Hypothyroidism

Summary

Also known as &underactive thyroid,& hypothyroidism is a disorder in which the thyroid gland does not produce enough thyroid hormone to meet the body's needs. A lack of thyroid hormone can cause a variety of symptoms affecting most of the body's major systems. These symptoms might include fatigue, weakness, lack of appetite and a low sex drive.

Hypothyroidism can be caused various factors. The most common cause in the United States is a disease called Hashimoto disease, in which the body's own immune system targets and destroys thyroid cells. It can also be caused by problems with the pituitary gland, which regulates function of the thyroid gland, as well as birth defects, surgical removal of the thyroid gland, irradiation of the neck or inflammation of the thyroid gland. Because the range of symptoms in hypothyroidism is so wide, it can be difficult for physicians to diagnose.

Generally, hypothyroidism is diagnosed with laboratory tests that measure the level of thyroid hormone and thyroid-stimulating hormone (TSH), which is responsible for causing the thyroid to increase production of thyroid hormone. In cases of hypothyroidism caused by problems with the thyroid gland itself, the TSH level will be elevated while the level of thyroid hormone is reduced. This reflects the body's attempts to stimulate the thyroid to secrete more hormone.

Hypothryoidism is also frequently classified into subclinical hypothyroidism and overt hypothyroidism. In cases of subclinical hypothryoidism, the level of TSH is elevated but the level of thyroid hormone may be normal. These patients may have only very slight symptoms or none at all. In overt hypothyroidism, TSH will be elevated and thyroid hormone levels are depressed. These patients are likely to have symptoms and signs consistent with hypothyroidism.

The majority of people with hypothyroidism will need to take manufactured versions of natural thyroid hormones for the rest of their lives to relieve their symptoms and to avoid serious long–term consequences. These consequences include a greater risk for hardening of the arteries (atherosclerosis), heart disease, heart attack, high blood pressure (hypertension), stroke, respiratory problems and anemia.

About hypothyroidism

Also known as &underactive thyroid,& hypothyroidism is a disorder in which the thyroid gland is not producing enough thyroid hormone to meet the body's needs. It is much more common than hyperthyroidism, in which too much thyroid hormone is produced.

The thyroid hormone is controlled by the pituitary gland, which secretes thyroid-stimulating hormone, or TSH. The pituitary, in turn, is controlled by the hypothalamus in the brain, which secretes thyrotroptin-releasing hormone, or TRH. Any problem along this hypothalamic-pituitary-thyroid access can result in hypothyroidism. Conditions that can affect this access include certain medications and diseases, radiation therapy, and surgeries. However, the vast majority of cases of hypothyroidism are due to problems with the thyroid gland itself. This is known as primary hypothyroidism.

In much of the world, primary hypothyroidism is closely linked to iodine consumption. Thyroid hormones depend on small amounts of iodine, an element present in seafood and agricultural products grown in iodine–rich soil. In developing countries or geographic areas with iodine–poor soil or otherwise limited access to foods containing sufficient amounts of iodine, the rate of hypothyroidism is 15 percent. In the 1920s, iodized salt was introduced in the U.S. to ensure adequate dietary intake of iodine.

In iodine–sufficient areas of the world, such as the United States, the most common cause of hypothyroidism is Hashimoto disease or &chronic autoimmune thyroiditis.& Hashimoto disease is an autoimmune disorder in which the body's own immune cells attack the thyroid gland. As a result, the gland cannot produce enough thyroid hormone to meet the body's demand. Women are five to eight times more likely to develop this condition than men, and older women are particularly at risk. About a quarter of the people who have Hashimoto disease also have mitral valve prolapse (a type of valvular regurgitation).

Another condition associated with hypothyroidism is a usually temporary disorder called silent thyroiditis. A type of silent thyroiditis called &postpartum thyroiditis& can affect new mothers within six months after they give birth. Silent thyroiditis can also affect men or women who have not had a recent pregnancy. The condition can cause either hypothyroidism or hyperthyroidism and it can be temporary.

