East African immigrants to Scandinavia are admitted to mental hospitals far more frequently than native Scandinavians. Most of these patients are admitted for psychosis, commonly ascribed to problems adapting to the new culture. However, psychosis is also known to be associated with hyperthyroidism, and the high frequency of psychosis among East Africans in Scandinavia may at least in part be due to hyperthyroidism rather than cultural problems. Large areas in East Africa are notorious for high natural concentrations of fluoride in water and plants. Fluoride inhibits the production of thyroid hormones. To maintain normal thyroxin levels the body increases the capacity for thyroxin production. Goitre is caused by such a compensatory mechanism, and endemic goitre is widespread in many high-fluoride areas, even where dietary access to iodine is adequate. When people from such areas arrive in a low-fluoride area, their elevated capacity to produce thyroid hormones may lead to hyperthyroidism and subsequently to psychosis.