Our knowledge regarding the causes of breast cancer and the factors that predispose to it is fragmentary. Endocrine, nutritional, genetic, immunologic, racial, and environmental factors should be considered in the evaluation of any study of the pathogenesis of human breast cancer.1 Although the association of a number of factors with a disease should prove helpful in understanding the disorder, it is clear that a causal relationship must not be assumed.
In this context studies suggesting that thyroid dysfunction and/or thyroxine supplementation predispose to the development of breast cancer must be viewed with cautious interest. As noted by Aldinger et al in this issue (p 1638), the literature in this field is confusing, inconclusive, and controversial. The most recent renewal of the controversy was prompted by Kapdi and Wolfe,2 who found a positive relationship between the incidence of thyroid supplementation and breast cancer. This finding has been challenged by