Hypothyroidism may also be caused by a disorder called De Quervain thyroiditis, which involves a painful inflammation of the thyroid gland. Although this condition initially causes hyperthyroidism, some people are left with permanent hypothyroidism once it has resolved.

Other causes of hypothyroidism include:

  • Irradiation of the neck. The effect is dose–dependent and it may take several years for hypothyroidism to develop. Full–body radiation has also been shown to have negative effects on the thyroid gland.
  • Birth defects (congenital defects). Congenital hypothyroidism occurs in about 1 in 4,000 live births in the United States and is one of the most common preventable causes of mental retardation. Congenital hypothyroidism can be detected by newborn screening, which is mandatory in the United States. Newborns who are treated promptly will usually have normal development.
  • Certain medications and nutritional supplements. These include some antidepressants, one of the antiarrhythmics and selenium (one of the antioxidants).
  • Treatments for hyperthyroidism. Attempts to make an overactive thyroid produce less thyroid hormone through medication, radiation or surgery often result in the thyroid gland becoming unable to produce enough hormone for normal functioning.
  • A problem in the pituitary gland. This type of hypothyroidism is known as secondary hypothyroidism. The condition results from the pituitary gland not producing enough thyroid–stimulating hormone (TSH). This hormone encourages the thyroid gland to produce thyroid hormones. However, the thyroid gland itself may be healthy.

Risk factors for hypothyroidism include:

  • Gender (women are affected more than men)
  • Advanced age (over 50)
  • Smoking
  • Obesity (body mass index greater than 30)
  • Previous thyroid surgery or radiation treatments
  • A family history of hypothyroidism or other thyroid disorder(s)

Subclinical hypothyroidism is a term used to describe patients whose thyroid tests indicate elevations of TSH and normal or near-normal levels of thyroid hormone. People with subclinical hypothyroidism often have no apparent symptoms of hypothyroidism. It is estimated that 3 to 5 percent of the U.S. population has subclinical hypothyroidism, including 10 percent of women over 60, making this a significant healthcare concern. At this point, widespread screening for hypothyroidism in the absence of symptoms is somewhat controversial. The American Thyroid Association, however, has recommended measurement of TSH at age 35 and every 5 years afterward, particularly for women. Other groups, such as the United States Preventive Services Task Force, have argued against thyroid screening based on the cost/benefit analysis.

Patients who have been diagnosed with subclinical hypothyroidism need to be monitored by a healthcare professional because subclinical hypothyroidism has been associated with an increased risk of heart disease and heart attacks. Hypothyroidism can be particularly harmful during pregnancy because it can harm the developing fetus.

Heart–related effects of hypothyroidism

Hypothyroidism has a number of effects in the body that are potentially damaging to the cardiovascular system. The condition is characterized by a decrease in the available oxygen, a decrease in the ability of the heart to contract, a decrease in the mass of the heart muscle and an increase in vascular resistance throughout the body. This means the output of the heart is decreased at the same time that resistance is increased in arteries throughout the body. People with hypothyroidism may experience the following heart-related complications:

  • High LDL (&bad&) cholesterol levels, which is a known risk factor for heart disease, heart attack and stroke. It has been shown that women over 65 years of age with high LDL levels are more likely to have undiagnosed hypothyroidism than same–aged women with normal LDLs. Older women with high LDLs are therefore encouraged to have their thyroid levels checked.
  • High blood pressure (hypertension), which puts additional strain on the heart and blood vessels. Up to 40 percent of patients with hypothyroidism suffer from elevated blood pressure, despite the reduction in cardiac output.
  • Decreased amount of oxygen–rich blood that the heart can pump to the rest of the body and slowed heart rate.
  • Enlarged heart. This condition would occur in long-standing cases of untreated hypothyroidism. In this situation, the heart muscle may eventually expand (dilate) in response to the additional stress placed on it. While this is a common symptom of heart failure, heart failure caused solely by hypothyroidism is rare.
  • Pericardial effusion.
  • Elevated homocysteine. Higher homocysteine levels are associated with increased risk of heart attack.

