In the United States, the most common cause of hypothyroidism is Hashimoto's thyroiditis, a condition that causes the body's natural defenses-the immune system-to produce antibodies that over time destroy thyroid tissue. As a result, the thyroid gland cannot make enough thyroid hormone.

Worldwide, iodine deficiency is the number one cause of hypothyroidism. Iodine added to salt, food, and water has eliminated this problem in the United States and other Western countries.

Other common causes of hypothyroidism include:

• Thyroid surgery. Part or all of the thyroid gland may be removed to treat disorders such as having too much thyroid hormone (hyperthyroidism), an enlarged thyroid gland (goiter) that makes swallowing difficult, thyroid cancer, or thyroid nodules that may be overactive or cancerous. Hypothyroidism results when the thyroid gland is removed or when remaining thyroid tissue does not function properly.
• Radioactive iodine therapy, which is often used to treat hyperthyroidism. Radioactive iodine therapy can destroy the thyroid gland, leading to hypothyroidism.
• External beam radiation, which is used to treat some cancers, such as Hodgkin's lymphoma. This radiation treatment can destroy the thyroid gland.

Less common causes include:

• Infections. Viral and bacterial infections can temporarily damage the thyroid gland, causing a short-term form of the condition. Hypothyroidism caused by infection usually does not result in permanent hypothyroidism.
• Medicines. Some medicines can interfere with normal production of thyroid hormone. Lithium is one of the most common medicines that causes hypothyroidism. Others include amiodarone (such as Amiodarone, Cordarone, and Pacerone) and interferon alfa (such as Infergen, Rebetron, and Wellferon).
• In rare instances, disorders of the pituitary gland or the hypothalamus (secondary and tertiary forms of hypothyroidism). The pituitary gland and hypothalamus produce hormones that control the thyroid and, as a result, affect its ability to produce thyroid hormone.
• Excessive iodine, which, in food or medicines, can reduce the function of the thyroid gland. This is usually temporary.
• Congenital hypothyroidism. About 1 in 4,000 infants is born without a properly functioning thyroid gland. All children born in a hospital in the United States are tested at birth for hypothyroidism.¹

Mild (subclinical) hypothyroidism is most often caused by inadequate treatment of hypothyroidism, Hashimoto's thyroiditis, or radioactive iodine therapy. But it may be caused by anything that causes hypothyroidism.

Autoimmune thyroiditis, an inflammation of the thyroid gland, sometimes occurs during pregnancy and can cause hypothyroidism. About 2% of pregnant women in the United States get hypothyroidism.²