Thyroid hormones regulate metabolism, or the way the body uses energy. If thyroxine levels are low, many of the body's functions slow down.
About 4.6 percent of the population aged 12 years and above in the United States (U.S.) has hypothyroidism.
The thyroid gland is found in the front of the neck below the larynx, or voice box, and has two lobes, one on each side of the windpipe.
It 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, TS3 and TS4.
- 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?
Hypothyroidism refers to the underproduction of hormones in the thyroid gland, and has a wide range of symptoms.
Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormones to meet the needs of the body.
The opposite is hyperthyroidism, where the thyroid produces too much thyroid hormone.
However, the link between hyperthyroidism and hypothyroidism is complex, and one can lead to the other, in certain circumstances.
The thyroid creates two thyroid hormones, triiodothyronine (T3) and thyroxine (T4).
These regulate metabolism, and they also affect:
- 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.
This, in turn, is regulated by the hypothalamus, a region of the brain. TSH ensures that enough thyroid hormones are made to meet the needs of the body.
Causes and risk factors
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 in which the immune system attacks the body's own cells and organs.
Thyroiditis is an inflammation of the thyroid gland. It causes thyroid hormones to leak into the blood, raising their overall levels and leading to hyperthyroidism. After 1 to 2 months, this may develop into hypothyroidism.
Thyroiditis can be caused by viral or bacterial infection, an autoimmune condition or following pregnancy.
In cases of congenital hypothyroidism, the thyroid gland does not function properly from birth.
This 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 as causes of hypothyroidism
Thyroid treatment and surgery can lead to hypothyroidism.
Hypothyroidism can occur after part of the thyroid is removed during surgery.
Radiation treatment of the thyroid can also lead to hypothyroidism. Radioactive iodine is a common treatment for hyperthyroidism. It works by destroying the cells of the thyroid gland and decreasing the production of T4.
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, and this can adversely affect the thyroid gland.
Sheehan's syndrome is a condition that involves damage to the pituitary gland.
If a woman loses a life-threatening amount of blood or has severely low blood pressure during or after childbirth, the gland can be damaged, causing it to under-produce pituitary hormones.
Iodine is needed for the production of the thyroid hormones, but the level must be balanced. Too much or too little iodine can lead to hypothyroidism or hyperthyroidism.
Other risk factors
The risk of hypothyroidism is higher in people with a family history of thyroid disease and those aged over 60 years.
It most commonly affects women from middle age onward, but it can occur at any age.
During and after pregnancy
Pregnancy can be a cause of hypothyroidism.
Increased demands on metabolism during pregnancy results in increased demands on the thyroid.
In one study, 85 percent of women who are pregnant and taking thyroid hormone replacement needed an additional intake of 47 percent on average, during pregnancy.
If hypothyroidism occurs during pregnancy, it is usually due to Hashimoto's disease. This condition affects between 3 and 5 out of every 1,000 women during pregnancy.
Uncontrolled hypothyroidism increases the risk of miscarriage, preterm delivery, and a rise in blood pressure during late pregnancy, or preeclampsia.
It can also affect brain development and growth rates.
Women who have been pregnant within the last 6 months have a higher risk of thyroiditis and hypothyroidism.
Signs and symptoms
Thyroid hormones affect multiple organ systems, so the symptoms of hypothyroidism are wide-ranging and diverse.
They commonly include, but are not limited to:
- weight gain
- cold intolerance
- slowed heart rate, movements, and speech
- joint and muscle pain, cramps, and weakness
- dry skin
- thin, brittle hair or fingernails
- decreased sweating
- pins and needles
- heavy periods, or menorrhagia
- high cholesterol
- puffy face, feet, and hands
- balance and co-ordination issues
- loss of libido
- recurrent urinary and respiratory tract infections
If left untreated, the following symptoms can manifest:
- puffiness in the face
- thinned or missing eyebrows
- slow heart rate
- hearing loss
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 develops slowly. Symptoms may go unnoticed for a long time, and they may be vague and general.
Symptoms vary a great deal between individuals, and they are shared by other conditions. The only way to obtain a concrete diagnosis is through a blood test.
Tests and diagnosis
Doctors usually carry out a physical examination, take a medical history, and sent to a laboratory for analysis.
The most common blood test is the TSH test. This detects the amounts of 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 or hypothyroidism.
The T4 test and the thyroid autoantibody test are additional blood tests used to confirm the diagnosis or determine its cause.
The doctor may 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.
There may also be tests to check cholesterol levels, liver enzymes, prolactin, and sodium.
Treatment and prevention
Treatment for hypothyroidism focuses on supplementing the thyroid hormone.
To replenish levels, doctors usually prescribe 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 T4 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 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.
They should inform their doctor if they are sensitive to the effects of iodine.
People with hypothyroidism should discuss any major dietary changes with their doctor, especially when starting a high fiber diet, or eating lots of soy or cruciferous vegetables.
Diet can affect the way in which the body absorbs thyroid medication.
During pregnancy, iodine requirements increase. Using iodized salt in the diet and taking prenatal vitamins can maintain the required levels of iodine.
Hypothyroidism can normally 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.