Possible causes of an underactive thyroid include autoimmune diseases, treatment for hyperthyroidism, thyroid surgery, radiation therapy and medications, according to Mayo Clinic. Less common causes of hypothyroidism include congenital disease, pituitary disorders, pregnancy and iodine deficiency. A thyroid gland that does not produce enough of the hormones triiodothyronine and thyroxine has a significant impact on the health, as these hormones regulate every aspect of the metabolism.
The autoimmune disease Hashimoto's thyroiditis is the most common cause of hypothyroidism, reports Mayo Clinic. This inflammatory disease causes the body to produce antibodies that attack the thyroid gland. As of 2015, the cause of this autoimmune disease remains under investigation; however, the antibodies it produces reduce the amount of hormones the thyroid produces.
Treatment for hyperthyroidism, with radioactive iodine or thyroid surgery, sometimes causes permanent hypothyroidism, warns Mayo Clinic. Patients with this condition must take supplemental thyroid hormones for the rest of their lives.
Radiation therapy for tumors in the head or neck can reduce the production of thyroid hormones, according to Mayo Clinic. Certain medications, including lithium for psychiatric disorders, also affect the thyroid.
The thyroid requires iodine, a trace mineral, to produce hormones, notes Mayo Clinic. In the United States, the addition of iodine to table salt has eliminated most cases of underactive thyroid due to iodine deficiencies, but consuming too much iodine increases the risk of overactive thyroid.