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 percent of the United States population aged 12 and above has hypothyroidism.
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 an endocrine gland, made up of special cells that make hormones. Hormones are chemical messengers that relay information to the organs and tissues of the body, controlling processes such as metabolism, growth, and mood.
Contents of this article:
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 most common cause of hypothyroidism in the U.S. is Hashimoto's disease
- Symptoms of hypothyroidism include fatigue, cold intolerance, and joint and muscle pain
What is hypothyroidism?
If the thyroid gland does not produce enough thyroid hormones, hypothyroidism will occur.
Hypothyroidism is a condition that occurs when the thyroid gland does not produce enough thyroid hormones to meet the needs of the body.
It is important to note the difference between hypothyroidism and hyperthyroidism; hyperthyroidism is a condition where the thyroid produces too much thyroid hormone, hypothyroidism is the reverse.
The thyroid creates two thyroid hormones, triiodothyronine (T3) and thyroxine (T4) that regulate metabolism. They also affect the following:
- 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, which in turn is regulated by the hypothalamus. TSH ensures that enough thyroid hormones are made to meet the needs of the body.
Causes of hypothyroidism
Hypothyroidism can occur if the thyroid gland fails to work properly, or if the thyroid gland is not stimulated properly by the hypothalamus or pituitary gland.
The most common cause of hypothyroidism in the U.S. is Hashimoto's disease, also known as chronic lymphocytic thyroiditis or autoimmune thyroiditis.
Hashimoto's disease is an autoimmune disease - a disorder where the immune system attacks the body's own cells and organs. The condition causes the immune system to attack the thyroid gland, leading to inflammation and interference in its ability to produce thyroid hormones.
Thyroiditis is an inflammation of the thyroid gland. It causes thyroid hormones to leak out into the blood, raising their overall levels and leading to hyperthyroidism. After 1-2 months, this may develop into hypothyroiditis. Thyroiditis can be caused by viral or bacterial infection, or it can result from an autoimmune condition or following pregnancy.
In cases of congenital hypothyroidism, babies are born with a thyroid gland 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 U.S. are screened for hypothyroidism.
Thyroid surgery and treatment
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, which can lead to damage of 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. If a woman loses a life-threatening amount of blood or has severely low blood pressure during/after childbirth, the gland can be damaged, causing it to under-produce pituitary hormones.
Iodine is needed for the production of the thyroid hormones. However, the level of iodine must be balanced. If the body has too much or too little iodine, hypothyroidism or hyperthyroidism can develop.
Signs and symptoms
Weight gain is one of the possible symptoms of hypothyroidism, caused by the slowing of the metabolism.
Thyroid hormones affect multiple organ systems, meaning that the symptoms of hypothyroidism are wide-ranging and diverse, affecting different people in different ways.
Symptoms of hypothyroidism include:
- 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:
- Decreased taste and smell
- Slow speech
- Thickening of skin
- Peripheral neuropathy
- Depression and memory impairment
- Birth defects
- Myxedema: a rare, life-threatening condition characterized by intense cold intolerance and profound lethargy
Hypothyroidism most commonly affects middle-aged and older women, but it can affect individuals of any age. If it develops in children or teenagers, the signs and symptoms are generally the same as adults; however, they may also experience:
- Poor growth
- Delayed development of teeth
- Poor mental development
- Delayed puberty
Hypothyroidism is a condition that develops slowly. Symptoms may go unnoticed for a long time and may be difficult to notice.
Certain groups of people are more likely to develop hypothyroidism than others.
People have an increased likelihood of developing the condition if they have had previous thyroid problems such as a goiter (enlargement of the thyroid gland) or thyroid treatment such as surgery.
Radiation treatment of the neck and chest can also increase the likelihood of thyroid dysfunction.
People have a greater risk of developing a thyroid disorder if they have conditions such as Turner syndrome or autoimmune diseases like lupus or rheumatoid arthritis.
There is also an increased risk of 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
Pregnancy increases the risk of hypothyroidism.
Increased demands on metabolism during pregnancy results in increased demands on the thyroid.
One study found that 85 percent of those who are pregnant required a median increase in thyroid hormones of 47 percent during pregnancy, which may explain why pregnant women are more susceptible to hypothyroidism.
Hypothyroidism is usually caused by Hashimoto's disease during pregnancy, affecting 3-5 out of every 1,000 pregnancies.
If uncontrolled, hypothyroidism increases the risk of miscarriage, preterm delivery, and a rise in blood pressure during late pregnancy (preeclampsia). It can also affect brain development and growth of the baby.
A form of thyroiditis called postpartum thyroiditis affects 4-9 percent of women in the first year after they have given birth, causing hyperthyroidism that commonly lasts for 1-2 months.
Tests and diagnosis
Doctors usually diagnose hypothyroidism by a thorough physical examination, medical history, and 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, the patient may have hypothyroidism; if TSH levels are below normal, the patient may 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 tests above, a blood sample is taken and sent to a laboratory for analysis.
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 doctor may also perform tests to check cholesterol levels, liver enzymes, prolactin, sodium, or perform a full blood count.
Treatment and prevention
Treatment for hypothyroidism focuses on replenishing the amount of thyroid hormone the body 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. Doctors will regularly monitor the patient's blood to determine if the dosage of synthetic thyroxine needs to be adjusted. Regular monitoring will be required, but the frequency of blood tests will likely decrease over time.
Iodine and nutrition
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. They should inform their doctor if they are sensitive to the effects of iodine.
Hypothyroidism can generally be managed appropriately by following the advice of a qualified healthcare 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.