Thyroid disease is an impairment in the normal functioning of the thyroid, an important gland located at the base of the neck. A major function of the thyroid is to regulate metabolism, the bio-chemical processes in the body. Thyroid disease may speed up or slow down metabolism, producing a wide range of physical and mental symptoms.
Thyroid-stimulating hormone (TSH)
The thyroid is an H-shaped gland that has two main parts, or lobes, that lie on either side of the trachea (TRAY-key-a) or windpipe. The lobes are connected by a narrow segment called the isthmus. The principal hormone* produced by the thyroid is thyroxine. Production of this hormone is in turn controlled by another hormone, called thyroid-stimulating hormone (TSH), secreted by the pituitary gland located at the base of the brain. Thyroxine is released into the bloodstream and controls the rate of metabolism*. In children, thyroid hormones are essential for normal growth and development.
- * hormones
- are chemicals that are produced by different glands in the body. A hormone is like the body’s ambassador: it is created in one place but is sent through the body to have specific regulatory effects in different places.
- * metabolism
- is the total of all the chemical activities in cells that release energy from nutrients or use energy to make other substances, such as proteins.
Disorders of the thyroid can cause overproduction of thyroid hormones (hyperthyroidism), or underproduction of thyroid hormones (hypothyroidism). Sometimes the thyroid becomes enlarged, a condition known as goiter.
Hyperthyroidism: a revving engine
The most common type of hyperthyroidism, or thyrotoxicosis (thy-ro-tox-i-KO-sis), is called Graves’ disease. It is an autoimmune disorder, a disturbance of the immune system. Antibodies stimulate the thyroid to produce excessive quantities of hormone, thereby raising the rate of metabolism. Graves’ disease can occur at any age, but the highest incidence of this disorder is in women between 20 and 40 years of age.
Symptoms of Graves’ disease include an increased heart rate, nervousness and irritability, tremor, loss of weight, enlarged thyroid gland (goiter), abnormalities of the menstrual periods, sweating and heat intolerance, restless overactivity, and sleeplessness. Sometimes there is also exophthalmos (eks-off-THAL-mus), a condition in which the eyeballs protrude (bulge outward).
Less commonly, hyperthyroidism results from a form of thyroiditis (thy-roid-EYE-tus) inflammation of the thyroid caused by a viral infection or by thyroid nodules (lumps or growths) that may produce excess hormones.
Hypothyroidism: a Slowing down
Whereas hyperthyroidism abnormally raises the metabolic rate, hypothyroidism slows it down too much. Many of the symptoms of hypothyroidism are thus the reverse of those seen in hyperthyroidism. The most common cause of hypothyroidism is Hashimoto’s thyroiditis, which occurs most often in young and middle-aged women.
Hashimoto’s thyroiditis, like Graves’ disease, is an autoimmune disorder. The immune system damages the thyroid rather than stimulating it, resulting in an underproduction of hormone. Symptoms of Hashimoto’s thyroiditis include a slow heart rate, tiredness, muscular weakness, weight gain, abnormal menstrual periods, intolerance of cold, dry skin, hair loss, hoarseness, enlarged thyroid (goiter), and mental dullness. In more severe cases, there may be myxedema (mik-se-DEE-ma), a thickening and puffiness of the skin most noticeable in the face.
Less often, hypothyroidism is caused by surgical removal of part or all of the thyroid gland to treat other thyroid conditions, or by insufficient iodine in the diet, which is now a rare occurrence in developed countries.
When hypothyroidism occurs in infancy and is not treated cretinism (KREET-in-izm) results. A child with cretinism has stunted growth and mental deficiency. Older children who become hypothyroid show slowing of growth and delayed sexual maturation.
Goiter is not itself a disease. The term simply refers to enlargement of the thyroid, sometimes visible as a swelling in the front of the neck. Enlargement of the thyroid can be a sign of hyperthyroidism or hypothyroidism. It even can occur when thyroid function is normal.