Furthermore, a recent study of postmenopausal women found a link between hypothyroidism and &hardening of the arteries& (atherosclerosis), which is directly related to heart disease, heart attack and stroke.

Signs and symptoms of hypothyroidism

Hypothyroidism can be difficult to diagnose without blood tests because of the wide variability of symptoms. In past years, hypothryoidism was often diagnosed by the presence of a goiter, or enlarged thyroid. This condition is rare in the United States today. Instead, hypothyroidism is often diagnosed much earlier after blood tests confirm elevated TSH and abnormal thyroid levels. In general, the symptoms caused by hypothyroidism are associated with the gradual slowing of the metabolism, including:

  • Fatigue
  • Constant weakness
  • Lack of appetite
  • Low sex drive
  • Thinning hair
  • Weight gain
  • Memory loss
  • Depression
  • Feeling cold
  • Heavy menstrual flow; longer and/or more frequent periods; severe menstrual cramps
  • Thin, brittle fingernails
  • Constipation
  • Joint or muscle pain
  • Hypersensitivity of the eyes to light

Rarely, an extreme form of advanced hypothyroidism may be marked by a condition called myxedema (a red, puffy face; dry, rough skin; and loss of hair). Other symptoms associated with severe or long–standing hypothyroidism include slow heart rate (bradycardia), hoarse throat and low body temperature. In rare cases, the condition can worsen into a myxedema coma in which the person's blood pressure, blood sugar, breathing rate and body temperature are low enough to be life–threatening.

Untreated, hypothyroidism could eventually lead to the development of diabetes and/or anemia.

Diagnosis methods for hypothyroidism

Thyroid hormone screening is widespread and easily available. If thyroid disease is suspected, the diagnosis usually includes the following:

  • Medical history. The physician will ask about any symptoms the patient may have been experiencing, such as fatigue or weakness. Because thyroid disease tends to run in families, knowledge of the medical background of close family members is also important in making a diagnosis.

  • Physical examination. Physicians will feel (palpate) the thyroid gland, located in the neck, to look for signs that it is enlarged (a goiter) or has abnormal nodules or growths on it. Physicians may ask the patient to sip water from a cup so they can feel the thyroid gland while the liquid is being swallowed. They may also use a stethoscope to listen to the blood flowing around and through the gland. The exam will also include a visual inspection of the skin, eyes and muscle tone, a testing of reflexes and measurements of blood pressure and heart rate.

  • Blood test. A thyroid test is a type of blood test that measures the levels of several hormones produced by the thyroid gland and one produced by the pituitary gland (thyroid-stimulating hormone, or TSH). It requires one tube of blood to be sent to a clinical laboratory for testing. Thyroid tests are used to diagnose thyroid conditions as well as to monitor treatment for thyroid disorders. There are several different types of thyroid hormones and the ones chosen for testing will depend on which type of thyroid disorder is suspected.
  • Chest x-ray. This painless imaging test can help the physician to determine if the patient has an enlarged heart.

  • Thyroid radioactive iodine uptake (RAIU, iodine uptake). Because the thyroid gland is normally the only area of the body that absorbs iodine, the physician can give an injection or a liquid dose of a radioactive iodine solution, and monitor how fast and how much iodine is absorbed by the thyroid gland. A healthy thyroid will absorb most of it. Low uptake indicates hypothyroidism.
  • Thyroid scan. This is usually performed in conjunction with the RAIU. Patients are injected with the radioactive iodine and then scanned with a gamma camera over a period of 20 to 30 minutes. The camera projects the images onto a computer screen and takes still pictures of the scan images. As with the RAIU, low uptake indicates hypothyroidism. The scan may also provide valuable information about whether a thyroid nodule or tumor is potentially malignant, as well as whether it is active (producing thyroid hormones) or inactive.

