Patient information from BMJ

Underactive thyroid

Last published:Aug 30, 2022

Having an underactive thyroid means that your body isn't making enough of the hormones you need to get energy from food. Some people with an underactive thyroid don't notice any symptoms. But other people feel tired and low. Your skin may be dry and you might lose some hair. Tablets can help replace the hormones in your body.

What is an underactive thyroid?

Your thyroid is a small gland in your neck. It makes hormones, which travel around your body in your blood. The main hormone your thyroid makes is thyroxine. If your thyroid gland doesn't make enough thyroxine you have what doctors call hypothyroidism.

Thyroxine helps to control how fast your body makes and uses energy from food. Your doctor can tell if you have low levels of thyroxine with a blood test.

If your thyroid stops making enough hormones it's usually because it has been damaged by your immune system. You're more likely to get this type of hypothyroidism if someone in your family has it.

There are other, less common reasons why the thyroid gland stops working properly. It can be damaged by surgery, radiotherapy, some medicines, or another medical problem. You might need tests to rule out other causes. Some women develop an underactive thyroid after they give birth. This is often temporary.

What are the symptoms?

It can be easy to miss the symptoms of an underactive thyroid gland. They are often mild and quite vague. Your doctor can order blood tests to check for an underactive thyroid.

If you have an underactive thyroid you may get some of these symptoms:

• Feeling tired or depressed

• Thinking more slowly or less clearly than usual

• Having dry skin

• Putting on weight

• Feeling the cold more than usual

• Having constipation

• Getting irregular periods, if you're a woman.

If an underactive thyroid isn't treated your symptoms may gradually get worse. After a few months or years, you may notice that you:

• have very dry, flaky skin

• have coarse hair, or lose some of your hair

• put on weight, even though you're not eating any more food than usual

• feel depressed

• feel sluggish and find it difficult to think clearly

• have a hoarse voice.

What treatments work?

An underactive thyroid gland is treated with hormone tablets. These tablets contain the hormone thyroxine. The full name for replacement thyroxine is levothyroxine. You'll probably need treatment for the rest of your life.

If you're getting symptoms from an underactive thyroid, taking levothyroxine will make you feel better.

• You usually take one levothyroxine tablet a day on an empty stomach. Most people take it before breakfast.

• The dose of levothyroxine you take needs to be adjusted carefully to make sure that you don't have too much. Your doctor will start you on a low dose and gradually increase it. You'll need regular blood tests to check the dose is right for you.

• It may take several weeks before you notice any difference in your symptoms. But levothyroxine should make you feel less tired and sluggish. If you were constipated or depressed, these symptoms should also improve.

It is important not to take too much thyroxine. If your dose is too high you may get an abnormal heartbeat (atrial fibrillation) or your bones may get too thin. Your doctor will treat you with the lowest possible dose to avoid these problems.

If your thyroid level is only slightly lower than normal, and you don't have any symptoms, talk to your doctor about what's best for you. You might not need to take thyroxine straight away. Instead, you may have your thyroid level checked regularly.

What will happen to me?

If your underactive thyroid isn't treated your symptoms may get gradually worse. But with treatment you should be able to lead a normal, active life.

Where to get more help

There are many organisations that offer help to people with thyroid conditions. For example, in the UK, the British Thyroid Foundation (btf-thyroid.org) provides support for people affected by thyroid problems.