What is hypothyroidism?

Hypothyroidism is when your thyroid does not make enough thyroid hormones. It is also called underactive thyroid. This slows down many of your body's functions, like your metabolism.

The most common cause of hypothyroidism in the United States is Hashimoto's disease. In people with Hashimoto's disease, the immune system mistakenly attacks the thyroid. This attack damages the thyroid so that it does not make enough hormones.

Hypothyroidism also can be caused by:

• Hyperthyroidism treatment (radioiodine)
• Radiation treatment of certain cancers
• Thyroid removal