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:
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.
Thyroid-stimulating hormone stimulates the thyroid gland to produce metabolism-regulating hormones T3 and T4.
A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.
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.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
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:
Weakness and fatigue are common symptoms of hypothyroidism.
A goiter 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 (goiter) 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 goiter in the throat.
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.
The extensive network of cells and structures that is responsible for activating and coordinating the body's functions, sensory input and cognition.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
There can be many causes of hypothyroidism:
This is the most common cause of hypothyroidism in the US. Inflammation damages cells within the thyroid that produce thyroid hormone.
Inflammation can be caused by:
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 iodized 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 the US, 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:
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:
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 tumor, 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.
These include:
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.
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:
Girls are twice as likely as boys to be born with an underactive thyroid.
A condition in which abnormal proteins accumulate in organs and tissues of the body, including the tongue, intestines, heart, skin, liver, kidneys and brain.
A medical condition in which the body's immune system abnormally targets substances that are normally found within the body.
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.
A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.
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).
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.
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.
A metal element that is used in medications that treat psychiatric illnesses.
A treatment that uses ionizing radiation to kill or control growth of malignant cancer cells.
An inflammatory condition that can affect the organs of the body, particularly the lungs, skin, liver and eyes.
A rare condition in which excess collagen fibers are produced by the body, affecting the skin and internal organs.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
A growth caused by an abnormal and uncontrolled reproduction of cells.
A mental disorder in which the emotions alternate between extreme highs and lows.
Risk factors for developing hypothyroidism can include:
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.
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.
A metal element that is used in medications that treat psychiatric illnesses.
A treatment that uses ionizing radiation to kill or control growth of malignant cancer cells.
An inflammatory condition that can affect the organs of the body, particularly the lungs, skin, liver and eyes.
A rare condition in which excess collagen fibers are produced by the body, affecting the skin and internal organs.
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 sound waves. 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 tumors. Ultrasound guided biopsy may then be used to obtain a tissue sample of the tumor for classification.
If your doctor suspects that the pituitary gland is causing your hypothyroidism, magnetic resonance imaging (MRI) may be performed to view the pituitary gland.
A protein molecule produced by the immune system. Antibodies bind specifically to foreign substances to neutralize them or target them for destruction.
A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.
A type of imaging that uses a magnetic field and low-energy radio waves, instead of X-rays, to obtain images of organs.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
A growth caused by an abnormal and uncontrolled reproduction of cells.
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:
For medication to be effective, it must be taken regularly, as directed by your doctor.
A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
Myxedema 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:
Myxedema coma can be triggered in people with untreated hypothyroidism by different infections and illnesses, cold exposure and some medications.
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.
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:
A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
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.
A chemical substance secreted in one part of an organism and transported to another part of that organism, where it has a specific effect.
A large gland located in the lower front part of the neck that produces hormones that regulate metabolism, growth and development, especially during childhood.
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.
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.