Hypothyroidism - Causes of underactive thyroid

An underactive thyroid (hypothyroidism) is when your thyroid gland doesn't produce enough of the hormone thyroxine, also called T4.

Most cases of underactive thyroid are due to either the immune system attacking the thyroid gland or a damaged thyroid.

Immune system

Most cases of underactive thyroid happen when the immune system, which normally fights infection, attacks the thyroid gland. This damages the thyroid, which means it is not able to make enough of the hormone thyroxine, and leads 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 is not clear what causes Hashimoto’s disease, but the condition runs in families. It is also common in people with another disorder related to the immune system, such as type 1 diabetes and vitiligo.

Previous thyroid treatment

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 overactive thyroid (where the thyroid gland produces too much hormone) or thyroid cancer.

Less common causes

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

In some cases, babies are born with an underactive thyroid because the gland does not develop properly in the womb. This is called congenital hypothyroidism and it is usually picked up during routine screening soon after birth. 

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

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

• lithium - a medication sometimes used to treat certain mental health conditions, including depression and bipolar disorder
• amiodarone - a medication sometimes used to treat irregular heartbeats (arrhythmias)
• interferons - a class of medication sometimes used to treat certain types of cancer and hepatitis C

Speak to your GP or specialist if you are concerned that a medication you are taking may be affecting your thyroid hormone levels.