A case of histiocytosis X involving the thyroid and hypothalamus is reported. A 16 year old female presented with amenorrhoea and diabetes insipidus. She subsequently developed a painful goitre with biochemical hypothyroidism, and stridor. The stridor and goitre responded to cyclophosphamide. Previous publications on the use of cytotoxics in histiocytosis X involving the thyroid are reviewed. We describe for the first time both the ultrasound appearances of the thyroid in this condition and the use of serial volumetric measurements to monitor therapy.