An underactive thyroid is when your thyroid does not produce enough thyroid hormones.
It is usually treated with daily tablets called levothyroxine. Levothyroxine replaces the thyroxine hormone (T4).
You'll usually take the tablets for the rest of your life. Levothyroxine eases symptoms and usually has few side effects.
Starting treatment
It can take time to get the dose of levothyroxine right. You may need regular blood tests to help with this.
Your GP may start you on a low dose and increase it gradually. This will depend on how you respond to the medicine. Some people start to feel better right away. Others do not notice an improvement for a few months.
When you're on the right dose, you'll have a blood test once a year. This is to check your hormone levels.
Taking levothyroxine
You need to take levothyroxine at the same time every day.
You're usually told to take them in the morning. But some people prefer to take them at night.
How and when to take levothyroxine
When you might not need medicine
Blood tests might suggest you have an underactive thyroid. But you might not have any symptoms or your symptoms are very mild.
You may not need to take tablets. Your GP usually checks your hormone levels every few months. They'll prescribe levothyroxine if you start to get symptoms.
If an underactive thyroid is not treated
If an underactive thyroid is not treated, it can lead to complications.
These include:
- heart disease
- goitre
- pregnancy problems
- myxoedema coma - a rare, but life-threatening condition
Heart problems
You are at greater risk of cardiovascular disease if you do not have treatment.
Low levels of thyroxine can lead to high cholesterol. This can lead to serious heart-related problems, such as angina and a heart attack.
Non-urgent advice: Contact your GP if:
- you're being treated for an underactive thyroid and you have chest pain
Goitre
Goitre is an abnormal swelling of the thyroid gland. It causes a lump to form in your throat. Goitre can develop in people with thyroid problems.
Underactive thyroid and pregnancy
An underactive thyroid should be treated properly before you become pregnant. This is important for you and your baby's health.
Tell your GP if you're pregnant or trying to become pregnant and you have an underactive thyroid. They may refer you to a specialist.
Pregnancy complications
If an underactive thyroid is not treated, there's a risk of problems.
These include:
- pre-eclampsia - this can cause high blood pressure, fluid retention 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 development
- premature birth or a low birth weight
- stillbirth or miscarriage
These problems can usually be avoided with treatment.
Myxoedema coma
Myxoedema coma is a rare life-threatening condition. It happens when the thyroid hormone levels become very low.
Myxoedema coma requires emergency treatment in hospital.
Emergency action required: Phone 112 or 999 if:
someone has symptoms of a myxoedema coma, such as:
- confusion
- hypothermia
- drowsiness
Content supplied by the NHS and adapted for Ireland by the HSE
