An underactive thyroid gland can be caused by:
- an immune system disorder
- previous thyroid treatment
Immune system disorder
An immune system disorder usually causes an underactive thyroid gland.
If there is a problem with your immune system, it can attack your thyroid gland and damage it. A damaged thyroid cannot make enough of the hormone thyroxine. You may show symptoms of an underactive thyroid.
This autoimmune disorder is known as "Hashimoto's disease".
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. For example, type 1 diabetes or vitiligo (an autoimmune disorder that affects the skin).
Previous thyroid treatment
Previous thyroid treatment can cause an underactive thyroid. It can happen during surgery or other treatment, such as radioactive iodine therapy. It can also happen as a complication after treatment.
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
Less common causes of an underactive thyroid include:
- a lack of iodine in your diet
- you are born with it (congenital hypothyroidism)
- a problem with the pituitary gland
- some viral infections
- some medicines
- 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 is usually picked up during newborn screening.
Pituitary gland problem
A problem with the pituitary gland can lead to an underactive thyroid.
The pituitary gland sits at the base of the brain. It makes thyroid-stimulating hormone (TSH), which controls the production of thyroid hormones.
If the pituitary gland is damaged, it may lead to an underactive thyroid.
Medicines
Some medicines may cause an underactive thyroid.
These include:
- lithium - used to treat some mental health conditions, including bipolar disorder
- amiodarone - used to treat irregular heartbeats (arrhythmia)
- interferons - used to treat some types of cancer and hepatitis C
Amiodarone can cause both an underactive and overactive thyroid.
Talk to your GP if you think medicines are affecting your thyroid.
Content supplied by the NHS and adapted for Ireland by the HSE
