Underactive thyroid

An underactive thyroid (hypothyroidism) is a condition where your thyroid gland doesn't make enough hormones. The most common signs are tiredness, weight gain and feeling depressed.

Most of the time, an underactive thyroid can be treated well with daily hormone tablets. They replace the hormones your thyroid isn't making.

There's no way to prevent it. In most cases the cause is one of two things: the immune system attacks the thyroid gland and damages it, or the thyroid is damaged during certain treatments for an overactive thyroid or for thyroid cancer.

Symptoms of an underactive thyroid are often similar to those of other conditions, and they usually develop slowly, so you may not notice them for years.

You should see your doctor and ask to be tested for an underactive thyroid if you have symptoms including:

• tiredness
• weight gain
• depression
• being sensitive to the cold
• dry skin and hair
• muscle aches

The only accurate way of finding out whether you have a thyroid problem is to have a thyroid function test, where a sample of blood is tested to measure your hormone levels.

Underactive thyroid affects both men and women, though it's more common in women. In the UK, it affects 15 in every 1,000 women and 1 in 1,000 men. Children can develop it too.

Around 1 in 3,500 to 4,000 babies are born with an underactive thyroid (congenital hypothyroidism). In the UK, every baby is screened for congenital hypothyroidism with a blood spot test when the baby is about 5 days old.

An underactive thyroid is treated with a hormone replacement tablet called levothyroxine, which you take every day to raise your thyroxine levels. In most cases you'll need the treatment for the rest of your life. But with proper treatment, you should be able to live a normal, healthy life.

If an underactive thyroid isn't treated, it can lead to complications. These include heart disease, goitre, pregnancy problems and a life-threatening condition called myxoedema coma (although this is very rare).

Many of the symptoms of an underactive thyroid (hypothyroidism) are the same as those of other conditions, so it's easy to mistake it for something else.

Symptoms usually come on slowly, and you may not realise you have a medical problem for several years.

Common symptoms include:

• tiredness
• being sensitive to cold
• weight gain
• constipation
• depression
• slow movements and thoughts
• muscle aches and weakness
• muscle cramps
• dry and scaly skin
• brittle hair and nails
• loss of libido (sex drive)
• pain, numbness and a tingling sensation in the hand and fingers (carpal tunnel syndrome)
• irregular periods or heavy periods

Elderly people with an underactive thyroid may develop memory problems and depression. Children may grow and develop more slowly. Teenagers may start puberty earlier than normal.

If you have any of these symptoms, see your doctor and ask to be tested for an underactive thyroid.

You're unlikely to have many of the later symptoms of an underactive thyroid, because the condition is usually picked up before the more serious symptoms appear.

Later symptoms of an underactive thyroid include:

• a low-pitched and hoarse voice
• a puffy-looking face
• thinned or partly missing eyebrows
• a slow heart rate
• hearing loss
• anaemia

An underactive thyroid (hypothyroidism) happens when your thyroid gland doesn't make enough of the hormone thyroxine, also called T4.

In most cases, the cause is the immune system attacking the thyroid gland and damaging it. The other common cause is damage that comes from treatments for thyroid cancer or for an overactive thyroid.

An underactive thyroid often happens when the immune system, which normally fights infection, turns on the thyroid gland and attacks it. The damage means the thyroid can't make enough of the hormone thyroxine, and that's what leads to the symptoms of an underactive thyroid.

The most common autoimmune cause is a condition called Hashimoto's disease.

Doctors don't know what causes Hashimoto's disease, but it runs in families. It's also common in people who have another immune system disorder, such as type 1 diabetes and vitiligo.

An underactive thyroid can also happen as a side effect, or a complication, of earlier treatment to the thyroid gland. This includes surgery, and a treatment called radioactive iodine therapy.

Doctors sometimes use these treatments for an overactive thyroid (where the thyroid gland produces too much hormone) or for thyroid cancer.

Worldwide, not getting enough iodine in the diet is a common cause of an underactive thyroid, because the body needs iodine to make thyroxine. In the UK, though, iodine deficiency is uncommon.

Some babies are born with an underactive thyroid because the thyroid gland doesn't develop properly in the womb. This is called congenital hypothyroidism. It's uncommon, affecting around 1 in 3,000 babies, and it's usually picked up during routine screening soon after birth.

A problem with the pituitary gland could also lead to an underactive thyroid. The pituitary gland sits at the base of the brain and regulates the thyroid, so damage to the pituitary gland may lead to the condition.

Some viral infections have also been linked to an underactive thyroid, as have some medicines used to treat other conditions, such as:

• lithium, a medicine sometimes used to treat certain mental health conditions, including depression and bipolar disorder
• amiodarone, a medicine sometimes used to treat irregular heartbeats (arrhythmias)
• interferons, a class of medicine sometimes used to treat certain types of cancer and hepatitis C

Speak to your doctor or specialist if you're concerned that a medicine you're taking may be affecting your thyroid hormone levels.

