Hypothyroidism is a condition in which a person's thyroid gland is not producing enough hormone. It may be caused by an autoimmune disorder, a genetic defect in a newborn, certain medications, surgical removal of the thyroid gland, radiation therapy for cancer, and other reasons. Hypothyroidism is sometimes categorized as either primary (caused by a problem in the thyroid gland itself) or secondary (caused by the lack of hormones that ordinarily stimulate the thyroid gland to produce thyroid hormone).
Hypothyroidism is an endocrine disorder. It is caused by underfunctioning of a gland that is part of the endocrine system—a group of small organs located throughout the body that regulate growth, metabolism, tissue function, and emotional mood. The thyroid gland itself is a butterfly-shaped organ that lies at the base of the throat below the Adam's apple.
Hypothyroidism is not easy to diagnose because its symptoms are found in a number of other diseases; it often comes on slowly; and it may produce few or no symptoms in younger adults. In general, hypothyroidism is characterized by a slowing down of both physical and mental activities.
About 3 percent of the general population in the United States and Canada have some form of hypothyroidism. Apart from cretinism, which affects one child in every 3,000 to 4,000, hypothyroidism is largely a disease of adults. The most common form of primary hypothyroidism in North America is Hashimoto disease, an autoimmune disorder that is diagnosed in about fourteen women out of every 1,000 and one man in every 2,000. Internationally, however, the most common cause of hypothyroidism is a lack of iodine in the diet.
Some people are at increased risk of hypothyroidism:
- Women. Women are two to eight times as likely to have hypothyroidism, depending on the age group being studied.
- Age over fifty. In one Massachusetts study, 6 percent of women over age sixty and 2.5 percent of men over age sixty were found to be hypothyroid.
- Race. According to the National Institutes of Health (NIH), the rates of hypothyroidism in the United States are highest among Caucasians (5.1 percent) and Hispanics (4.1 percent) and lowest among African Americans (1.7 percent).
- People who have close relatives with an autoimmune disease.
The most common causes of hypothyroidism are:
- Hashimoto disease. This is an autoimmune disorder in which the patient's immune system attacks the thyroid gland, leading to tissue destruction.
- Treatment for hyperthyroidism. People who have been treated for an oversupply of thyroid hormone with radioactive iodine may lose their ability to produce enough thyroid hormone.
- Surgery on the thyroid gland.
- Radiation therapy for the treatment of head or neck cancer.
- Medications. Lithium, given to treat some psychiatric disorders, and certain heart medications may affect the functioning of the thyroid gland.
- Pregnancy. As many as 10 percent of women may become hypothyroid in the first year after childbirth, particularly if they have diabetes.
An Olympic Champion's Story
Carl Lewis (1961–), a track and field superstar, was the first athlete to equal Jesse Owens's (1913–1980) feat of winning four gold medals in a single Olympic Games in 1936. In 1984 Lewis won his four medals at the Games held in Los Angeles. He continued to dominate track and field events into the 1990s.
Shortly before competing in the 1996 Olympic Games in Atlanta, however, Lewis was diagnosed with hypothyroidism. His early symptoms were easy to miss, such as a weight gain that could be explained as the result of a muscle-building program that was part of his training for the games. He began treatment with synthetic thyroid hormone at once. In July 1996, he won the long jump at the Atlanta Games and became one of only several athletes in the history of the Olympics to win a total of nine gold medals in the course of his career.
Lewis later published a book about his condition in which he said, “As is the case with most people with thyroid conditions, I had no clue that I had the condition at all. The fact that I was checked was a fortunate accident…. I knew nothing about thyroid problems before discovering that I had one
myself. Educate yourself, and follow prescribed treatment. As I showed in the Olympics, you can go back to being 100 percent. I feel even better now than I did in Atlanta, now that my stress level is on a more even keel and my medication levels are just right.”
- Viral infections. These can cause a short-term inflammation of the thyroid gland in some people.
- A tumor in the pituitary gland. The pituitary gland produces a hormone called thyroid-stimulating hormone or TSH. Low levels of TSH can lead to secondary hypothyroidism.
- Congenital. About one baby in every 3,000 to 4,000 is born with a defective thyroid gland or no gland at all.
- Too little iodine in the diet. This cause of hypothyroidism is most common in developing countries; it is rare in North America and Europe.
Hypothyroidism can be difficult to diagnose because many of its early symptoms are not unique to it. In addition, the symptoms typically come on gradually. The person may simply feel tired or less energetic than usual, or develop dry, itchy skin and brittle hair that falls out easily. The classic symptoms of hypothyroidism—sensitivity to cold, puffy complexion, decreased sweating, and coarse skin—may occur in only 60 percent of patients. It may take months to years before the person or his or her doctor begins to suspect a problem with the thyroid gland.
Not every patient with an underactive thyroid has the same symptoms or has them with the same severity. Common symptoms of hypothyroidism, however, include the following:
- Increased sensitivity to cold weather
- Dry, itchy skin and a pale or yellowish complexion
- Dry brittle hair that falls out easily and nails that break or split
- Goiter (swelling in the front of the neck caused by thyroid enlargement)
- Hoarse voice and puffy facial skin
- Unexplained weight gain of 10–20 pounds (4.5–9 kilograms), most of which is fluid
- Sore and aching muscles, most commonly in the shoulders and hips
- In women, extra-long menstrual periods or unusually heavy bleeding
- Weak leg muscles
- Decreased sweating
- Memory loss or difficulty concentrating
- Slowed heart rate (less than 60 beats per minute) and lowered blood pressure
The diagnosis of hypothyroidism is usually made by tests of the patient's thyroid function following a careful history of the patient's symptoms. The first test is a blood test for thyroid-stimulating hormone, or TSH.
