A 95-year-old woman had been treated over the past 8 years for progressive dysphagia. When her condition worsened, blood tests revealed the presence of leucocytosis and eosinophilia in the absence of anaemia or thrombocytopenia. Within 11 days of diagnosis, the patient died of respiratory failure. Necropsy showed normal thyroid tissue and an absence of infectious disease. However, an upper mediastinal tumour was found and was histopathologically diagnosed as a neoplastic transformation of the ectopic thyroid. Only 1% of endothoracic goitres present as ectopic or autonomous goitre with no parenchymal or vascular connection to the thyroid gland. This case represents a very rare situation in which a leukemoid reaction and peripheral hypereosinophilia were observed as a manifestation of an anaplastic thyroid carcinoma in an ectopic mass.