An underactive thyroid gland (hypothyroidism) is where your thyroid gland does not 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.