Overactive thyroid (also known as hyperthyroidism) is a relatively common hormonal condition that occurs when there is too much thyroid hormone in the body.

Excess levels of thyroid hormones can then speed up the body’s metabolism, triggering a range of symptoms, such as:

• nervousness and anxiety
• hyperactivity – where a person can’t stay still and is full of nervous energy
• unexplained or unplanned weight loss 
• swelling of the thyroid gland, which causes a noticeable lump, known as a goitre, to form in the throat

The severity, frequency and range of symptoms can vary from person to person.

Read more about the symptoms of an overactive thyroid gland.

What causes an overactive thyroid gland?

The thyroid gland is found in the neck. It produces hormones that are released into the bloodstream to control the body's growth and metabolism. These hormones are called thyroxine and triiodothyronine.

They affect processes such as heart rate and body temperature, and help convert food into energy to keep the body going.

In hyperthyroidism, the thyroid gland produces too much thyroxine or triiodothyronine, which speeds up the body's metabolism.

There are several possible underlying causes, the most common being Graves' disease, in which the body's immune system targets the thyroid gland and causes it to produce too much of the thyroid hormones.

Read more about the causes of an overactive thyroid gland.

Treatment

An overactive thyroid usually responds well to treatment, and most people are able to control their symptoms.

The three most widely used treatments for an overactive thyroid gland are:

• thionamides – a group of medications, including carbimazole and methimazole, that stop the thyroid gland producing too much thyroid hormone
• radioiodine treatment – a radioactive substance called iodine that helps shrink the thyroid gland, reducing its activity (the radiation contained in iodine is a very low dose and does not pose a threat to health)
• surgery – in a small number of cases surgery may be required to remove some or all of the thyroid gland, particularly if there is a large goitre

Beta-blockers may also sometimes be used to temporarily relieve many symptoms of an overactive thyroid gland, although it doesn't target the thyroid gland itself.

It's common for treatment to lead to the thyroid not producing enough hormones. This is known as having an underactive thyroid gland (hypothyroidism). However, an underactive thyroid is not usually serious and is easily treated.

Read more about the treatment of an overactive thyroid gland.

Complications

Around 1 in 20 people with Graves' disease will also develop symptoms affecting their eyes, such as:

• double vision 
• sensitivity to light (photophobia)
• tearing (excess production of tears)

This is known as Graves' ophthalmopathy and should be seen by a doctor who specialises in treating eye conditions (an ophthalmologist).

A rarer and more serious complication is a sudden and severe flare-up of symptoms, known as a thyroid storm. A thyroid storm can be life-threatening, as it causes severe dehydration and heart problems.

Read more about the complications of an overactive thyroid gland.

Who is affected

Women are 10 times more likely to have an overactive thyroid gland than men.

It is estimated that around 1 in 50 women in England currently live with an overactive thyroid gland.

In most cases, symptoms will begin somewhere between the ages of 20 and 40, though they can start at any age, including in childhood.

An overactive thyroid gland occurs most frequently in white and Asian people, and less frequently in African-Caribbean people.