An underactive thyroid, also called hypothyroidism, means your thyroid gland does not produce enough chemicals called hormones.
Common signs of an underactive thyroid are tiredness, weight gain and feeling depressed.
An underactive thyroid is not usually serious. It can often be treated successfully by taking daily hormone tablets to replace the hormones your thyroid isn't making.
An underactive thyroid cannot be prevented. Most cases are caused either by your immune system attacking your thyroid or by damage to your thyroid that can occur during some treatments for thyroid cancer and an overactive thyroid.
Read more about the causes of underactive thyroid.
When to see your GP
Symptoms of an underactive thyroid are often confused for something else by patients and doctors and they develop slowly so you may not notice them for years.
You should see your GP and ask to be tested for an underactive thyroid if you have symptoms including:
- weight gain
- being sensitive to the cold
- dry skin and hair
- muscle aches
The only accurate way to find out if you have a thyroid problem is to have a blood test to measure your hormone levels.
Read more about the symptoms of underactive thyroid and testing for an underactive thyroid.
Who is affected
Both men and women can have an underactive thyroid, although it's more common in women. In the UK, it affects 15 in every 1,000 women and 1 in 1,000 men. The condition can also develop in children.
Around 1 in 3,500-4,000 babies is born with an underactive thyroid (called congenital hypothyroidism). All babies born in the UK are screened for congenital hypothyroidism using a heel-prick blood test when the baby is about five days old.
Treating an underactive thyroid
Treatment for an underactive thyroid involves taking daily hormone-replacement tablets, called levothyroxine, to raise your thyroxine levels. You will usually need treatment for the rest of your life. However, with proper treatment you should be able to lead a normal, healthy life.
If it's not treated, an underactive thyroid can lead to complications, including heart disease, goitre (a lump in the throat caused by a swollen thyroid), pregnancy problems and a life-threatening condition called myxoedema coma (although this is very rare).
Read more about treating underactive thyroid and the complications of underactive thyroid.