Underactive thyroid

Also known as hypothyroidism

Underactive thyroid (hypothyroidism) is a condition in which your thyroid gland doesn’t make enough thyroid hormone. It can leave you feeling low in energy and tired.

Key points about underactive thyroid

  1. Many of your body functions slow down when your thyroid doesn’t produce enough thyroid hormones.
  2. Common causes of an underactive thyroid include Hashimoto’s disease, thyroid cancer and its treatment, medicines such as lithium or amiodarone, iodine deficiency, pituitary gland problems or childbirth.
  3. Symptoms of an underactive thyroid include tiredness and fatigue, low mood, weight gain, feeling cold all the time, constipation, dry skin or dry hair.
  4. Treatment for underactive thyroid involves taking a hormone tablet called thyroxine to replace the hormone your body isn't making.

What is the thyroid gland?


Your thyroid is a small, butterfly-shaped gland in your neck. It produces two 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.

Who is affected?

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 within 48 hours of birth. 

What causes an underactive thyroid?

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 and rheumatoid arthritis.

Other causes of an underactive thyroid include:

  • thyroid cancer or its treatment
  • medicines such as lithium or amiodarone
  • iodine deficiency
  • pituitary gland problems
  • childbirth. 



Thyroid cancer or its treatment
  • Thyroid cancer may affect your thyroid gland and make it produce fewer or no making thyroid hormones.
  • An operation to remove your thyroid gland is the main treatment for thyroid cancer.
  • If your whole thyroid gland is removed (total thyroidectomy), you nearly always get hypothyroidism.
  • Cancer radiotherapy can also interfere with the functioning of your thyroid gland, making it produce fewer or no thyroid hormones. 
  • Certain medicines, such as lithium for bipolar disorder and amiodarone for heart conditions, can interfere with the normal production of thyroid hormones.
Iodine deficiency 
  • A lack of iodine in your diet can prevent your thyroid gland from producing enough thyroid hormones.
  • A symptom of insufficient iodine is an enlarged thyroid gland, known as goitre.
  • Iodine deficiency is becoming increasingly common in Aotearoa New Zealand, partly due to eating more commercially prepared foods and eating less salt.
  • Read more about how to eat less salt but more iodine.
Pituitary gland problems
  • Your pituitary gland, which is located at the base of your brain, regulates your thyroid gland.
  • If your pituitary gland is not functioning properly it can lead to an underactive thyroid.
  • A short period of hypothyroidism can occur in women after giving birth.

What are the symptoms of an underactive thyroid?

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 GP or doctor for a proper diagnosis.

Common initial symptoms Later symptoms if underactive thyroid is untreated
  • ongoing tiredness
  • feel sluggish and unable to think clearly
  • unexplained weight gain
  • difficulty losing weight
  • feeling cold most of the time
  • constipation or hard stools
  • irregular menstrual periods
  • dry, flaky skin
  • coarse hair
  • low mood or feeling depressed
  • hoarse voice
  • hair loss
  • joint or muscle pain
  • puffy face
  • slow speech
  • painful tingling in hands or symptoms of carpal tunnel syndrome
  • thickening of your skin 
  • thinning of your eyebrows

If you are pregnant and have these symptoms, see your GP or doctor. If an underactive thyroid is left untreated, it can cause problems such as pre-eclampsia and premature labour.

How are the causes of an underactive thyroid diagnosed?

See your GP or doctor for a proper diagnosis if you have any of the symptoms of an underactive thyroid. They will do the following:

  • Ask you questions about:
    • your symptoms, such as tiredness, weight gain, constipation, cold sensitivity and changes to your hair, skin or nails
    • any family history of thyroid problems
    • any operations, x-rays or illnesses you have had
    • medicines you may be taking.
  • Do a physical check, including:
    • weighing you
    • examining your hair and nails for brittleness
    • checking your pulse and blood pressure
    • looking for swelling of your thyroid gland
    • testing your muscle strength.
  • Take blood tests to assess:
    • thyroid hormone levels, which include T3, T4 and TSH (thyroid stimulating hormone)
    • 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).  

How is an underactive thyroid treated?

Treatment depends on the cause of your underactive thyroid. If you have low thyroid hormone levels but no symptoms, your doctor might suggest more blood tests every 6 or 12 months to monitor your thyroid function. If you have symptoms of underactive thyroid, your doctor might recommend you take regular tablets of thyroid hormone (called T4, also known as thyroxine or levothyroxine).


  • Thyroxine needs to be started slowly, so the dose you take and how often are gradually built up to reach the right level. This usually takes about 6 weeks.
  • Your symptoms will probably go away within a few months but may stay for up to 6 months after starting the medicine.
  • Having regular blood tests every 6–12 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 also important not to take more thyroxine than has been recommended by your doctor because this can cause symptoms of hyperthyroidism and contribute to osteoporosis.

Talk to your doctor if you are using thyroxine and are planning to or have become pregnant. This is because the amount of thyroxine you need may need to be increased.

How can I care for myself with an underactive thyroid?

If you have been prescribed thyroxine tablets for an underactive thyroid, keep taking it at the same time each day in the morning. You will need to avoid taking iron or calcium tablets at the same time as they can reduce the amount of thyroxine that gets into your blood. You will also need to have regular blood tests and appointments with your doctor to find out whether you are on the right dose of thyroxine.

What support is available with an underactive thyroid?

The NZ Thyroid Support Group provides support for individuals to meet, talk and share information about thyroid disorders. Visit their website or contact them by phone: 09 480 2680.

Learn more

The following links provide further information about underactive thyroid. Be aware that websites from other countries may have information that differs from New Zealand recommendations.   

Underactive thyroid (hypothyroidism) HealthInfo Canterbury, NZ
Hypothyroidism (underactive thyroid) – a patient’s guide Family Doctor, NZ
Hypothyroidism HealthDirect, Australia
Hypothyroidism & Hashimoto’s Disease The Australian Thyroid Foundation, Australia
Thyroid – hypothyroidism Better Health Channel, Australia
Causes of hypothyroidism Mayo Clinic, US
Underactive thyroid
Underactive thyroid gland Patient Info, UK
Thyroid information American Thyroid Association, US


  1. Hypothyroidism 3D Regional HealthPathways, NZ, 2021
  2. Management of thyroid dysfunction in adults BPAC, NZ, 2010
  3. Hypothyroidism Patient Info, UK
Credits: Health Navigator Editorial Team.