Goiter and other thyroid disorders sometimes are the result of too little iodine in the diet. In ancient Greece, iodine-rich seaweed was eaten in response to enlarged thyroid glands. In 1811, the French chemist Courtois identified iodine, which began being used internally in the treatment of thyroid disorders in 1821.
In 1922, the Swiss Goiter Commission introduced the first program of adding iodine to salt as a preventive measure against goiter in Switzerland. Also in 1922, Michigan physician David Murray Cowie expressed interest in eliminating goiter by means of iodized salt. Cowie worked with the Michigan State Medical Society to have iodized salt placed on Michigan grocery shelves and, eventually, in stores across the United States. In areas with such programs, iodine deficiency is rarely seen.
A goiter can be seen in hyperthyroidism of Graves’ disease, in which the thyroid enlarges due to stimulation of the gland by the malfunctioning immune system. In hypothyroidism, it enlarges as part of the body’s attempt to produce enough hormone to compensate for damage done to it by the disease, or because of inflammation* caused by the disease, or both.
- * inflammation
- (in-fla-MAY-shun) is the body’s reaction to irritation, infection, or injury that often involves swelling, pain, redness, and warmth.
Goiter also can occur in parts of the world where there is inadequate iodine in the diet. Found in seafood and most table salt preparations, iodine is an element essential for the formation of thyroid hormones in the body.
Distinct swellings or lumps within the thyroid are called nodules. They are most common in women and their incidence increases with age. The large majority of thyroid nodules are benign, but some may be cancerous. They therefore require prompt medical evaluation.
Sometimes the thyroid temporarily enlarges slightly during puberty or pregnancy, without impairing its function or causing any other symptoms.
To diagnose a suspected thyroid disorder, the doctor will take a medical history and perform a physical examination. Blood samples usually are taken to measure the levels of thyroid hormones and TSH, the pituitary hormone that stimulates the thyroid. The thyroid also may be checked using various scanning techniques. If a thyroid tumor is suspected, a sample of thyroid tissue may be removed for examination.
Most thyroid diseases are highly treatable. Hyperthyroidism may be treated with a single dose of radioactive iodine, which destroys overactive thyroid cells. Alternatively, antithyroid medications may be prescribed to suppress formation of thyroid hormones. Surgical removal of most of the thyroid is another treatment. Hypothyroidism is treated with hormone replacement medication, which usually must be continued for life.
George and Barbara Bush
When George Bush was President of the United States (1989-1993), he and his wife, Barbara, both were diagnosed with Graves’ disease, a type of hyperthyroidism (overactive thyroid).
Because only 1 or 2 of every 100 women, and even fewer men, get Graves’ disease, the likelihood of any husband and wife having the disorder at the same time is extremely rare. Since one person cannot catch Graves’ disease from another, this was an amazing medical coincidence.
A goiter of uncertain cause may disappear on its own, or it may be small and not need treatment. Goiters caused by thyroid disease usually shrink with treatment. Occasionally, surgery is needed for removing a very large goiter. Thyroid disease is not contagious. It often runs in families, and there is no way a person can prevent it. People who live in parts of the world where seafood is scarce and table salt is not iodized, however, need to make sure they take in sufficient amounts of iodine to avoid hypothyroidism and goiter.
Wood, Lawrence O, M.D., David S. Cooper, M.D., and E. Chester Ridgway, M.D. Your Thyroid: A Home Reference, third edition. New York: Bailamme Books, 1996.
Rubenfeld, Sheldon, M.D. Could It Be My Thyroid. Houston: Thyroid Society for Education and Research, 1996.
American Thyroid Association, Montefiore Medical Center, 111 East 210 Street, Bronx, NY 10467. The American Thyroid Association works with the Thyroid Foundation of America to publish patient education brochures and to provide referrals to patient support organizations. http://www.thyroid.org
National Graves’ Disease Foundation, 2 Tsitsi Court, P.O. Box 1969, Brevard, NC 28712. Telephone 828-877-5251 http://www.ngdf.org/index.htm
Columbia-Presbyterian Medical Center in New York has an online Thyroid Center with information about hyperthyroidism and hypothyroidism. http://cpmcnet.columbia.edu/dept/thyroid/