The parathyroid glands are four small glands found at the back of the thyroid gland in the neck. The…
What is hypothyroidism?
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.
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;
- 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 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.
There can be many causes of hypothyroidism:
Inflammation of the thyroid gland (thyroiditis)
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).
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.
As well as the medications mentioned above, some drugs can cause damage to the thyroid gland and lead to an underactive thyroid.
- Lithium (which can be used to treat bipolar disorder), and;
- Some chemotherapy drugs (such as thalidomide and sunitinib).
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.
Risk factors for developing hypothyroidism can include:
- Being female (women are significantly more likely to develop most forms of hypothyroidism);
- Being older, particularly over 60 years of age;
- Having other autoimmune conditions such as pernicious anaemia, vitiligo and Addison's disease;
- Having a family history of hypothyroidism or other autoimmune conditions;
- Taking in too much or too little iodine;
- Viral infections including hepatitis C and common colds;
- Surgery or treatment for hyperthyroidism;
- Surgery on the pituitary gland;
- Radiotherapy to the neck or brain;
- Having an infiltrative condition such as scleroderma or sarcoidosis, and;
- Taking some medications such as lithium and thalidomide.
Methods for diagnosis
The doctor will discuss symptoms and look for physical signs of hypothyroidism, including examining your neck for signs of thyroid gland enlargement.
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.
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
Types of treatment
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.
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.
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:
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.
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.