Hypothyroidism means a low level of thyroid hormone (called thyroxine, or T4) in the blood.
The level of thyroxine is low because the thyroid gland does not make enough of it. This is why the condition is also known as underactive thyroid.
The role of the 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 to turn food into energy to keep the body going.
Normally, the levels of thyroxine and triiodothyronine in the blood are carefully controlled so that these processes happen at a stable rate.
However, when the thyroid gland does not produce enough thyroxine, the body's metabolism slows down.
This may cause symptoms such as:
- weight gain,
- muscle aches, and
- tiredness (See Symptoms for a full list).
Hypothyroidism can be classed as either:
- subclinical, when the levels of thyroid hormone are still within normal limits and symptoms are mild or not even noticeable, or
- overt, where you have symptoms.
Why does it happen?
The most common cause of hypothyroidism is an autoimmune reaction, which means your body's immune system attacks the thyroid gland, causing inflammation (thyroiditis). For more information, see Causes.
Hypothyroidism can also be a side effect of medical treatment for hyperthyroidism (an overactive thyroid) or for thyroid cancer.
These two types of hypothyroidism account for well over 90% of all cases.
Who is affected
Hypothyroidism can occur in anyone, but is most common in women and occurs more often with increasing age.
It affects 15 in every 1,000 women and one in 1,000 men.
Congenital hypothyroidism (where it is present at birth) occurs in about one in 4,500 live births in the UK.
Hypothyroidism is usually not serious, and taking thyroxine tablets will restore the lack of thyroxine in your blood and cure your symptoms. However, you will have to take this medication for the rest of your life.