Hypothyroidism, also known as underactive thyroid, is a condition where the thyroid gland does not create enough of a thyroid hormone called thyroxine. Thyroid hormones regulate the way in which the body uses energy - metabolism - and without enough thyroxine many of the body's functions slow down.
About 4.6% of the US population aged 12 and above has hypothyroidism.1
The thyroid gland is found in the front of the neck below the larynx (voice box) and has two lobes, one on each side of the windpipe.
The thyroid is one of the endocrine glands, special groups of cells that make hormones. Hormones are chemical messengers that provide instruction to the organs and tissues of the body in order to control processes such as metabolism, growth and mood.2
It is important to note the difference between hypothyroidism and hyperthyroidism - a condition where the thyroid produces too much thyroxine.
Contents of this article:
You will also see introductions at the end of some sections to any recent developments that have been covered by MNT's news stories. Also, look out for links to information about related conditions.
Fast facts on hypothyroidism
Here are some key points about hypothyroidism. More detail and supporting information is in the main article.
- The thyroid gland produces two thyroid hormones.
- These hormones regulate the body's metabolism.
- The thyroid gland is stimulated by the hypothalamus and the pituitary gland.
- Hypothyroiditis can occur if the thyroid gland does not function correctly, or if it is not stimulated properly.
- The most common cause of hypothyroidism in the US is Hashimoto's disease.
- Symptoms of hypothyroidism include fatigue, cold intolerance and joint and muscle pain.
- Pregnant women are more susceptible to hypothyroiditis as the body needs more thyroid hormones during pregnancy.
- Hypothyroiditis is usually diagnosed by physical examination followed by a blood test.
- Hypothyroiditis can be treated with a medicine called synthetic thyroxine.
- Hypothyroiditis can be controlled well with lifelong medication.
What is hypothyroidism?
Hypothyroidism is a condition that occurs when the thyroid gland does not produce enough thyroid hormones to meet the needs of the body. The thyroid creates two thyroid hormones, triiodothyronine (T3) and thyroxine (T4) that regulate metabolism. They also affect the following:1
If the thyroid gland does not produce enough thyroid hormones, hypothyroidism will occur.
- Brain development
- Heart and nervous system functions
- Body temperature
- Muscle strength
- Skin dryness
- Menstrual cycles
- Cholesterol levels.
The production of thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is made by the pituitary gland and in turn regulated by the hypothalamus. TSH ensures that enough thyroid hormones are made to meet the needs of the body.
What causes hypothyroidism?
Hypothyroidism can occur due to the thyroid gland failing to work properly, or due to thyroid gland not being stimulated properly by the hypothalamus or pituitary gland.3
The most common cause of hypothyroidism in the US is Hashimoto's disease,1 also known as chronic lymphocytic thyroiditis or autoimmune thyroiditis.4
It is an autoimmune disease - a disorder where the body's immune system attacks the body's own cells and organs. Hashimoto's disease causes the immune system to attack the thyroid gland, leading to inflammation and interference in its ability to produce thyroid hormones.
Thyroiditis is the inflammation of the thyroid gland. It causes thyroid hormone to leak out into the blood, raising their overall levels and leading to hyperthyroidism. After 1-2 months, this usually develops into hypothyroiditis.
Thyroiditis can be caused by viral or bacterial infection, and can result from an autoimmune condition and pregnancy.
In cases of congenital hypothyroidism, babies are born with a thyroid gland that does not function properly. Congenital hypothyroidism can lead to problems with physical and mental growth problems, but early treatment can prevent these complications. Most newborns in the US are screened for hypothyroidism.
Thyroid surgery and treatment1
Several conditions can be treated by partially or fully removing the thyroid gland, this can lead to hypothyroidism. Hyperthyroidism, goiters, thyroid nodules, and thyroid cancer can all be treated in this way.
Radiation treatment of the thyroid can lead to hypothyroidism. Radioactive iodine is a common treatment for hyperthyroidism and works by destroying the cells of the thyroid gland, thereby decreasing the production of thyroxine. Radiation is also used to treat people with head and neck cancers, Hodgkin's disease and other lymphomas, and this can damage the thyroid gland.
A number of drugs can interfere with thyroid hormone production. These include amiodarone, interferon alpha, interleukin-2, lithium and tyrosine kinase inhibitors.
Pituitary gland abnormalities
If the pituitary gland stops functioning properly, the thyroid gland may not produce the correct amount of thyroid hormone. Pituitary tumors or pituitary surgery can affect the function of the pituitary gland, which may then adversely affect the thyroid gland.
Sheehan's syndrome is a condition that causes damage to the pituitary gland.5 If a woman loses a life-threatening amount of blood or has severe low blood pressure after childbirth then the gland can be damaged, leading it to under-produce pituitary hormones.6
Iodine is needed for the production of the thyroid hormones. If the body has too much or too little iodine then this can lead to hypothyroidism, or other complications such as hyperthyroidism and goiter.
