Underactive thyroid

Also known as hypothyroidism

Underactive thyroid (hypothyroidism) is a condition in which the thyroid gland does not make enough thyroid hormone. It can leave you feeling low in energy and tired.

Treatment for underactive thyroid involves taking a hormone tablet to replace the hormone your body isn't making.

The thyroid gland

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

Thyroid hormones help the body use energy, stay warm and keep the brain, heart, muscles, and other organs working as they should.

Many of the body's functions slow down when the thyroid doesn't produce enough of these hormones.

Who is affected?

Both men and women 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 New Zealand). To detect this, most babies have a blood spot test within 48 hours of birth. 

What causes underactive thyroid?

The most common cause of underactive thyroid is the autoimmune condition called Hashimoto’s disease. This is when the body’s immune system attacks and destroys the cells of the thyroid gland. Underactive thyroid is also more common in people who have type 1 diabetes and rheumatoid arthritis (although these automimmune conditions do not cause hypothyroidism). 

Other Causes

Causes Description
Thyroid cancer treatment
  • Surgical removal of the thyroid gland is the main treatment for thyroid cancer. If the whole thyroid gland is removed (total thyroidectomy), hypothyroidism nearly always occurs.
  • Cancer radiotherapy can also interfere with the functioning of the thyroid gland.
  • Certain medications such as lithium for bipolar disorder and amiodarone for heart conditions can interfere with the normal production of thyroid hormone.
Iodine deficiency disorder
  • The lack of enough iodine in the diet can prevent the thyroid gland from producing hormones.
  • A symptom of insufficient iodine is an enlarged thyroid gland, known as goitre.
  • Iodine deficiency is becoming increasingly common in New Zealand, partly due to an increase in consumption of commercially prepared foods and a reduction in consumption salt.²
  • See also: how to eat less salt but more iodine.
Pituitary gland problems
  • The pituitary gland (located at the base of the brain) regulates the thyroid gland. If the pituitary gland is not functioning properly it can lead to an underactive thyroid.
  • Brief hypothyroidism can occur in women after giving birth.

What are the symptoms of 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 confuse them for symptoms of other conditions. As the condition worsens, your body's functioning continues to slow and the signs and symptoms become more obvious.  

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

How is underactive thyroid diagnosed?

Many symptoms of underactive thyroid are the same as other diseases, so diagnosis cannot be based just on symptoms. To diagnose underactive thyroid your doctor will:

  • Ask 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:
    • measuring your weight
    • 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
    • thyroid antibodies in your blood
    • cholesterol levels because underactive thyroid can cause the body’s LDL cholesterol level to increase (a risk factor for heart disease).  

How is underactive thyroid treated?

Your doctor will take findings from your medical history, physical check and blood tests into consideration when deciding whether treatment is needed.

If you have low thyroid hormone level but no symptoms, your doctor might suggest more blood tests every 6 or 12 months to monitor your thyroid function. If you have low thyroid hormone level and symptoms, your doctor might recommend you take regular tablets of thyroid hormone (specifically one 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.
  • Most people start to feel better in a week or two. Your symptoms will probably go away within a few months. But you will likely need to keep taking the pills from now on.
  • Having regular blood tests every 6 to 12 months is important because it 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 to not take more thyroxine than has been recommended by your doctor because this may contribute to osteoporosis and in older people, there is debate if it increases heart risks and other problems.  

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 can increase.

Learn more

Causes of hypothyroidism Mayo Clinic (US), 2012
Underactive thyroid
 NHS (UK), 2013
Thyroid information American Thyroid Association (US)
Thyroid support group NZ thyroid support group

Credits: Health Navigator Editorial Team. Reviewed By: Mark Bolland, Auckland DHB (27 March 2017)