  • Biopsy. A tiny needle is inserted through the skin to remove a sample of a thyroid nodule or tumor to be examined for possible malignancy. A biopsy is always performed when any form of thyroid cancer is suspected.

  • Electrocardiogram (EKG). If the patient is experiencing heart–related symptoms, such as palpitations or chest pain, an EKG will likely be performed. The EKG is a recording of the heart's electrical activity as a graph on a moving strip of paper or video monitor. The highly sensitive electrocardiograph machine helps to detect heart irregularities, disease and damage by measuring the heart's rhythms and electrical impulses.

Treatment and prevention of hypothyroidism

Most patients with hypothyroidism will require lifelong treatment with thyroid hormone replacement. The only exceptions are hypothyroidism that is temporary, as in the case of hypothyroidism caused by an iodine–containing drug that can be discontinued or postpartum thyroiditis. In general, thyroid hormone treatment is very successful.

Patients on thyroid replacement therapy require synthetic thyroid hormones to replace the lost natural thyroid hormone. Synthetic thyroid hormones are administered as pills, usually for the rest of the patient's life. Patients generally start with the lowest possible dose and then increase the dose gradually until thyroid hormone levels stabilize.

As the therapy begins, the physician will closely monitor thyroid hormone levels with periodic thyroid tests, usually every few weeks at the beginning of therapy. Most patients can maintain a proper balance with 100 to 150 micrograms of synthetic thyroid medication each day, although the range varies from 50 to 200 micrograms a day. Patients taking these medications are advised to refrain from taking calcium or iron supplements within six to 12 hours of their thyroid medication. Calcium and iron can interfere with the medication's absorption.

In general, elderly people and heart patients should be treated conservatively. Thyroid hormone increases the oxygen demands of the heart muscle. In elderly patients and those with coronary artery disease (CAD) or CAD risk factors, thyroid hormone has a small risk of causing abnormal heart rhythms (arrhythmias), chest pain (angina) or even heart attack (myocardial infarction).

Pregnant women may require higher doses of thyroid hormone during pregnancy and more frequent monitoring.

Synthetic thyroid hormone can also be dangerous to people with normal thyroid function. It is vital that thyroid tests confirm the presence of hypothyroidism before the patient begins taking synthetic thyroid hormones. Taking thyroid hormone medications without having a thyroid hormone deficiency has been found to lead to heart failure in some patients with heart disease.

Little is known about what causes thyroid disease. Patients cannot make lifestyle changes to minimize their risk of developing thyroid disease and there is no immunization against this disorder. Early detection, however, can significantly impact the severity of the disorder and treatment options, especially among newborns with congenital hypothyroidism, who may suffer from permanent brain damage if not treated promptly.

Patients with mild thyroid disorders may not experience any symptoms. Therefore, patients who have a sibling, parent, aunt or uncle with thyroid disease are urged to have screening tests performed to see if they, too, have the condition. Not only can these tests help diagnose existing thyroid conditions, but they can also serve as a baseline for comparing future changes in thyroid function.

Questions for your doctor on hypothyroidism

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about hypothyroidism:

  1. Can my condition be expected to worsen with time?
  2. How often will I need blood testing to monitor my hormone levels?
  3. Is there another, more effective combination of hormone therapy that I can use?
  4. Are there are lifestyle and dietary changes I can make to help my condition? Are there any habits that can aggravate my condition?
  5. Does this raise my chances of contracting other diseases, such as cancer and heart disease?
  6. What is your opinion of desiccated thyroid hormones? I've read they are a "natural alternative" to synthetic thyroid hormones preparations?
  7. Will this affect any of my normal activities, such as exercise, sex, sleeping, and appetite?
  8. Can I use a generic version of the same hormone I'm taking?
  9. What symptoms should cause me to seek professional help?
  10. Will these hormones interfere with any other prescription medications?
  11. Are there any nutrients or supplements that might help my condition? Any I should avoid?
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