Objective To explore the association between bipolar disorder and thyroid function. Methods A total of 59 bipolar disorder patients diagnosed according to DSM-Ⅳ -TR and 41 controls were included. The levels of thyroid hormone， including serum total triiodothyronine (TT3)， free triiodothyronine (FT3)， total thyroxine (TT4)， free thyroxine (FT4) and thyroid stimulating hormone(TSH) were measured by enzyme linked immunosorbent assay. Hamilton Depression Scale (HAMD)， Hamilton Anxiety Scale (HAMA) and Bech-Rafaelsen scale (BRMS) were used to assess the clinical symptoms of the patients. Results The mania group showed significantly higher values of TT4 and FT4 than control group， and lower value of FT3 compared with depression group (P＜ 0.01). While the depression group showed significant lower values of TT3 and FT3， and higher value of FT4 compared with control group (P＜ 0.05). Level of FT4 was significantly higher in female mania group than control group (P＜ 0.05). Level of TT3 was significantly lower in female depression group compared with mania group and control group(P ＜ 0.05). Female depression group showed significant higher level of FT4， and lower TT3 compared with control group (P＜ 0.05). Only level of TT4 was significantly higher in male mania group than control group (P＜ 0.05). In bipolar depression patients， the level of FT4 was negatively correlated with HAMD score (r=-0.34，P=0.03). Conclusions Thyroid function in patients with bipolar disorder shows certain changes. Changes in the level of thyroid hormone were different between mania patients and depression patients， especially in female.