Underactive thyroid (hypothyroidism)

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 body’s 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.

What causes hypothyroidism?

The most common cause of hypothyroidism is Hashimoto’s disease, an autoimmune disorder. With Hashimoto’s disease, the body produces antibodies that attack and destroy the thyroid gland.

Other causes of hypothyroidism include:

A viral infection

Radiotherapy 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, 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.

Too little iodine in the diet. The thyroid needs iodine to produce thyroid hormone. Your body doesn't make iodine, so you need to get it through your diet. Iodised table salt is rich in iodine. Other food sources of iodine include shellfish, saltwater fish, eggs, dairy products and seaweed. In the UK, iodine deficiency used to be a common problem until, in the early 1900s, salt manufactures started to add small amounts of iodine to salt, which reduced the number of cases. However, in recent years, the number of people experiencing iodine deficiency in the UK has started to rise. This could be due to an increased number of people preferring a low-salt, non-dairy diet.

Pregnancy. The reason isn’t clear, but sometimes, inflammation of the thyroid occurs after pregnancy. This is called postpartum thyroiditis. Women with this condition usually have a severe increase in thyroid hormone levels followed by a sharp drop in thyroid hormone production. Most women with postpartum thyroiditis will regain their normal thyroid function.