TSH is a hormone produced by the pituitary gland in the brain that stimulates the thyroid gland to produce thyroid hormone. When the thyroid gland is not producing enough hormone, the pituitary gland secretes more TSH; thus a high level of TSH in the blood indicates that the thyroid gland is not as active as it should be.
The TSH test, however, does not always detect borderline cases of hypothyroidism. The doctor may order additional tests to measure the levels of thyroid hormone as well as TSH in the patient's blood. If the doctor thinks that the patient may have Hashimoto disease, he or she may test for the presence of abnormal antibodies in the blood. Because Hashimoto disease is an autoimmune disorder, there will be two or three types of anti-thyroid antibodies in the patient's blood in about 90 percent of cases.
In some cases, the doctor may also order an ultrasound study of the patient's neck in order to evaluate the size of the thyroid gland or take a small sample of thyroid tissue in order to make sure that the gland is not cancerous.
Treatment for hypothyroidism consists of a daily dose of a synthetic form of thyroid hormone sold under the trade names of Synthroid, Levothroid, or Levoxyl. The patient is told that the drug must be taken as directed for the rest of his or her life.
In the early weeks of treatment, the patient will need to see the doctor every four to six weeks to have his or her TSH level checked and the dose of medication adjusted. After the doctor is satisfied with the dosage level and the patient's overall health, checkups are done every six to twelve months. The reason for this careful measurement of the medication is that too much of the synthetic hormone increases the risk of osteoporosis in later life or abnormal heart rhythms in the present.
Congenital hypothyroidism or cretinism is also treated with synthetic thyroid hormone. Most hospitals now screen newborns for thyroid problems, because untreated hypothyroidism can lead to lifelong physical and mental developmental disorders.
The prognosis for patients with hypothyroidism is very good, provided they take their medication as directed. They can usually live a normal life
with a normal life expectancy. Children with congenital hypothyroidism have a good prognosis if the disorder is caught and treated early. Some develop learning disorders, however, in spite of early treatment.
The chief risks to health are related to a lack of treatment for hypothyroidism. If low levels of thyroid hormone are not diagnosed and treated, patients are at increased risk of goiter, an enlarged heart, and severe depression. In addition, women with untreated hypothyroidism have a higher risk of giving birth to babies with cleft palate and other birth defects.
One rare but potentially life-threatening complication of long-term untreated hypothyroidism is myxedema coma. In this condition, which is usually triggered by stress or illness, the person becomes extremely sensitive to cold, may be unusually drowsy, or lose consciousness. Heart rate, blood pressure, and breathing may all be abnormally low. Myxedema coma requires emergency treatment in a hospital with intravenous thyroid hormone and intensive care nursing.
There are no proven ways to prevent hypothyroidism because the disorder has so many possible causes.
Research in hypothyroidism in the early 2000s has a number of different goals. One is to look for specific genes that may be linked to hypothyroidism. Another area of research is to discover reasons for the high female/male ratio. Still another goal is to discover a cure for the condition that will do away with the need for lifetime treatment with synthetic thyroid hormone.
SEE ALSO Hashimoto disease
WORDS TO KNOW
Congenital: Present at birth.
Cretinism: A form of hypothyroidism found in some newborns.
Endocrine system: A system of small organs located throughout the body that regulate metabolism, growth and puberty, tissue function, and mood. The thyroid gland is part of the endocrine system.
Goiter: A swelling in the neck caused by an enlarged thyroid gland.
Hyperthyroidism: A disease condition in which the thyroid gland produces too much thyroid hormone.
Hypothyroidism: A disease condition in which the thyroid gland does not produce enough thyroid hormone.
Metabolism: The chemical changes in living cells in which new materials are taken in and energy is provided for vital processes.
Myxedema: A synonym for hypothyroidism. Myxedema coma is a condition in which a person with untreated hypothyroidism loses consciousness. It is potentially fatal.
Rosenthal, M. Sara. The Hypothyroid Sourcebook. New York: McGraw-Hill, 2002.
Skugor, Mario. Thyroid Disorders: A Cleveland Clinic Guide. Cleveland, OH: Cleveland Clinic Press, 2006.
Pérez-Peña, Richard. “Cases: Heeding Thyroid's Warnings.” New York Times, October 7, 2003. Available online at http://www.query.nytimes.com/gst/fullpage.html?res=980DE0D6133CF934A35753C1A9659C8B63 (accessed June 14, 2008). The writer of the article was diagnosed with hypothyroidism after several puzzling episodes of memory loss and falling asleep at work.
American Thyroid Association (ATA). Hypothyroidism FAQs. Available online at http://www.thyroid.org/patients/faqs/hypothyroidism.html (updated May 2008; accessed June 14, 2008).
Hormone Foundation. Hormones and You: Hypothyroidism. Available online in PDF format at http://www.hormone.org/Resources/Thyroid/upload/bilingual_Hypothyroidism.pdf (updated 2004; accessed June 14, 2008).
Mayo Clinic. Hypothyroidism (Underactive Thyroid). Available online at http://www.mayoclinic.com/health/hypothyroidism/DS00353 (updated June 12, 2008; accessed June 14, 2008).
Virtual Medical Centre. What Is Hypothyroidism? Available online at http://www.virtualendocrinecentre.com/diseases.asp?did=143 (accessed June 14, 2008). This is an animation that explains the functions of the thyroid gland as well as the symptoms of hypothyroidism. It takes about a minute and a half to play.