The present case report refers to a multiorgan autoimmune disease manifesting following thymectomy performed for a benign thymoma. This disease is characterized by hypothyroidism, severe myasthenia, polymyositis and alopecia which are organ-specific diseases probably with a different time of onset but which are all an expression of the same immunopathologic process occurring in individuals who have a genetic predisposition. Characteristic of the present case is not only the association of the different immunopathologic clinical pictures but also the rather difficult differential diagnosis between a hypothyroidism-related myopathy and polymyositis. It was possible to formulate the diagnosis by integrating the results of clinical and laboratory evaluation with the therapeutic outcome. The onset of the syndrome was attributed to the withdrawal, following surgery, of the inhibitory effects of the thymoma on some clones of autoreactive lymphocytes.