The relationship of mental phenomena to hyperthyroidism has been recognized since the times of Graves and Basedow. In the past, surgical domination of studies of disease of the thyroid gland has hindered progress from the psychiatric standpoint in this field, so that only in recent years has proper consideration been given to the psychiatric aspect of the problem.In this study we have attempted to analyze the mental phenomena occurring in 134 cases of hyperthyroidism, and to determine whether the occurrence of mental phenomena can be directly attributed to the hyperthyroidism.It is apparent from this study that hyperthyroidism is a constitutional disease which may manifest any of the usual types of psychic reaction. Thus, there is no type of psychic reaction that can be considered characteristic of hyperthyroidism. However, we found three types of mental reaction dominant in hyperthyroidism; namely, toxic exhaustion psychosis, acute delirium reaction, and manic depressive reaction.The cases of toxic exhaustion psychosis and of acute delirium reaction constitute 91 per cent of our material. Toxic exhaustion psychosis occurred most frequently in the cases of exophthalmic goiter, whereas the acute delirium reaction occurred in a high percentage of the cases of hyperfunctioning adenomatous goiter. It appears that hyperthyroidism may act as a definite etiologic factor in the production of mental manifestations in these two groups. The mental manifestations usually occur during a state of crisis which results during a period of severe intoxication. However, the mental manifestations do not differ essentially from those occurring in association with other states of marked exhaustion, or with other types of intoxication.The cases of manic depressive reaction constitute the third large group of this series. Many of these patients gave histories of previous mental disturbance. The coexistence of mental manifestations and hyperthyroidism in many of these cases was probably a coincidence. However, hyperthyroidism does appear to play a part as a precipitating factor in the production of mental manifestations in those cases which fall in the class of symptomatic depression.Undoubtedly, the nervous stability of an individual is a dominant factor in determining the occurrence of mental manifestations. Either the existence of a state of hyperthyroidism, or an operative procedure, may serve as a means of precipitating a severe mental reaction in a mentally unstable individual.The administration of compound solution of iodine in adequate doses in the preoperative and postoperative treatment of exophthalmic goiter has prevented or controlled crisis, and hence has reduced the incidence of mental reactions. Unfortunately, the administration of compound solution of iodine has not produced similar results in cases of hyperfunctioning adenomatous goiter. The management of cases of hyperthyroidism in which there is evidence of mental manifestations is a medical and psychiatric problem and surgical interference is definitely contraindicated in the course of a severe psychic reaction.