What is hypothyroidism?

Hypothyroidism is a condition resulting from an underactive thyroid gland, which is associated with an undersupply of hormones that control metabolism in the body.

The thyroid gland plays a key role in the endocrine system, which produces the hormones that control bodily functions such as metabolism. If the hormone levels are not within a normal range, this can affect the whole body, causing a wide range of symptoms and health problems.

The thyroid is located at the front of the throat, just above the hollow between the upper ends of the collarbones and below the Adam's apple (the larynx). It has two lobes that sit on either side of the windpipe. It is controlled by the pituitary gland, which sits at the base of the brain and secretes thyroid-stimulating hormone (TSH).

The thyroid produces two hormones:

  • Tri-iodothyronine (T3), and;
  • Thyroxine (T4).

The thyroid hormones circulate throughout the bloodstream, regulating metabolism. The lower the levels of T3 and T4 are, the slower metabolism will be.

Low levels of T3 and T4 in the bloodstream are detected by the pituitary gland, which then secretes more TSH to try to stimulate the thyroid gland to produce more thyroid hormones.

Cross-section of the head and throat showing the brain, the pituitary gland, the thyroid gland and the production of thyroid-stimulating hormone and thyroid hormones.Thyroid-stimulating hormone stimulates the thyroid gland to produce metabolism-regulating hormones T3 and T4. 

Hormones

A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.

Metabolism

The sum of all chemical changes that take place within an organism to maintain growth and development and convert food into energy and building blocks.

Pituitary gland

A small gland located at the base of the brain which produces many hormones important for the healthy function of the body.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Signs and symptoms

Hypothyroidism can affect the whole body and the symptoms can vary greatly between people. Symptoms may develop suddenly, or very slowly over years. They may be so slight that they are difficult to notice, or so severe they are life-threatening. In older people, symptoms can be difficult to tell apart from the normal changes that come with ageing.

Because hypothyroidism causes a slowing of metabolism, it is associated with general weakness, with the systems and muscles of the body (such as the heart, lungs, digestive system and nervous system) not working as well as they should.

Symptoms can include:

  • Lack of energy and feelings of weakness and fatigue;
  • A decreased tolerance to the cold;
  • Weight gain;
  • Constipation;
  • Puffy face, eyes, hands and feet;
  • Skin and hair changes, including dry skin and hair, thinning eyebrows and brittle nails;
  • Joint and muscle pains;
  • Carpal tunnel syndrome and other nerve compression disorders, which can cause pins and needles, pain and possibly weakness in the hands;
  • Breathlessness and fatigue with exercise;
  • An enlarged thyroid gland (goitre);
  • Swelling of the tongue and a hoarse or husky voice;
  • Difficulty with memory and concentration;
  • Depression, and;
  • Irregularities with the menstrual cycle.

A woman slumping with fatigue.Weakness and fatigue are common symptoms of hypothyroidism. 

Goitre

A goitre is an enlargement of the thyroid gland and can appear as a lump at the front of the throat.

While enlargement of the thyroid gland (goitre) can occur in hypothyroidism, it can also occur for other reasons, including an overactive thyroid (hyperthyroidism), thyroid cancer, benign thyroid nodules and sometimes even when your thyroid hormones are at normal levels.

A person with a goitre.A goitre in the throat. 

Digestive system

The series of organs within the body that contribute to the digestion of food. It begins at the mouth and ends at the anus, and includes the stomach, small and large intestines as well as the pancreas, gallbladder and liver.

Menstrual cycle

A monthly cycle of changes that a woman's body goes through to prepare for a potential pregnancy.

Metabolism

The sum of all chemical changes that take place within an organism to maintain growth and development and convert food into energy and building blocks.

Nervous system

The extensive network of cells and structures that is responsible for activating and coordinating the body's functions, sensory input and cognition.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Causes

There can be many causes of hypothyroidism:

Inflammation of the thyroid gland (thyroiditis)

This is the most common cause of hypothyroidism in Australia. Inflammation damages cells within the thyroid that produce thryoid hormone.

Inflammation can be caused by:

  • An autoimmune disorder in which the immune system attacks the thyroid (Hashimoto thyroiditis);
  • Viral infections including the common cold and hepatitis C, and;
  • Pregnancy (sometimes called postpartum thyroiditis as symptoms tend to occur in the months after birth).

Iodine

The nutrient iodine is very important in the production of the thyroid hormones T3 and T4. It is found in a range of foods including dairy products, seafood, bread and iodised salt.

