Hypothyroidism is when the thyroid gland does not make enough hormones. The thyroid is a butterfly-shaped gland in the front of the neck. A healthy thyroid makes the hormones thyroxine (T4) and triiodothyronine (T3). These play many roles in the body, such as:

• How many calories a person burns and how much they weigh
• The speed at which the heart beats
• The temperature of a person's body
• How quickly the body breaks down food

If it is not treated, it can lead to many health problems, such as obesity and heart disease.

The Thyroid Gland

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Hashimoto thyroiditis is the most common cause. A problem with the immune system causes it to attack thyroid gland cells.

Other causes are:

• Subacute thyroiditis —Inflammation of the thyroid gland after a viral upper respiratory tract infection
• Medicines —Such as those used to treat hyperthyroidism, lithium for mental health problems, and some heart medicines
• Medical treatments —Medicines, radiation therapy, or surgery to remove part of the thyroid gland
• Idiopathic thyroid atrophy —The thyroid tissue shrivels up for reasons that are not known
• Iodine deficiency —The thyroid gland does not get enough iodine to make hormones (rare in the United States)
• Iodine excess —Foods (such as shellfish) and some medicines (such as cough medicine) that have large amounts of iodine can stop the thyroid from making hormones (rare)
• Certain illnesses —Cancers and certain infections
• Pituitary adenoma —A non-cancerous tumor of the pituitary gland causes a problem signaling the thyroid to make hormones
• Postpartum thyroiditis —The thyroid becomes inflamed after giving birth and does not work as it should
• Congenital hypothyroidism —When a baby is born with problems making normal amounts of thyroid hormones