An underactive thyroid gland (hypothyroidism) is where your thyroid gland doesn't produce enough hormones.
Common signs of an underactive thyroid are tiredness, weight gain and feeling depressed.
An underactive thyroid can often be successfully treated by taking daily hormone tablets to replace the hormones your thyroid isn't making.
There's no way of preventing an underactive thyroid. Most cases are caused either by the immune system attacking the thyroid gland and damaging it, or by damage to the thyroid that occurs during some treatments for an overactive thyroid or thyroid cancer.

Symptoms of an underactive thyroid are often similar to those of other conditions, and they usually 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:
• tiredness
• weight gain
• depression
• being sensitive to the cold
• dry skin and hair
• muscle aches
The only accurate way of finding out whether you have a thyroid problem is to have a thyroid function test, where a sample of blood is tested to measure your hormone levels.

Treatment for an underactive thyroid involves taking daily hormone replacement tablets, called levothyroxine, to raise your thyroxine levels. You'll usually need treatment for the rest of your life. However, with proper treatment, you should be able to lead a normal, healthy life.
If an underactive thyroid isn't treated, it can lead to complications.