After total thyroidectomy because of hyperthyroidism, hypothyroidism developed in a 78-year-old woman. Despite replacement therapy with levothyroxine sodium in continuously increasing doses, worsening hypothyroidism led to respiratory failure, necessitating artificial ventilation. The addition of liothyronine resulted in complete recovery. Impaired conversion of thyroxine to triiodothyronine by the dejodase was responsible for the manifestations of hypothyroidism.