Abstract: Objective To analyze the main causes of myasthenia gravis（MG） accompanying low T₃ syndrome. Methods Twenty-five MG patients accompanying low T₃ syndrome and 40 MG patients with normal thyroid hormone were enrolled in this study.Their thyroid hormone levels including APOAb,TT₃,TT4,FT₃,FT4,and TSH were measured. Results The main factor for ocular MG accompanying low T₃ syndrome was the age irrespective of MG itself.The important factor for moderate general MG are continual glucocortical hormone therapy and hypoalimentation. Conclusion MG involves different muscles.The mechanism underlying MG accompanying low T₃ syndrome is also different.