Hypothyroidism, also called underactive thyroid disease, is a common disorder. With hypothyroidism, your thyroid gland does not make enough thyroid hormone.

The thyroid gland is located in the front lower part of your neck. Hormones released by the gland travel through your bloodstream and affect nearly every part of your body, from your heart and brain, to your muscles and skin.

The thyroid controls how your body's cells use energy from food, a process called metabolism. Among other things, your metabolism affects your bodys temperature, your heartbeat, and how well you burn calories. If you don't have enough thyroid hormone, your body processes slow down. That means your body makes less energy, and your metabolism becomes sluggish.

The most common cause of hypothyroidism is Hashimoto's thyroiditis. "Thyroiditis" is an inflammation of the thyroid gland. Hashimoto's thyroiditis is an autoimmune disorder. With Hashimotos, your body produces antibodies that attack and destroy the thyroid gland. Thyroiditis may also be caused by a viral infection.

Other causes of hypothyroidism include:

Radiation therapy to the neck area. Treating certain cancers, such as lymphoma, requires radiation to the neck. Radiation damages the cells in the thyroid. This makes it more difficult for the gland to produce hormone.

Radioactive iodine treatment. This treatment is commonly prescribed to people who have an overactive thyroid gland, a condition known as hyperthyroidism. However, radiation destroys the cells in the thyroid gland. This usually leads to hypothyroidism.

Use of certain medications. Certain medicines to treat heart problems, psychiatric conditions, and cancer can sometimes affect the production of thyroid hormone. These include amiodarone (Cordarone), lithium, interferon alpha, and interleukin-2.

Thyroid surgery . Surgery to remove the thyroid will lead to hypothyroidism. If only part of the thyroid is removed, the remaining gland may still be able to produce enough hormone for the body's needs.

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Hypothyroid Symptoms, Causes, and Treatments - WebMD