Underactive thyroid

Your thyroid is a small, butterfly-shaped gland in your neck. It produces 2 thyroid hormones: tri-iodothyronine (T3) and thyroxine (T4). 

These thyroid hormones help your body use energy, stay warm and keep your brain, heart, muscles and other organs working as they should. Many of your body's functions slow down when your thyroid doesn't produce enough of these hormones.

Anyone can have an underactive thyroid, but it is more common in women and in people over 50 years of age.

Some babies are born with an underactive thyroid, although this is very rare (about 20 babies every year in Aotearoa New Zealand). To detect this, most babies have a blood spot test(external link) within 48 hours of birth. 

The most common cause of underactive thyroid is an autoimmune condition called Hashimoto’s disease. This is when your body’s immune system attacks and destroys the cells of your thyroid gland, making it produce fewer or no thyroid hormones.

It is not clear what triggers this, but it sometimes runs in families. Underactive thyroid is also more common in people who have autoimmune conditions such as type 1 diabetes or rheumatoid arthritis.

Other causes of an underactive thyroid include:

• medicines such as lithium or amiodarone
• previous treatment to your thyroid 
• pregnancy or childbirth
• too much or too little iodine
• thyroid cancer or its treatment
• pituitary gland problems.

The symptoms of an underactive thyroid are often mild and easy to miss. They develop slowly over a number of months or even years. You may put symptoms such as feeling tired or gaining weight down to poor diet or simply getting older. It's also easy to mistake the symptoms for other conditions. Therefore, it is important to see your healthcare provider for a diagnosis.

If you are pregnant and think you might have an underactive thyroid, see your healthcare provider.  If an underactive thyroid is left untreated, it can cause problems such as pre-eclampsia and premature labour. 

See your healthcare provider for a diagnosis if you have any of the symptoms of an underactive thyroid. They will do the following:

Ask you questions about:

• your symptoms
• any family history of thyroid problems
• any operations, X-rays or illnesses you have had in the past
• medicines you may be taking.

Do a physical check, which may include:

• weighing you
• examining your hair and nails
• checking your pulse and blood pressure
• looking for swelling of your thyroid gland
• testing your muscle strength.

Arrange blood tests to assess:

• TSH (thyroid stimulating hormone) – if this is high then further tests will be done to check:
  • thyroid hormone levels (T4)
  • thyroid antibodies in your blood
  • cholesterol levels, because underactive thyroid can cause your body’s LDL cholesterol level to increase (which is a risk factor for heart disease).  

Treatment depends on the cause of your underactive thyroid. If you have symptoms of underactive thyroid, you will probably need to take thyroid hormone tablets every morning (known as thyroxine or levothyroxine).

Thyroxine

• Thyroxine needs to be started slowly, so the dose you take is gradually built up to reach the right level. This usually takes about 6 weeks. 
• Your symptoms will probably get better within a few months of starting the medicine but may take up to 6 months to go away completely.
• Having regular blood tests every 6 to 12 months allows your doctor to check your thyroid hormone level and adjust the dosage of your thyroxine if necessary.
• The amount of thyroxine you need can change over time because the function of your thyroid gland may change over time.
• It is important to take the dose recommended by your doctor because this can cause too much thyroxine can cause hyperthyroidism, heart disease or osteoporosis.
• Talk to your doctor if you are taking thyroxine and are planning to or have become pregnant. This is because your body may need a different amount of thyroid hormone during pregnancy.

If your blood test shows a high level of the pituitary hormone TSH, but a normal level of the thyroid hormone T4 this is called subclinical hypothyroidism. The causes are the same as for underactive thyroid (hypothyroidism). Because your body has a normal amount of thyroid hormone there are usually no symptoms. Your healthcare provider will arrange for follow up blood tests every 2 to 12 months because a small number of people (1–5%) do go on to develop hypothyroidism.

A small number of people with subclinical hypothyroidism will need to be treated with thyroid hormone even though their T4 levels are not low. This applies to people who:

• are pregnant or planning a pregnancy
• have positive thyroid antibodies or very high TSH levels
• have symptoms of an underactive thyroid.

Ask your healthcare provider if you think this might be you.

If you've been prescribed thyroxine tablets for an underactive thyroid, keep taking it every morning at the same time. You shouldn't take iron or calcium tablets at the same time as they can reduce the amount of thyroxine that gets into your blood. You will need to continue having regular blood tests to check you're on the right dose of thyroxine.

Apps reviewed by Healthify

You may find it useful to look at some self-management and healthy living apps.

The NZ Thyroid Support Group provides support for individuals to meet, talk and share information about thyroid disorders. Visit their website(external link) or call 09 480 2680.