The purpose of the thyroid gland is to produce and release hormones to control metabolism, which regulates the body's many processes that use energy. Metabolism controls processes like breathing, circulation, muscle movements, excretion of waste and digestion of food. If the thyroid gland doesn't produce enough thyroid hormone, a number of symptoms result--and treatment is needed to replace those hormones. There are a number of different causes of hypothyroidism, or underactive thyroid.
Autoimmune Diseases
Most commonly, an autoimmune disease is the underlying cause of hypothyroidism, according to the U.S. National Library of Medicine. The autoimmune disease Hashimoto's thyroiditis is the most frequent autoimmune disease that results in underactive thyroid. No one understands what causes autoimmune diseases, but they occur when the body's own immune system attacks itself--in this case, the thyroid gland.
Birth Defects
Some people may just be born with a congenital defect (present at birth) of the thyroid gland that prevents it from producing sufficient amounts of thyroid hormone. It's possible to be born with underactive thyroid. Underactive thyroid is a congenital defect affecting about one in 3,000 babies born in the U.S. according to the Mayo Clinic, and the condition may be inherited.
Damage to the Thyroid
The thyroid can become damaged from radiation treatment (typically to treat cancer) administered in the area of the neck and throat. The thyroid gland can also be destroyed during treatment for hyperthyroidism, or overactive thyroid. Radioactive iodine absorbed by the thyroid gland can destroy it and prevent it from producing enough thyroid hormone. The thyroid can also be surgically removed to treat hyperthyroidism, which results in lack of thyroid hormone production.
Medication Use
The U.S. National Library of Medicine reports that underactive thyroid hormone production is a side effect of a number of different medications, like lithium (often used to stabilize mood), amiodarone (to treat irregular heartbeat) and medications to treat hyperthyroidism.
Pregnancy
Underactive thyroid hormone production during or following pregnancy is common, sometimes because of the production of antibodies that affect the thyroid gland, according to the Mayo Clinic. Hypothyroidism needs to be treated during pregnancy to prevent complications to the fetus and the mother's health.
Iodine Deficiency
The thyroid gland requires the mineral iodine to function, so if the diet is deficient in iodine, underactive thyroid can result. Though uncommon in the U.S., iodine deficiency is a common cause of underactive thyroid in other areas of the world.
