AbstractHyperthyroidism is associated with various movement disorders, such as chorea and tremors. We report on a young Chinese woman with an unusual presentation of myoclonus, involving both platysmal muscles, in association with subclinical hyperthyroidism. The myoclonus was preceded by symptoms of hyperthyroidism, namely weight loss, menstrual disturbances, and heat intolerance. The movements abated with clonazepam and hyperthyroidism was treated with carbimazole. The myoclonus recurred briefly when she stopped taking clonazepam, but she has since remained well and euthyroid. © 2005 Movement Disorder Society