Underactive thyroid

Also known as hypothyroidism

Key points about underactive thyroid

  • Underactive thyroid (hypothyroidism) is when your thyroid gland doesn’t make enough thyroid hormone.
  • Many of your body functions slow down when your thyroid doesn’t produce enough thyroid hormones.
  • Symptoms of an underactive thyroid include tiredness, low mood, weight gain, feeling cold all the time, constipation, dry skin or dry hair.
  • Common causes include autoimmune disease, operations or radioiodine treatment to your thyroid, medicines such as lithium or amiodarone, iodine deficiency or excess, or pregnancy.
  • It's treated with thyroxine which you take regularly to replace the hormone your body isn't making.
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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.



  • Certain medicines, such as lithium for mood disorders and amiodarone for heart conditions, can interfere with the normal production of thyroid hormones.
Previous treatment to your thyroid
  • This could include an operation, radioiodine treatment or radiotherapy.
  • Hypothyroidism can happen for a short period after giving birth.
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.
Thyroid cancer or its treatment
  • Rarely, thyroid cancer may affect your thyroid gland and make it produce fewer or no 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 change how your thyroid gland works, making it produce fewer or no thyroid hormones.
Pituitary gland problems
  • Your pituitary gland, which is found at the base of your brain, controls your thyroid gland.
  • If your pituitary gland is not working properly it can lead to 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 healthcare provider for a 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 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:

  • 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).  

Treatment depends on the cause of your underactive thyroid. If you have low thyroid hormone levels but no symptoms, your doctor might suggest checking blood tests every 6 or 12 months to monitor your thyroid function. If you have symptoms of underactive thyroid, you will probably need to take thyroid hormone tablets every morning (known as thyroxine or levothyroxine).


  • 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 6–12 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 you have been prescribed thyroxine tablets for an underactive thyroid, keep taking it every morning at the same time. You should not 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 are on the right dose of thyroxine.  

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

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


  1. Hypothyroidism(external link) 3D Regional HealthPathways, NZ, 2021
  2. Management of thyroid dysfunction in adults(external link) BPAC, NZ, 2010
  3. Hypothyroidism(external link) Patient Info, UK

Clinical guidelines and resources

Hypothyroidism(external link) 3D Regional HealthPathways, NZ, 2021
Hypothyroidism(external link) Patient Info, UK
Subclinical hypothyroidism(external link) Patient Info, UK
Management of thyroid dysfunction in adults(external link) BPAC, NZ, 2010
Congenital hypothyroidism(external link) National Screening Unit, NZ, 2016
Levothyroxine brand information(external link) Medsafe NZ

Continuing professional development

Primary hypothyroidism in adults(external link) Research Review Educational Series E-Learning Module, NZ

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Credits: Healthify editorial team. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Dr Alice Miller, GP, FRNZCGP, Wellington

Last reviewed:

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