Very little of fundamental significance is known about the aetiology of thyrotoxicosis. Heredity may play some part but its importance is in no way clearly defined. The incidence of thyrotoxicosis is world wide, but there is some concentration of toxic goitres in areas where simple goitres are endemic. There is, as far as I know, no satisfactory explanation why it is about 7 times commoner in women than in men, but almost all agree that stress, either physical or mental, tends to precipitate hyperthyroidism, and this is particularly true of emotion. It is natural, therefore, that workers should turn to possible relationships between the thyroid, the central nervous system and the other endocrine glands when considering the aetiology of the thyrotoxic state.