hormone levels were measured. Results The serum FT3 levels in patients with type 2 diabetes mellitus were significantly lower than that of controls (P<0.05). There was no significantly difference in terms of FT4 and TSH between two groups (P>0.05). The serum FT3 levels in patents with type 2 diabetes mellitus with glycated hemoglobin in the range of 5.5%-6.5%, 6.6%-9.5%, >9.5% were significantly lower than that of <5.5% group (P <0.05). However, there was no significantly difference in term of FT4 and TSH between those groups (P>0.05). Conclusion The levels of serum thyroid hormone in diabetes patients mainly showed FT3 level reduce, which was directly proportional to the severity of the diabetes. Therefore, the serum FT3 could be used as an important index in the evaluation of severity of diabetes.