What causes Hypothyroidism ? 

Hypothyroidism has several causes, including
                
- Hashimotos disease  - thyroiditis, or inflammation of the thyroid  - congenital hypothyroidism, or hypothyroidism that is present at birth  - surgical removal of part or all of the thyroid  - radiation treatment of the thyroid  - some medications
                
Less commonly, hypothyroidism is caused by too much or too little iodine in the diet or by abnormalities of the pituitary gland.
                
Hashimotos Disease
                
Hashimotos disease, also called chronic lymphocytic thyroiditis, is the most common cause of hypothyroidism in the United States.1 Hashimotos disease is a form of chronic inflammation of the thyroid gland. Hashimotos disease is also an autoimmune disorder.
                
Normally, the immune system protects the body against foreign invaderssuch as viruses and bacteriathat can cause illness. But in autoimmune diseases, the immune system attacks the bodys own cells and organs. With Hashimotos disease, the immune system attacks the thyroid, causing inflammation and interfering with its ability to produce thyroid hormones.
                
More information is provided in the NIDDK health topic, Hashimotos Disease.
                
Thyroiditis
                
Thyroiditis causes stored thyroid hormone to leak out of the thyroid gland. At first, the leakage raises hormone levels in the blood, leading to hyperthyroidismwhen thyroid hormone levels are too highthat lasts for 1 or 2 months. Most people then develop hypothyroidism before the thyroid is completely healed.
                
Several types of thyroiditis can cause hyperthyroidism followed by hypothyroidism:
                
- Subacute thyroiditis. This condition involves painful inflammation and enlargement of the thyroid. Experts are not sure what causes subacute thyroiditis, but it may be related to a viral or bacterial infection. The condition usually goes away on its own in a few months.  - Postpartum thyroiditis. This type of thyroiditis develops after a woman gives birth. For more information, see the section titled What happens with pregnancy and thyroid conditions?  - Silent thyroiditis. This type of thyroiditis is called silent because it is painless, as is postpartum thyroiditis, even though the thyroid may be enlarged. Like postpartum thyroiditis, silent thyroiditis is probably an autoimmune condition and sometimes develops into permanent hypothyroidism.
                
Congenital Hypothyroidism
                
Some babies are born with a thyroid that is not fully developed or does not function properly. If untreated, congenital hypothyroidism can lead to mental retardation and growth failure. Early treatment can prevent these complications, so most newborns in the United States are screened for hypothyroidism.
                
Surgical Removal of the Thyroid
                
When part of the thyroid is removed, the remaining part may produce normal amounts of thyroid hormone, but some people who have this surgery develop hypothyroidism. Removal of the entire thyroid always results in hypothyroidism.
                
Part or all of the thyroid may be surgically removed as a treatment for
                
- hyperthyroidism  - a large goiter, which is an enlarged thyroid that may cause the neck to appear swollen and can interfere with normal breathing and swallowing  - thyroid nodules, which are noncancerous tumors, called adenomas, or lumps in the thyroid that can produce excess thyroid hormone  - thyroid cancer
                
Radiation Treatment of the Thyroid
                
Radioactive iodine, a common treatment for hyperthyroidism, gradually destroys the cells of the thyroid. Most people who receive radioactive iodine treatment eventually develop hypothyroidism. People with Hodgkins disease, other lymphomas, and head or neck cancers are treated with radiation, which can also damage the thyroid.
                
Medications
                
Some drugs can interfere with thyroid hormone production and lead to hypothyroidism, including
                
- amiodarone, a heart medication  - interferon alpha, a cancer medication  - lithium, a bipolar disorder medication  - interleukin-2, a kidney cancer medication