Hyperthyroidism refers to excessive secretion of thyroid hormones. Iodine overload, a goiter, thyroid nodules or certain congenital diseases can cause hyperthyroidism. However, the main cause is Graves' disease, an autoimmune disease that mostly affects women. It is characterized by the presence of antibodies that stimulate the production of thyroid hormones. Graves' disease is sometimes linked to other autoimmune diseases (such as diabetes) or stress.