Objective To investigate the correlation between thyroid hormone and patient’s condition, course of disease and complication in patients with diabetes mellitus (DM). Methods 100 patients with DM diagnosed between January 2010 and December 2012 were divided into different groups according to disease type, condition, and complication of DM. The serum level of free serum thyroid hormone FT₃ and FT₄ in MD patients and 50 healthy controls were determined by RIA. Results The percentage of abnormal thyroid function in DM group was 24%, and the ratio in female patients was higher than that of male patients (P＜0.05). The serum level of FT₃ and FT₄ in DM group was significantly lower than that of in healthy control group, and FT₃ and FT₄ levels in Type 1DM was lower than that of in Type 2 DM (P<0.01). With the course of disease prolongs, there was a significant decreasing in FT₃ level, while FT₄ showed almost no changes (P>0.05). The serum FT₃ and FT₄ levels in complicated group were significantly lower than that of with diabetic nephropathy and DKA. Conclusion The serum levels of FT₃ and FT₄ in patients with diabetes mellitus show some degree of decrease, and they may be used as an indicator for the diagnosis and evaluation of treatment of patients with diabetes mellitus.