The recommended daily intake of iodine for adults who are not pregnant is 150mcg. Taking in too little and occasionally too much iodine can cause hypothyroidism. In Australia, most people receive sufficient iodine through their diets. During pregnancy, women require more iodine and are often advised to take a supplement by their doctor.

The most common causes of taking in excess iodine include:

  • Dyes containing iodine used for medical imaging tests;
  • Overconsumption of foods with high iodine content, such as kelp and nori seaweed, and;
  • Medications high in iodine such as amiodarone (commonly used to treat heart irregularities).

Treatments for hyperthyroidism

Treatments for an overactive thyroid (hyperthyroidism) can lead to hypothyroidism. This can occur at the time of treatment, or develop years afterwards.

These hyperthyroidism treatments include:

  • Medications including carbimazole and propylthiouracil; 
  • Surgical removal of part or all of the thyroid gland (thyroidectomy), and;
  • Radioactive iodine treatment (which intentionally destroys tissue in the thyroid).

Pituitary gland and hypothalamus dysfunction

TSH is produced by the pituitary gland, which in turn stimulates the thyroid to produce hormones. The hypothalamus (part of the brain just above the pituitary gland) influences the function of the pituitary gland.

If either the pituitary gland or hypothalamus do not work properly, the pituitary gland may not produce enough TSH and an underactive thyroid can result. This is sometimes called secondary hypothyroidism and can occur because of a tumour, surgery or radiotherapy.

Sheehan syndrome can also lead to hypothyroidism. It is a very rare condition that can occur when a woman bleeds severely during childbirth, causing damage to the pituitary gland.

Medications

As well as the medications mentioned above, some drugs can cause damage to the thyroid gland and lead to an underactive thyroid.

These include:

Infiltrative diseases

Some diseases (such as scleroderma, sarcoidosis and amyloid disease) cause a growth or accumulation of tissues and cells that don't usually belong in the thyroid. This can lead to damage to the hormone-producing cells.

Congenital and neonatal hypothyroidism

Babies can be born with an underactive thyroid (called congenital hypothyroidism), or develop it soon after birth (neonatal hypothyroidism).

The causes of hypothyroidism in babies include:

  • A poorly developed or missing thyroid gland;
  • The thyroid gland producing hormones that do not function properly, and;
  • The pituitary gland not stimulating the thyroid gland to produce thyroid hormones.

Girls are twice as likely as boys to be born with an underactive thyroid.

Amyloid disease

A condition in which abnormal proteins accumulate in organs and tissues of the body, including the tongue, intestines, heart, skin, liver, kidneys and brain.

Autoimmune disorder

A medical condition in which the body's immune system abnormally targets substances that are normally found within the body.

Chemotherapy

A medication-based treatment, usually used in the treatment of cancers. There are numerous, different types of chemotherapy drugs that can be prescribed by a specialist. These can commonly be used alongside other cancer treatments such as surgery and radiotherapy.

Hormone

A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.

Hypothalamus

A small part of the brain that carries out many functions, in particular the communication between the nervous system and the endocrine system (via the pituitary gland).

Inflammation

A body’s protective immune response to injury or infection. The accumulation of fluid, cells and proteins at the site of an infection or physical injury, resulting in swelling, heat, redness, pain and loss of function.

Iodine

A chemical element important for hormone development that is found in foods including dairy products, seafood, eggs, bread and some vegetables. It is often used in medicine as a dye.

Lithium

A metal element that is used in medications that treat psychiatric illnesses.

Pituitary gland

A small gland located at the base of the brain which produces many hormones important for the healthy function of the body.

Radiotherapy

A treatment that uses ionising radiation to kill or control growth of malignant cancer cells.

Sarcoidosis

An inflammatory condition that can affect the organs of the body, particularly the lungs, skin, liver and eyes.

Scleroderma

A rare condition in which excess collagen fibres are produced by the body, affecting the skin and internal organs.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Tumour

A growth caused by an abnormal and uncontrolled reproduction of cells.

Bipolar disorder

A mental disorder in which the emotions alternate between extreme highs and lows.

Risk factors

Risk factors for developing hypothyroidism can include:

Addison's disease

A chronic condition in which the adrenal glands are damaged, leading to a reduction of the hormones cortisol and/or aldosterone. These hormones are important for daily bodily functions and their deficiency can lead to a wide variety of symptoms.

Autoimmune conditions

A medical condition in which the body's immune system abnormally targets substances that are normally found within the body.

Iodine

A chemical element important for hormone development that is found in foods including dairy products, seafood, eggs, bread and some vegetables. It is often used in medicine as a dye.

