Abstract: Objective  To observe the levator muscles using MRI at the different functional eye positions and to expose the main cause of eyelid retraction in thyroid associated ophthamopathy (TAO). Design Prospective case-control study. Participants Forty-two TAO patients. Methods TAO patients were divided into 2 groups: group A, with eyelid retraction; group B, without eyelid retraction. Normal controls were also included in this project, as group C. Area of each functional point along the levator muscle was measured at primary, down-gaze, up-gaze positions. Main Outcome Measures The area of each point along the levator muscle. Results Hypertrophy were presented in eyelid retraction group (group A 3.08±0.20, group B 2.60±0.20, control 1.41±0.06), highly related to the enlargement of superior rectus muscle(r=0.995).  Both systolic rate and diastolic rate were found significantly reduced, to 40% and 60% of the controls respectively. Muscle belly was the point that contributed the most of the reduction of muscle retraction function. The systolic curve of group B was quite close to that of the controls, whereas the diastolic curve matched that of group A. Conclusion In spite of the hypertrophy of levator, both contraction and relaxation functions of levtator are reduced in TAO patients.  Furthermore, the deficiency of the contraction may contribute the most to the eyelid retraction. (Ophthalmol CHN, 2014, 23: 326-331)

Key words: thyroid associated ophthalomopathy, eyelid retraction, levator muscles, magnetic resonance imaging