A young women presenting with hyperthyroidism proved to have diffuse infiltration of the thyroid with carcinoma probably from a primary breast adenocarcinoma. The gland was diffusely infiltrated with tumour although the thyroid follicles were intact. Blood thyroid hormone levels were raised but thyroid uptake of iodine was undetectable. It is suggested that the tumour released a locally active agent which stimulated hormone release but not iodine uptake, the latter being very low due to suppression of TSH.