Thyroid Disease

Thyroid disease is an impairment in the normal functioning of the thyroid, an important gland located below the chin at the base of the neck. A major function of the thyroid is to regulate metabolism, the biochemical processes in the body. Thyroid disease may speed up or slow down metabolism, producing a wide range of physical and mental symptoms.

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.

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.

The most common type of hyperthyroidism, or thyrotoxicosis (thy-ro-tox-i-KO-sis), is Graves’ disease, an autoimmune disease * , 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 in people of any age, but the highest incidence 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), an inflammation of the thyroid caused by a viral infection or by thyroid nodules (lumps or growths) that may produce excess hormones.

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 disease. The immune system damages the thyroid rather than stimulates 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 rare 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 have hypothyroidism show slowing of growth and delayed sexual maturation.

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. Goiter can also 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. Thus they 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 takes a medical history and performs a physical examination. Blood samples are usually taken to measure the levels of thyroid hormones and TSH, the pituitary hormone that stimulates the thyroid. The thyroid may also 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 typically is continued for life.

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.

See also Dietary Deficiencies • Growth and Growth Disorders • Metabolic Disease • Thyroid Cancer

* hormone is a chemical substance that is produced by a gland and sent into the bloodstream carrying messages that have certain effects on other parts of the body.

* metabolism (meh-TAB-o-liz-um) is the process in the body that converts foods into the energy necessary for body functions.

* autoimmune disease (awtoh-ih-MYOON) is a disease in which the body's immune system attacks some of the body's own normal tissues and cells.

* immune system (im-YOON SIStem) is the system of the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.

* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.

* inflammation (in-fla-MAY-shun) is the body's reaction to irritation, infection, or injury that often involves swelling, pain, redness, and warmth.

* benign (be-NINE) refers to a condition that is not cancerous or serious and will probably improve, go away, or not get worse.

* puberty (PU-ber-tee) is the period during which sexual maturity is attained.