This invention relates to a composition for use in the treatment of iodine deficiency diseases such as fibrocystic dysplasia of the breast, breast cancer, endometriosis and premenstrual syndrome. The composition may also be used for the prophylaxis of breast cancer. More particularly, this invention relates to a composition for the treatment of iodine deficiency diseases, the composition comprising a solution of elemental iodine (I.sub.2), which may also be referred to as metallic iodine or iodi...