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Underactive thyroid (hypothyroidism)

Illustration showing the thyroid gland, larynx (voice box) and trachea (windpipe)Your thyroid is a small, butterfly‑shaped gland in your neck. It produces two thyroid hormones. These hormones are tri‑iodothyronine (T3) and thyroxine (T4).

Thyroid hormones help your body use energy and stay warm. They keep your brain, heart, muscles and other organs working as they should.

Underactive thyroid (hypothyroidism) is when your thyroid doesn't make enough of the thyroid hormones. This slows down your all your body functions (your metabolism).


Symptoms of an underactive thyroid

If you have an underactive thyroid you may:

These symptoms generally come on slowly over months or even years so it can take a long time to realise something is wrong.

If you're pregnant and your underactive thyroid isn't treated, it can cause problems such as pre-eclampsia and premature labour.

Causes of an underactive thyroid

The commonest cause is a condition called autoimmune thyroiditis. Your immune system usually fights off infection but when you have an autoimmune disease, your immune system attacks your own cells. Your body makes a tiny protein (antibody) that attaches to your thyroid, stopping it from making enough thyroid hormone. We don't know what triggers this, but it sometimes runs in families. It's more common in women and people over the age of 50.

Treatment for an overactive thyroid or thyroid cancer can sometimes make your thyroid underactive.

Rare causes include some medicines such as lithium or amiodarone, a lack of iodine, problems with your pituitary gland and childbirth.

Diagnosing an underactive thyroid

A blood test measures your thyroid hormones and TSH (thyroid stimulating hormone).

TSH is a hormone which controls how much thyroxine you make and is made by your pituitary gland at the base of your brain. If your thyroid isn't making enough thyroxine your TSH will be high.

A blood test can also check for thyroid antibodies (a protein attacking your thyroid cells).

Treating an underactive thyroid

You can take a thyroxine tablet daily to replace the thyroxine your body isn't making. Your dose will start low and increase until your blood test shows you're on the right dose. Most people will need to take thyroxine for the rest of their life. If your underactive thyroid happened after childbirth, you may not need to stay on treatment.

Because thyroxine tablets replace what your body naturally produces it's rare to have any side effects. If you take too much thyroxine, you can get symptoms of an overactive thyroid. These include sweating, anxiety, shakes and heart racing.

Self‑care for an underactive thyroid

Take your thyroxine regularly. It's best to take your tablet about the same time each day in the morning.

Avoid taking iron or calcium tablets at the same time as they reduce the amount of thyroxine that will get into your blood.

Have regular blood tests at least once a year to check you're still on the right dose of thyroxine.

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Written by HealthInfo clinical advisers. Last reviewed May 2020.

See also:


Thyroid eye disease

Understanding your thyroid function results

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