Recent developments on hypothyroidism causes from MNT news
According to a new study published in JAMA Otolaryngology-Head & Neck Surgery, increased incidence of thyroid cancer may not be because of an increase in the disease, but an increase in diagnosis.
Iodine deficiency during pregnancy may have a negative effect on babies' mental development, according to research published in The Lancet.
Signs and symptoms
Thyroid hormones affect numerous bodily systems, meaning that the symptoms of hypothyroidism are wide-ranging and diverse, affecting different people in different ways.
Weight gain is one of the possible symptoms of hypothyroidism, caused by the slowing of the metabolism.
Symptoms of hypothyroidism include:135
- Weight gain
- Cold intolerance
- Joint and muscle pain
- Dry skin
- Thin, brittle hair or fingernails
- Decreased sweating
- Heavy periods (menorrhagia)
- Slowed heart rate
- High cholesterol
- Puffy face, feet and hands.
If left untreated, the following symptoms can manifest:58
- Decreased taste and smell
- Slow speech
- Thickening of skin
- Peripheral neuropathy
- Mental health issues: including depression and memory impairment
- Birth defects
- Myxedema: a rare, life-threatening condition characterized by intense cold intolerance and profound lethargy.
Hypothyroidism is a condition that develops slowly, meaning that symptoms may go unnoticed for a long period of time and can be difficult to spot.
Certain groups of people are more likely to develop hypothyroidism than others.1
People have an increased likelihood of developing the condition if they have had previous thyroid problems or treatment on their thyroids, such as a goiter or have undergone treatment that affects the thyroid, such as surgery. Radiation treatment of the neck and chest can also increase the likelihood of thyroid dysfunction.
There is also an increased propensity for hypothyroidism in people with a family history of thyroid disease, people aged over 60 and people who have been pregnant within the past 6 months.
During and after pregnancy
Increased demands on metabolism during pregnancy also means that the demands on the thyroid increase. One study found that 85% of those who are pregnant required a median increase in thyroid hormones of 47% during pregnancy, which is thought to explain why pregnant women are more susceptible to hypothyroidism.3
Pregnancy increases the risk of hypothyroidism.
Hypothyroidism is usually caused by Hashimoto's disease during pregnancy, affecting 3-5 out of every 1,000 pregnancies.1
If uncontrolled, hypothyroidism increases the risk of miscarriage, preterm delivery and a rise in blood pressure during late pregnancy (preeclampsia). It can also affect the brain development and growth of the baby.
A form of thyroiditis called postpartum thyroiditis affects 4-9% of women in the first year after they have given birth, this causes hyperthyroidism that commonly lasts for 1-2 months.
Tests and diagnosis135
Health care providers usually diagnose hypothyroidism by a thorough physical examination, medical history and through blood tests.
The most common blood test used is the TSH test. It detects the amounts of thyroid stimulating hormone (TSH) in the blood. If the TSH reading is above normal then the patient is usually considered to have hypothyroidism; if TSH levels are below normal then the patient is considered to have hyperthyroidism.
Additional blood tests used to confirm the diagnosis or determine the cause of hypothyroidism are the T4 test and the thyroid autoantibody test. It is often helpful for a doctor to run a complete thyroid panel, testing levels of T3 and T4, TSH and thyroid autoantibodies in order to fully establish the health and activity of the thyroid gland. This can help reduce the likelihood that a single normal reading for, say, thyroxine, masks an underlying issue with TSH or another key element of thyroid function.
For all of the aforementioned tests a blood sample is taken and sent to a laboratory for analysis.9
As many of the symptoms of hypothyroidism are the same as other conditions, the doctor will not be able to make a diagnosis based solely on symptoms.
The health care provider may also perform tests to check cholesterol levels, liver enzymes, prolactin, sodium or perform a full blood count.
Treatment and prevention
Courses of treatment for hypothyroidism aim to give the body the amount of thyroid hormone that it is lacking. This is achieved through synthetic thyroxine, a medication that is identical to the T4 hormone.
Dosage is determined by the patient's history, symptoms and current TSH level. Health care providers will regularly monitor the patient's blood to ensure that their dosage does not need to be adjusted. Monitoring will remain regular but gradually become less frequent over time.
Iodine and nutrition15
Iodine is an essential mineral for the production of thyroid hormone.
Iodine is an essential mineral for thyroid function, but people with autoimmune thyroid disease can be particularly sensitive to the effects of iodine, meaning that it can trigger or worsen hypothyroidism.
Iodine requirements increase in pregnancy in order to supply the growing fetus with sufficient nutrition. Those who are pregnant can ensure adequate iodine intake by including iodized salt in their diet and by taking prenatal vitamins.
Diet can affect the way in which the body absorbs thyroid medication. People with hypothyroidism should discuss any major dietary changes with their doctor, especially if such changes involve starting a high fiber diet or eating lots of soy or cruciferous vegetables, or if they are sensitive to the effects of iodine.
Hypothyroidism is usually able to be managed by following the advice of a qualified health care practitioner. With appropriate treatment, thyroid hormone levels should return to normal. In most cases, medications for hypothyroidism will need to be taken for the rest of the patient's life.