Several complications can occur if you have an underactive thyroid that is not treated.

Heart problems

Having an untreated underactive thyroid can increase your risk of developing cardiovascular disease (disease of the heart or blood vessels).

This is because having low levels of the hormone thyroxine can lead to increased levels of cholesterol in your blood. Having high levels of cholesterol can cause fatty deposits to build up in your arteries, restricting the flow of blood.

You should call your doctor if you are being treated for an underactive thyroid and you develop chest pain, so that any problems can be detected and treated if necessary. 

Goitre

A goitre is an abnormal swelling of the thyroid gland that causes a lump to form in the throat. They can develop in people with an underactive thyroid when the body tries to stimulate the thyroid to produce more thyroid hormones.

Pregnancy complications

If underactive thyroid is not treated during pregnancy, there is a risk of problems including:

• pre-eclampsia - which can cause high blood pressure and fluid retention in the mother, and growth problems in the baby
• anaemia in the mother
• an underactive thyroid in the baby
• birth defects
• bleeding after birth
• problems with the baby's physical and mental development
• premature birth or a low birthweight
• stillbirth or miscarriage

These problems can usually be avoided with proper treatment by a specialist, so you should tell your GP if you have an underactive thyroid and you are pregnant or planning a pregnancy.

Myxoedema coma

In very rare cases, a severe underactive thyroid may lead to a life-threatening condition known as myxoedema coma. This is where the thyroid hormone levels become very low, causing symptoms such as confusion, hypothermia, and drowsiness.

Myxoedema coma requires emergency treatment in hospital. It is usually treated with thyroid hormone replacement medication given directly into a vein. In some cases, other treatments such as mechanical breathing support, antibiotics and steroid medication (corticosteroids) are also required.