Iodine intake influences the health of Americans today, but in different ways than it has in the past. In the early 1900s, iodine deficiency was responsible for widespread endemic goiter in the United States. The incidence of goiter was markedly reduced by the introduction of iodine supplementation of salt and other foods in the 1920s. By the middle of this century iodine-related problems in the Unites States were more likely to be associated with too much rather than too little of this essential nutrient. Recent and disturbing new data suggest that there has been a sharp decline in iodine intake during the last 20 years, especially in women of reproductive age. A brief comparison of thyroid response to insufficient and excessive intakes in normal and diseased thyroid tissue is presented. The population affected and the implications for therapy are also reviewed.