Clinical observations on thyrotoxicosis coexisting with myotonic dystrophy

Two cases are presented and a third is cited from the literature in which the onset of hyperthyroidism exacerbated acute weakness in previously undiagnosed myotonic dystrophy. In two of the cases a return to the euthyroid state after treatment was associated with marked clinical improvement in muscle weakness. In the third case, severe bulbar and respiratory muscle weakness, which was apparently aggravated by the thyrotoxicosis, was followed by aspiration pneumonitis and death. Possible mechanisms for the effect of excessive thyroxine on muscle function are reviewed. Early diagnosis and aggressive treatment of the hyperthyroidism is extremely important.