Background: Several concomitant disorders especially thyroid abnormalities have been reported in patients with myasthenia gravis (MG). We aimed to estimate the frequency and pattern of thyroid disorders in Iranian patients with MG.All consecutive patients with MG referred to neurology clinic of Rasool-e-Akram Hospital during 2006-2007 were enrolled. Methods: All patients underwent clinical assessment of thyroid gland as well as thyroid function test. AChR Ab titer was measured as well. Nerve conduction study (NCS), Electromyography (EMG), and Repetitive Nerve Stimulation (RNS) was done by a same neurologist. The diagnosis of MG was made on the basis of clinical examinations, an edrophonium chloride test and electrophysiological studies. The diagnosis of thyroid disorders were based on clinical presentation as well as thyroid function tests.Results:FIFTY EIGHT PATIENTS (MEAN AGE [SD]: 37.1 [16.9], range: 10-80; female: 65.5%) were enrolled in this 12-month study. Four patients (6.9%) had abnormal thyroid function tests (Hypothyroidism: 3 [5.2%]; 4 females; 3 with hypothyroidism and 1 with hyperthyroidism). The mean age (SD) in men and women were 41.4 (21.3) and 34.9 (13.8) years (P: N.S.), respectively. In addition, once the MG patients are younger than 50, female gender is dominant while they are more than fifty, male is the dominant gender.Conclusion: Our results show that Iranian patients with MG tend to be female and young. Before sixth decade of life, women are the most presenting patients thereafter, men are the predominant gender. About 7 percent of them may suffer from concomitant thyroid problem especially hypothyroidism.