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Causes - Underactive thyroid (hypothyroidism)

An underactive thyroid gland can be caused by:

  • an immune system disorder
  • treatment for thyroid cancer or an overactive thyroid

Immune system

An immune system disorder usually causes an underactive thyroid gland.

If your immune system attacks the thyroid gland, it can damage it. A damaged thyroid cannot make enough of the hormone thyroxine. This can cause symptoms of an underactive thyroid.

Hashimoto's disease is an autoimmune disorder. It's the most common disorder 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 other immune system disorders. This includes type 1 diabetes or vitiligo.

Symptoms of an underactive thyroid

Previous thyroid treatment

Previous thyroid treatment can also cause an underactive thyroid. It can be a side effect or complication. This can happen during surgery, or treatment such as radioactive iodine therapy.

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

Less common causes

These include:

  • a lack of dietary iodine
  • congenital hypothyroidism
  • a problem with the pituitary gland
  • viral infections
  • some medicines
  • underactive thyroid after pregnancy

Congenital hypothyroidism

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

Pituitary gland

A problem with the pituitary gland can lead to an underactive thyroid. The pituitary gland sits at the base of the brain. It regulates the thyroid. If it's damaged it may lead to an underactive thyroid.


Some medicines may cause an underactive thyroid.

These include:

  • lithium – used to treat certain mental health conditions, including bipolar disorder
  • amiodarone – used to treat irregular heartbeats
  • interferons – used to treat certain types of cancer and hepatitis C

Amiodarone can cause both an underactive and overactive thyroid.

Talk to your GP if you think that medicine may be affecting your thyroid hormone levels.

Preventing an underactive thyroid

There's no way to prevent an underactive thyroid.

How to treat an underactive thyroid gland

Content supplied by the NHS and adapted for Ireland by the HSE

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This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.

Page last reviewed: 25 March 2021
Next review due: 25 March 2024