Thyroid Supplements and Breast Cancer

To the Editor.—  The study by Drs Kapdi and Wolfe purporting to show an increased incidence of breast carcinoma among those taking thyroid supplements for thyroid disease has unfortunately produced a great deal of concern among physicians and patients.We feel that this report can be criticized in several particulars. The study used only patients referred for mammography and determined the incidence of breast carcinoma among those who gave a history of receiving thyroid supplements. Preselection is clearly implicit in this method of choosing patients. Part of the higher incidence of breast carcinoma in women receiving thyroid supplements could be explained by the fact that those so treated for presumed hypothyroidism or goiter would be expected to have frequent or regular follow-up visits. Those not receiving medication (and hence less closely observed) may have widely disseminated or clinically obvious breast cancer not necessitating mammography by the time they come to