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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 pills to replace the hormones your body isn't making.

Doctors call an underactive thyroid hypothyroidism.

• An underactive thyroid is common in women. About 1 in 10 women over 60 have a mild form of the condition.^{1 2}
• The most common cause of an underactive thyroid is when the body's immune system attacks its own thyroid gland.²
• You can take pills to replace the hormones your thyroid should be making. You'll usually need to take these for the rest of your life.

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.

There 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 which 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) decides 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?

If you have symptoms, such as feeling tired or putting on weight, and blood tests show you have an underactive thyroid, doctors call your condition overt hypothyroidism.

But you may not notice any symptoms. If you don't have symptoms, but blood tests show you have a slightly underactive thyroid, your doctor may say you have mild hypothyroidism.

There are two causes of an underactive thyroid.

• A problem with the thyroid gland itself. This is the most common cause.
• Something else, such as a tumor, stops the pituitary gland working properly. So the pituitary gland doesn't produce the hormones that encourage the thyroid to work. This is less common.

Here are some of the things that can go wrong with the thyroid itself.

This is the most common reason for your thyroid to stop working properly. It means your body's immune system, which normally helps your body fight infections, attacks your thyroid gland. If you have this type of hypothyroidism, your thyroid gland may grow larger. But not everyone gets this.

We don't know why your immune system can attack your thyroid gland. But your genes (the genetic material you inherit from your parents) could be a cause. You're more likely to get this type of hypothyroidism if a brother or sister has it.³

Your thyroid can be damaged by surgery or X-ray treatment. This could mean there isn't enough healthy gland left to make the hormones your body needs. You might have surgery to remove a growth or cancer near your thyroid.

If you have an overactive thyroid (hyperthyroidism) you might have radiation treatment to destroy some of your thyroid gland. If too much tissue is destroyed by accident, it can make your thyroid underactive.

Some medicines can stop your thyroid gland from working properly. Drugs that can do this include lithium (used to treat some kinds of depression), amiodarone (used to treat abnormal heart rhythms) and interferon (used to treat hepatitis).

About 5 in 100 pregnant women get an underactive thyroid after they have their baby.⁴ It usually clears up by itself after a few months.

You're more likely to get an underactive thyroid if you have another condition where your immune system attacks part of your body. This is called autoimmune disease. Addison's disease (a disease of the adrenal gland) and vitiligo (where you get white patches on your skin) are examples of autoimmune disease.⁴

Your thyroid gland needs iodine from your food to make hormones. But in some parts of the world, there's not enough iodine in the soil, water and food supply. This can make people's thyroid glands become diseased and grow bigger. An enlarged thyroid is called a goiter. This problem is more common in developing countries than the West. It can be treated by adding iodine to flour or salt.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.