Hashimoto’s disease is an autoimmune condition that can lead to hypothyroidism, or an underactive thyroid. In rare cases, it may also result in hyperthyroidism, or an overactive thyroid. The immune system produces antibodies that attack the thyroid gland.There is a buildup of large numbers of white blood cells, which are part of the immune response, within the thyroid.The thyroid becomes damaged, impairing its ability to produce sufficient thyroid hormones.