Exacerbation of myasthenia gravis shortly after administration of methimazole for hyperthyroidism.

We report a case of myasthenia gravis (MG) which became worse shortly after the administration of methimazole (MMI) for hyperthyroidism. The activation of immune responses was found during the worsening of MG. The findings suggests the possibility that the worsening of MG might be induced by MMI, presumably by its immunomodulatory property.