Summary

Thyroid cancer with concomitant hyperthyroidism is rare. Most foci of malignancy are small and seen postoperatively as incidental findings after surgery for hyperthyroidism. Thyroid masses with clinical features of malignancy and concomitant hyperthyroidism are less-commonly reported. We report two cases of multinodular toxic goitre or Plummer's disease with clinical features of malignancy. Both patients had large multinodular goitres with evidence of metastasis to the manubrium for the first patient and to the lymph node and lungs for the second patient. Both were clinically euthyroid but with free hormone excess and suppressed thyroid stimulating hormone (TSH) on laboratory testing. Both patients received methimazole prior to thyroidectomy. Histopathology revealed follicular variant of papillary cancer with metastasis to the manubrium for the first patient and follicular thyroid cancer with lymph node metastasis for the second. While rare, thyrotoxicosis can occur with malignancy, Plummer's disease may harbour cancer and behave aggressively.