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Underactive thyroid (hypothyroidism)

Mate repe tenga

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, 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 do not 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 that controls how much thyroxine you make. It's 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 the 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.

Slightly underactive thyroid

If you have a slightly raised TSH level but normal T3 and T4 levels, it means your thyroid is slightly underactive but still able to produce enough hormones. This is known as subclinical hypothyroidism.

Subclinical hypothyroidism may get worse and develop into an underactive thyroid.

Tell your general practice team if you develop the symptoms of an underactive thyroid. You should also have blood tests every six to 12 months to monitor your thyroid hormone levels.

If your TSH remains raised or you develop symptoms, your general practice team may suggest starting thyroxine tablets.

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 gets into your blood.

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

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Written by HealthInfo clinical advisers. Last reviewed July 2023.


See also:


Thyroid eye disease

Understanding your thyroid function results

Page reference: 70690

Review key: HITHY-49064