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Underactive thyroid - What is an underactive thyroid?

BMJ Group Medical Reference


Your thyroid gland makes hormones. If you have an underactive thyroid, it doesn't make enough of these hormones. This can make you feel low. You may also put on weight and get dry skin. There are good treatments for this condition.

We've brought together the best research about an underactive thyroid and weighed up the evidence about how to treat it. You can use our information to talk to your doctor and decide which treatments are best for you.

If your thyroid gland is underactive, it doesn't make enough hormones. These hormones control how much energy your body uses.

An underactive thyroid can make you feel tired and depressed. You may also put on weight or have dry skin or hair. But you can have an underactive thyroid without noticing any symptoms.

Treatment for an underactive thyroid works well. You can take hormone tablets to replace the hormones your body isn't making.

Doctors call an underactive thyroid hypothyroidism.

Key points for people with an underactive thyroid

  • An underactive thyroid is common in women. It affects about 1 in 10 women in the UK.[1]

  • The most common cause of an underactive thyroid is when the body's immune system attacks its own thyroid gland.[2]

  • You can take tablets to replace the hormones your thyroid should be making. You'll usually need to take these for the rest of your life.

What is your thyroid?

Your thyroid is a small gland in your neck. It's just below your voice box (larynx), in front of your windpipe (trachea).[3] It's shaped like a butterfly.

thyroid-gland_default.jpgThere are two thyroid hormones.

  • The main one is thyroxine, also called T4. It's made by your thyroid gland and pumped into your blood.

  • The second hormone is triiodothyronine. It's sometimes called T3. Your liver and kidneys make T3 out of T4.

These hormones are chemicals that travel around your body in your blood. They control how fast your body makes and uses energy from food.

Problems with your thyroid can make it underactive or overactive. Here, we're looking at an underactive thyroid. This is called hypothyroidism.

A hormone called thyroid-stimulating hormone (TSH) controls how much of the thyroid hormones your body needs. TSH is made by your pituitary gland, which is in your brain.

  • If the amount of T4 and T3 in your blood drops too low, your pituitary gland releases more TSH. This tells your thyroid to make more hormones.

  • If the amount of T4 and T3 is too high, your pituitary gland stops releasing TSH. Your thyroid then stops making T3 and T4.

If your doctor thinks there might be a problem with your thyroid, he or she may do some blood tests to measure your levels of T4, T3, and TSH. To read more, see How do doctors diagnose an underactive thyroid?

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Last Updated: June 21, 2012
This information does not replace medical advice.  If you are concerned you might have a medical problem please ask your Boots pharmacy team in your local Boots store, or see your doctor.
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