Thyrotoxic myopathy mimicking myasthenic syndrome associated with thymic hyperplasia

A 41-year-old man with progressive limb weakness manifested fluctuating muscle weakness as seen in myasthenia gravis (MG). Laboratory investigations revealed hyperthyroidism without the complication of MG. Electrophysiological studies demonstrated abnormal features of neuromuscular transmissions resembling those of the Lambert-Eaton myasthenic syndrome rather than those of MG. A CT scan showed a mediastinal mass that suggested thymic hyperplasia which often complicates MG or hyperthyroidism. Medical treatment of hyperthyroidism resulted in resolution of MG-like symptoms and regression of thymic hyperplasia on CT concomitant with normalization of thyroid function. This case highlights the fact that careful investigations are needed to differentiate MG-like symptoms from genuine MG in cases of hyperthyroidism with thymic lesions.