The case study of a non-small cell lung carcinoma confirmed by complete necropsy study is reported in an old man with apathetic hyperthyroidism, atrial fibrillation and heart failure. The authors emphasize diagnostic challenges related to effects of hyperthyroidism on nutritional status versus under nutrition caused by unsuspected development of a coexistent malignancy. Moreover, main features and diagnostic pitfalls of apathetic hyperthyroidism are commented. The aim of this report is to enhance the suspicion index of primary health care workers about the possibility of concomitant benign and malignant conditions with similar manifestations.