Hypothyroidism, also known as underactive thyroid, is a condition where the thyroid gland does not enough thyroid hormone. Thyroid hormone regulates the way in which the body uses energy - metabolism - and without enough of this hormone 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 and travel to the various organs and tissues of the body 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 thyroid hormone.
Contents of this article
What is hypothyroidism?
What causes hypothyroidism?
Signs and symptoms
Tests and diagnosis
Treatment and prevention
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 s two thyroid hormones, triiodothyronine (T3) and thyroxine (T4) that regulate metabolism. They also affect the following:1
Woman holding glowing red throat
If the thyroid gland does not produce enough thyroid hormones, hypothyroidism will occur.
Brain development
Breathing
Heart and nervous system functions
Body temperature
Muscle strength
Skin dryness
Menstrual cycles
Weight
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
Hashimoto's disease
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.
Thyroiditis14
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, as a result of an autoimmune condition and pregnancy.
Congenital hypothyroidism1
Babies can be born with a thyroid that does not function properly. Congenital hypothyroidism can lead to 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 destroys the cells of the thyroid gland. 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.
Medication3
A number of drugs can interfere with thyroid hormone production. These include amiodarone, interferon alpha, interleukin-2, lithium, 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 pituitary gland's functioning and have a knock-on effect with 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 imbalance7
Iodine is needed for the production of the thyroid hormone. If the body has too much or not enough iodine then this can lead to hypothyroidism, or other complications such as hyperthyroidism and goiter.
Recent developments on hypothyroidism causes from MNT news
'Epidemic of diagnosis' associated with thyroid cancer
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 in pregnant women may harm babies' IQs
Iodine deficiency during pregnancy may have a negative effect on babies' mental development, according to research published in The Lancet.
Signs and symptoms
As the thyroid hormones affect a great deal of the body, the symptoms of hypothyroidism are varied, and it can affect individuals very differently from each other.
Measuring overweight belly
Weight gain is one of the possible symptoms of hypothyroidism, caused by the slowing of the metabolism.
Symptoms include:135
Fatigue
Weight gain
Cold intolerance
Joint and muscle pain
Constipation
Dry skin
Thin, brittle hair or fingernails
Decreased sweating
Heavy periods (menorrhagia)
Slowed heart rate
Weakness
High cholesterol
Puffy face, feet and hands.
If left untreated, the following symptoms can manifest:58
Decreased taste and smell
Hoarseness
Slow speech
Thickening of skin
Peripheral neuropathy
Mental health issues: including depression and memory impairment
Infertility
Birth defects
Myxedema: a rare, life-threatening condition characterized by intense cold intolerance and profound lethargy.
Hypothyroidism is a condition that develops slowly and so the appearance of these symptoms may not be obvious and difficult to spot at first.
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 surgery. Receiving radiation to the neck and chest also increases the likelihood.
Having other conditions such as Turner syndrome, or another autoimmune disease such as lupus or rheumatoid arthritis increases the chances of developing a thyroid disorder.
Other groups who are vulnerable are those 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
The body requires more thyroid hormones during pregnancy, with one study finding that 85% of pregnant patients required a median increase of 47%. This makes pregnant women more susceptible to the condition.3
Pregnant tummy
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 followed by a blood test. A medical history will probably be taken as well.
The most common blood test used is the TSH test. It detects the amounts of TSH in the blood: usually if the TSH reading is above normal then the patient has hypothyroidism, and if it is below, then the patient has hyperthyroidism.
Additional blood tests used to confirm the diagnosis or determine the cause of hypothyroidism are the T4 test and the thyroid autoantibody test.
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
Synthetic thyroxine15
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 nutrition15Iodine in water
Iodine is an essential mineral for the production of thyroid hormone.
Iodine is an essential mineral for the thyroid. People with autoimmune thyroid disease can be sensitive to the harmful side effects of iodine, however, and it can cause or worsen hypothyroidism as a result.
Women need more iodine when pregnant as the baby receives iodine from the mother's diet. It can easily be acquired through supplements such as iodized salt and prenatal vitamins.
Diet can affect the way in which the body absorbs thyroid medication. People with hypothyroidism should discuss with their doctor any changes in diet such as starting a high fiber diet or eating lots of soy. The doctor should also be consulted if a person is sensitive to the side effects of iodine.
If the health care provider's instructions are followed then hypothyroidism can usually be completely controlled. The patient's hormone levels should return to normal. In most cases, this medicine will need to be taken for the rest of the patient's life.
