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

If you have an untreated underactive thyroid, your risk of developing cardiovascular disease is increased.

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

Contact your doctor if you're being treated for an underactive thyroid and you develop chest pain, so that any problems can be detected and treated, if necessary.

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

If an underactive thyroid is not treated during pregnancy, there's a risk of problems occurring. These include:

• 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 treatment under the guidance of a specialist in hormone disorders (an endocrinologist). Therefore, tell a GP if you have an underactive thyroid and you're pregnant or trying to get pregnant.

In very rare cases, a severe underactive thyroid may lead to a life-threatening condition called 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's usually treated with thyroid hormone replacement medicine given directly into a vein. In some cases, other treatments such as breathing support, antibiotics and steroid medicine (corticosteroids) are also required.