The thyroid gland is a small butterfly-shaped gland in the neck, just in front of the windpipe (trachea).

One of its main functions is to produce hormones that help regulate the body's metabolism. These hormones are called triiodothyronine (T3) and thyroxine (T4).

Many of the body's functions slow down when the thyroid does not produce enough of these hormones.

Having too much of these hormones can cause serious problems that may need treatment.

Underactive thyroid (hypothyroidism).

Hypothyroidism means your thyroid gland is not producing enough hormones.

An underactive thyroid can often be treated by taking hormone tablets. These are used to replace the hormones your thyroid is not making.

There's no way of preventing an underactive thyroid.

Both men and women can have an underactive thyroid, but it's more common in women.

Most cases are caused either by:

• the immune system attacking the thyroid gland and damaging it

• damage to the thyroid during treatment for an overactive thyroid or thyroid cancer

Symptoms of an underactive thyroid are often like those of other conditions. They usually develop slowly, so you may not notice them for years.

Talk to your GP and ask to be tested for an underactive thyroid if you have symptoms including:

• tiredness

• weight gain

• depression

• feeling the cold more than usual

• dry skin and hair

• muscle aches

The only accurate way of finding out if you have a thyroid problem is to have a thyroid function test. This is where a sample of blood is tested to measure your hormone levels.

Treatment for an underactive thyroid involves taking daily hormone replacement tablets.

Levothyroxine tablets raise your thyroxine levels.

You'll have regular blood tests until the correct dose of levothyroxine is reached. This can take a little while to get right. When you're taking the correct dose, you'll have a blood test once a year to check your hormone levels.

You'll usually need treatment for the rest of your life. But with proper treatment, you can lead a normal, healthy life.

Overactive thyroid (hyperthyroidism)

An overactive thyroid produces too much thyroid hormone. The condition is also known as hyperthyroidism or thyrotoxicosis.

An overactive thyroid can affect anyone. It's about 10 times more common in women than men. It usually happens between 20 and 40 years of age.

An overactive thyroid can cause a wide range of symptoms, including:

• nervousness, anxiety and irritability

• mood swings

• difficulty sleeping

• persistent tiredness and weakness

• sensitivity to heat

• swelling in your neck from an enlarged thyroid gland (goitre)

• an irregular or unusually fast heart rate (palpitations)

• twitching or trembling

• weight loss

See your GP if you have symptoms of an overactive thyroid.

They'll ask about your symptoms. They can arrange for a blood test to check how well your thyroid is working.

If you have an overactive thyroid, you may be sent for more tests to find out the cause.

Graves' disease

This is a condition where your immune system mistakenly attacks and damages the thyroid. About 3 in every 4 people with an overactive thyroid have Graves' disease.

Lumps (nodules) on the thyroid

Extra thyroid tissue can produce thyroid hormones, causing your levels to be too high.

Medicines

Some medicines may cause your thyroid to become overactive. For example, amiodarone, which can be used to treat an irregular heartbeat (arrhythmia).

An overactive thyroid is usually treatable.

The main treatments are:

• medicine that stops your thyroid producing too much of the thyroid hormones

• radioiodine treatment – where a type of radiotherapy is used to destroy cells in the thyroid, reducing its ability to produce thyroid hormones

• surgery to remove some or all of your thyroid, so that it no longer produces thyroid hormones

Each of these treatments has benefits and drawbacks. A specialist in hormonal conditions (endocrinologist) will discuss which treatment is best for you.