An underactive thyroid (hypothyroidism) should be diagnosed as early as possible. When the body has low levels of thyroid hormones, such as triiodothyronine (T3) and thyroxine (T4), it can change the way it processes fat.

That can lead to high cholesterol and atherosclerosis, which means clogging of the arteries. In time, this can cause serious heart problems, such as angina and heart attack.

So if you keep having symptoms of an underactive thyroid, see your doctor and ask for a blood test.

A blood test that measures your hormone levels is the only accurate way to find out whether there's a problem. It's called a thyroid function test, and it looks at the levels of thyroid-stimulating hormone (TSH) and thyroxine (T4) in your blood.

A high level of TSH with a low level of T4 could mean you have an underactive thyroid. If your results show raised TSH but normal T4, you may be at risk of developing an underactive thyroid in the future. In that case your doctor may suggest a repeat blood test every so often, to see whether you eventually do develop it.

Blood tests are sometimes used for other measurements too, such as the level of a hormone called triiodothyronine (T3). This isn't routine, though, because T3 levels can often stay normal even when you have a significantly underactive thyroid.

The Lab Tests Online UK website has more information about the different types of thyroid function tests.

Your doctor may send you to an endocrinologist (a specialist in hormone disorders) if you:

• are younger than 16
• are pregnant or trying to get pregnant
• have just given birth
• have another health condition, such as heart disease, which may complicate your medicine
• are taking a medicine known to cause a reduction in thyroid hormones, such as amiodarone or lithium

An underactive thyroid (hypothyroidism) is usually treated with daily hormone replacement tablets called levothyroxine. The tablets replace thyroxine, the hormone your thyroid doesn't make enough of.

At first you'll have regular blood tests until the correct dose of levothyroxine is reached. It can take a little while to get this right. You may begin on a low dose, and the doctor may raise it gradually, depending on how your body responds. Some people start to feel better soon after treatment begins, while others don't notice any improvement in their symptoms for several months.

Once you're on the correct dose, you'll usually have a blood test once a year to monitor your hormone levels.

If your blood tests suggest you may have an underactive thyroid but you have no symptoms, or only very mild ones, you may not need any treatment. In that case, your doctor will usually monitor your hormone levels every few months and prescribe levothyroxine if you develop symptoms.

If your doctor prescribes levothyroxine for an underactive thyroid, take 1 tablet at the same time every day. Most people are advised to take it in the morning, though some prefer to take it at night.

Other medicines, supplements or foods can change how well the tablets work. So swallow the tablet with water on an empty stomach, and don't eat for 30 minutes afterwards.

If you forget a dose, take it as soon as you remember, as long as that is within a few hours of your usual time. If you don't remember until later than that, skip the missed dose and take the next one at the usual time, unless your doctor has told you otherwise.

An underactive thyroid is a lifelong condition, so you'll usually need to take levothyroxine for the rest of your life.

Levothyroxine doesn't usually cause side effects, because the tablets simply replace a hormone your body is missing.

Side effects mostly happen only when you're taking too much levothyroxine. Too high a dose can cause problems including sweating, chest pain, headaches, diarrhoea and vomiting.

If you develop new symptoms while taking levothyroxine, tell your doctor. Let them know too if your symptoms get worse or don't improve.

If you have an underactive thyroid, you may hear about combination therapy, which means using levothyroxine and triiodothyronine (T3) together. In the UK it isn't routinely used, because there isn't enough evidence to show it works better than levothyroxine alone (monotherapy).

In most cases, doctors should avoid suppressing thyroid-stimulating hormone (TSH) with a high dose of thyroid replacement therapy. It carries a risk of adverse side effects, such as atrial fibrillation (an irregular and abnormally fast heart rate), strokes, osteoporosis and fracture.

There is one exception. This type of treatment may sometimes be recommended if you have a history of thyroid cancer and there's a significant risk of it coming back.

If you have an underactive thyroid and it isn't being treated, your risk of developing cardiovascular disease goes up. The reason is that low levels of the hormone thyroxine can push up the cholesterol in your blood. High cholesterol can cause fatty deposits to build up in your arteries, and these restrict the flow of blood.

If you're being treated for an underactive thyroid and you develop chest pain, contact your doctor. That way any problems can be found and treated, if they need to be.

A goitre is an abnormal swelling of the thyroid gland, and it shows up as a lump in the throat. People with an underactive thyroid can develop a goitre. This happens when the body tries to push the thyroid to produce more thyroid hormones.

If an underactive thyroid isn't treated during pregnancy, problems can occur. These include:

• pre-eclampsia (this 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). So if you have an underactive thyroid and you're pregnant or trying to get pregnant, tell your doctor.

In very rare cases, a severe underactive thyroid can lead to a life-threatening condition called myxoedema coma. It happens when thyroid hormone levels fall very low, and this can cause symptoms such as confusion, hypothermia and drowsiness.

Myxoedema coma needs emergency treatment in hospital. The usual treatment is thyroid hormone replacement medicine, given straight into a vein. Some people also need other treatments, such as breathing support, antibiotics and steroid medicine (corticosteroids).