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What causes an underactive thyroid?

In the UK, the most common cause of an underactive thyroid is autoimmune thyroiditis, which is known as Hashimoto’s thyroiditis.

“This is when your immune system attacks your thyroid gland, damaging it and stopping it producing enough thyroid hormone,” explains Bupa.

According to the health body, if you’ve had your thyroid gland removed or radioactive iodine treatment for an overactive thyroid, this can cause an underactive thyroid too.

“Radiotherapy for thyroid cancer can also lead to an underactive thyroid,” it adds.