An underactive thyroid means your thyroid gland, located in the neck, does not produce enough hormones.
Common signs of an underactive thyroid are tiredness, weight gain and feeling depressed.
Medically known as hypothyroidism, an underactive thyroid is not usually serious. It is easily treated by taking hormone tablets to replace the hormones that your thyroid isn't making.
The thyroid produces a hormone called thyroxine, which controls how much energy your body uses. When the thyroid does not produce enough thyroxine, many of the body's functions slow down.
An underactive thyroid cannot be prevented. Most cases of underactive thyroid are caused either by your immune system attacking your thyroid or a damaged thyroid.
Find out more about the causes of underactive thyroid.
When to see your GP
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
Find out more about the symptoms of underactive thyroid.
Symptoms of an underactive thyroid are often confused for something else, by patients and doctors. Symptoms also usually begin slowly and you may not notice them for several years. The only accurate way to find out if you have a thyroid problem is to get a blood test to measure your hormone levels.
Find out more about testing for an underactive thyroid.
Who can it affect?
Both men and women can have an underactive thyroid. However, it's more common in women. In the UK, it affects 15 in every 1,000 women and 1 in 1,000 men.
One in 4,500 babies are born with an underactive thyroid (called congenital hypothyroidism). All babies born in the UK are screened for congenital hypothyroidism by having a heel-prick blood sample taken after the first week.
Underactive thyroid is usually not serious, and taking hormone-replacement tablets, called levothyroxine, will raise your thyroxine levels. You will usually need treatment for the rest of your life. However, with careful management, you should be able to lead a normal, healthy life.
Find out more about treatment for underactive thyroid.
If it is not treated, an underactive thyroid can lead to complications, including swelling of the thyroid (a condition called goitre), heart disease, mental health problems and infertility.
Find out more about complications of underactive thyroid.