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Underactive thyroid (hypothyroidism)

Hypothyroidism occurs when the thyroid gland doesn't make enough thyroid hormones. This slows down the body's metabolism, leading to symptoms including fatigue and weight gain. It's more common in older people, and women are more likely to be affected than men.

The thyroid gland 

Your thyroid is an endocrine gland. This means it secretes hormones into your bloodstream. Hormones are chemicals produced by your body to help regulate how your cells and organs work. They are sometimes called chemical messengers.

Your thyroid gland is in your neck, in front of your windpipe. It's about 2cm wide and 4cm high.

Underactive Thyroid

The position of the thyroid gland and surrounding structures

Your thyroid gland makes two hormones: thyroxine (also called T4) and triiodothyronine (also called T3). Together, these hormones regulate your body's growth and metabolism. Your metabolism is how quickly your body burns energy and how quickly reactions in your body happen. It affects lots of things, including your weight and how much you sleep.

The production of thyroid hormones is controlled by another hormone called thyroid stimulating hormone (TSH). This is made by an endocrine gland in your brain called the pituitary gland.

Symptoms of hypothyroidism 

Hypothyroidism usually develops gradually. The symptoms are mild, and you may not even notice them at first. This is called subclinical hypothyroidism.

Hypothyroidism cause a general slowing down of your body's functions. Some of the symptoms include:

  • feeling tired and sleeping a lot
  • feeling the cold easily
  • dry and/or pale skin
  • coarse, thinning hair and brittle nails
  • sore muscles, slow movements and weakness
  • a hoarse or croaky voice
  • depression
  • problems with memory and concentration
  • weight gain
  • constipation
  • fertility problems and increased risk of miscarriage
  • heavy, irregular or prolonged menstrual periods
  • a slow heart rate.

You may also have swelling of your thyroid gland. This may be visible as a lump on your neck called a goitre.

Very rarely, untreated hypothyroidism can lead to coma, called myxoedema coma. So if you have some or all of these symptoms, see your GP.

Causes of hypothyroidism 

Hashimoto's thyroiditis

The most common cause of hypothyroidism in Australia is Hashimoto's thyroiditis, (also known as chronic lymphocytic thyroiditis or autoimmune thyroiditis). It's an autoimmune disease, which means it's caused by antibodies from your immune system attacking your body. Rather than attacking foreign bodies such as bacteria or viruses, antibodies destroy your thyroid gland cells. This prevents your thyroid gland from producing enough hormones.

Hashimoto's thyroiditis is the most common type of autoimmune hypothyroidism. It can make your thyroid gland swell up and cause a goitre. If you don't have a goitre, the condition may be called atrophic thyroiditis or primary myxoedema.

Other causes

Around one in 4,000 babies is born with congenital hypothyroidism. This means the thyroid gland fails to develop or it doesn't produce enough thyroid hormones. Babies in Australia are screened for this when they're two to three days old. A blood sample is taken from a prick on the heel. With treatment, babies who have congenital hypothyroidism develop normally.

Disorders of your hypothalamus (the part of your brain that links your nervous system to your endocrine system) and pituitary gland can also lead to hypothyroidism. This is because these parts of your brain are involved in the regulation and production of thyroid hormones. Your body needs a substance called iodine to make thyroid hormones. Not having enough iodine in the diet is the leading cause of hypothyroidism worldwide. Studies suggest that some people in Australia have a mild to moderate iodine deficiency. To help prevent this, iodised salt is now used in all bread (except organic) made in Australia.

Some treatments for hyperthyroidism (an overactive thyroid gland), such as surgery, anti-thyroid drugs and radio-iodide therapy can cause hypothyroidism. Some medicines can affect the normal functioning of your thyroid gland and cause hypothyroidism. These include lithium carbonate (for bipolar disorder) and amiodarone (for heart rhythm problems).

Diagnosis of hypothyroidism 

Your GP will examine you and ask about your symptoms. You may need to have a blood test.

Hypothyroidism can be diagnosed by monitoring the levels of TSH and thyroid hormones in your blood. A diagnosis of autoimmune hypothyroidism is usually confirmed by the presence of particular antibodies in your blood.

Treatment of hypothyroidism 

Occasionally, hypothyroidism gets better without treatment. In general, however, the symptoms get progressively worse if it isn't treated. Hypothyroidism can be treated with a thyroxine replacement medicine called levothyroxine (also known as L-thyroxine, thyroxine, and T4 — all of these terms refer to the same chemical). This is taken as tablets and it can take some time to get the dose right. Usually you will start on a low dose that's gradually built up every few months. Your GP will adjust your dose according to the results of your blood tests.

Once the correct dose of levothyroxine has been established, you will usually have a thyroid function test every year to check the levels of your thyroid hormones.

You will usually feel much better once you're taking levothyroxine. Side-effects are unusual because a missing hormone is simply being replaced. However, if you take too much, you may develop symptoms of an overactive thyroid, such as anxiety and weight loss. You should let your GP know if you notice such symptoms as your dose of levothyroxine may need to be adjusted.

If you have subclinical hypothyroidism (which means you don't have any symptoms but your thyroid hormone levels are disrupted) you may not need any treatment at first. Your GP will usually monitor how your thyroid hormone levels change every few months. If you develop symptoms, your GP will suggest that you start treatment.

Special considerations 

Your thyroid gland can change during pregnancy. In the first half of pregnancy it's normal for your total amount of thyroid hormone to be slightly higher than usual. If you have hypothyroidism you'll need more frequent checkups during and after pregnancy, as your thyroxine requirements tend to be higher at these times.

Further information 

The Australian Thyroid Foundation
http://www.thyroidfoundation.com.au/

Sources 

Clinical Knowledge Summaries. Hypothyroidism. [online] London: National Institutes for Health and Clinical Excellence. 2010 [last updated Feb 2011, accessed 4 Jul 2011] Available from:
http://www.cks.nhs.uk/hypothyroidism

Food Standards Australia New Zealand (FSANZ). Iodine in Food. [online] Canberra, ACT: FSANZ. c2010 [Accessed 30 June 2011] Available from:
http://www.foodstandards.gov.au/Pages/default.aspx

Gallego G Goodall S Eastman CJ. Iodine deficiency in Australia: is iodine supplementation for pregnant and lactating women warranted? Medical Journal of Australia. 2010; 192(8): 461-463.

Genetics Education in Medicine (GEM) Consortium. Genetics in Family Medicine: The Australian Handbook for General Practitioners – Newborn Screening. [online] Canberra, ACT: Commonwealth of Australia. 2007 [accessed 4 Jul 2011] Available from:
http://www.nhmrc.gov.au/_files_nhmrc/file/your_health/egenetics/practioners/gems/sections/04_newborn_screening.pdf (PDF, 914Kb)

Topliss DJ Eastman CJ. Diagnosis and management of hyperthyroidism and hypothyroidism. Medical Journal of Australia. 2004; 180(4): 186-193.

Last published: 31 July 2011

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