Lithium

A metal element that is used in medications that treat psychiatric illnesses.

Pituitary gland

A small gland located at the base of the brain which produces many hormones important for the healthy function of the body.

Radiotherapy

A treatment that uses ionising radiation to kill or control growth of malignant cancer cells.

Sarcoidosis

An inflammatory condition that can affect the organs of the body, particularly the lungs, skin, liver and eyes.

Scleroderma

A rare condition in which excess collagen fibres are produced by the body, affecting the skin and internal organs.

Methods for diagnosis

Physical examination

The doctor will discuss symptoms and look for physical signs of hypothyroidism, including examining your neck for signs of thyroid gland enlargement.

Blood tests

A blood test can detect a range of hormone levels, including TSH, T3 and T4. Blood tests may be repeated over a period of weeks or months to confirm that the hormone levels are consistently low.

A blood test can also detect thyroid antibodies that can cause thyroiditis.

Thyroid ultrasound

An ultrasound produces a picture of the thyroid using soundwaves. This enables identification of any changes to or enlargement of the thyroid gland. Further investigations of any observed changes may be recommended to diagnose other conditions such as tumours. Ultrasound guided biopsy may then be used to obtain a tissue sample of the tumour for classification. 

Magnetic resonance imaging

If your doctor suspects that the pituitary gland is causing your hypothyroidism, magnetic resonance imaging (MRI) may be performed to view the pituitary gland.

Antibodies

A protein molecule produced by the immune system. Antibodies bind specifically to foreign substances to neutralise them or target them for destruction.

Biopsy

The removal of a tissue sample for microscopic laboratory examination. It is used to determine the presence, cause and type of the disease.

Hormone

A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.

Magnetic resonance imaging

A type of imaging that uses a magnetic field and low-energy radio waves, instead of X-rays, to obtain images of organs.

Pituitary gland

A small gland located at the base of the brain which produces many hormones important for the healthy function of the body.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Tumours

A growth caused by an abnormal and uncontrolled reproduction of cells.

Types of treatment

Hypothyroidism is treated by medications that compensate for the removed or underactive thyroid hormone. The most commonly-used medication is thyroxine, which is usually taken orally daily.

In most cases, once you start treatment for hypothyroidism, you will need to continue taking the medication for the rest of your life and have your thyroid hormone levels measured regularly.

Because other foods and medications can interfere with how thyroid replacement medications are absorbed into the body, you may be asked by your doctor to:

  • Take the medication at a particular time of day, when your stomach is empty, and;
  • Avoid taking other drugs, multivitamins and supplements and even coffee for a period of time after taking your medication.

For medication to be effective, it must be taken regularly, as directed by your doctor.

Hormone

A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Potential complications

Myxoedema coma

Myxoedema coma is a rare but severe form of hypothyroidism that occurs as a result of very low levels of thyroid hormones. This is a life-threatening condition that requires urgent medical treatment.

Symptoms can include:

  • Low body temperature;
  • Confusion and unpredictable or unusual changes of mood, and;
  • Sleepiness, unresponsiveness or loss of consciousness (coma).

Myxoedema coma can be triggered in people with untreated hypothyroidism by different infections and illnesses, cold exposure and some medications.

Hypothyroidism and pregnancy

Hypothyroidism affects the reproductive system and can cause problems with fertility. Women with untreated hypothyroidism have an increased risk of miscarriage. Pregnant women who are already being treated for the condition require higher levels of thyroid replacement medications during pregnancy.

Cretinism

If hypothyroidism in babies is not treated, it can affect their growth and development and lead to intellectual disability. Early diagnosis and treatment within the first month of life with thyroid hormone replacement medication can help to prevent developmental problems.

Complications of thyroxine

Side effects are not common when the correct dose of thyroid medication is used. Symptoms that can occur with an excessive dose of medication can include: 

  • Tremors;
  • Nervousness;
  • Increased hunger;
  • Increased heart rate or palpitations;
  • Diarrhoea, and;
  • Feelings of tiredness and fatigue.

Hormones

A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Prognosis

For most people, treatment with thyroid replacement medications restores thyroid hormone levels to normal and relieves symptoms. Most people with hypothyroidism will need to continue with medications for the rest of their lives.

Hormone

A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.

Thyroid

A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.

Prevention

With the exception of hypothyroidism that is triggered by ingesting too low or too high quantities of iodine, it is not possible to prevent the condition. 

Iodine

A chemical element important for hormone development that is found in foods including dairy products, seafood, eggs, bread and some vegetables. It is often used in medicine as a dye.