Grave’s disease is the most common cause of hyperthyroidism in children. Symptoms of hyperthyroidism associated with Grave’s disease include goiter, tachycardia, tremor or diplopia caused by exophthalmos. Marked fatigue and weight loss may be observed due to hypermetabolic state. Myasthenia Gravis is far less frequent, with juvenile onset in fewer than 10% of the patients. Ocular symptoms: ptosis or diplopia, and weakness and fatigability of limb and bulbar muscles are hallmarks of myasthenia. In some of the patients Grave’s disease and myasthenia may mimic each other though coincidence of both diseases can be also observed. We report the case of a girl with abrupt onset of hyperthyroidism with severe dysphagia and weight loss. Family history positive for Grave’s disease and laboratory results confirming hyperthyroidism delayed diagnosis and treatment of her severe myasthenia.