Most clinicians assume that a thyroid mass which appears to be malignant is most likely a tumour arising from thyroid cells. We present a case where the thyroid malignancy was associated with a degree of systemic clinical and biochemical disturbance which suggested alternative diagnoses, and in which fine-needle aspiration revealed that the patient had a thyroid lymphoma. Gastric biopsy showed that this was a primary gastric diffuse B cell lymphoma. Subsequent management provided a number of challenging and overlapping problems, and the patient eventually died from this high-grade malignancy. We review the prevalence, presentation, diagnosis and management of lymphomas presenting as thyroid masses, and underline the problems in management when there is multi-system disease.