An underactive thyroid (hypothyroidism) is when your thyroid gland does not produce enough of the hormone thyroxine (also called T4).

Most cases of an underactive thyroid are caused by the immune system attacking the thyroid gland and damaging it, or by damage that occurs as a result of treatments for thyroid cancer or an overactive thyroid.

An underactive thyroid often occurs when the immune system, which usually fights infection, attacks the thyroid gland. This damages the thyroid, which means it's not able to make enough of the hormone thyroxine, leading to the symptoms of an underactive thyroid.

A condition called Hashimoto's disease is the most common type of autoimmune reaction that causes an underactive thyroid.

It's not clear what causes Hashimoto's disease, but it runs in families. It's also common in people with another immune system disorder, such as type 1 diabetes and vitiligo.

An underactive thyroid can also occur as a side effect or complication of previous treatment to the thyroid gland, such as surgery or a treatment called radioactive iodine therapy.

These treatments are sometimes used for an overactive thyroid (where the thyroid gland produces too much hormone) or thyroid cancer.

Worldwide, a lack of dietary iodine is a common cause of an underactive thyroid, because the body needs iodine to make thyroxine. However, iodine deficiency is uncommon in the UK.

Babies are sometimes born with an underactive thyroid because the thyroid gland does not develop properly in the womb. This is called congenital hypothyroidism and is uncommon. It's usually picked up during routine screening soon after birth.

The Great Ormond Street Hospital for Children has more information about congenital hypothyroidism

A problem with the pituitary gland could lead to an underactive thyroid. The pituitary gland sits at the base of the brain and regulates the thyroid. Therefore, damage to the pituitary gland may lead to an underactive thyroid.

An underactive thyroid has also been linked to some viral infections or some medicines used to treat other conditions, such as:

• lithium – a medicine sometimes used to treat certain mental health conditions, including depression and bipolar disorder
• amiodarone – a medicine sometimes used to treat an irregular heartbeat (arrhythmia)
• interferons – a class of medicine sometimes used to treat certain types of cancer and hepatitis C

Speak to a GP or specialist if you're concerned that a medicine you're taking may be affecting your thyroid hormone levels.