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The Merck Manual of Medical Information--Home Edition
Section 13. Hormonal Disorders
Chapter 145
Thyroid Gland Disorders
The thyroid is a small gland, measuring about 2 inches across, that lies just under the skin below the Adam's apple in the neck. The two halves (lobes) of the gland are connected in the middle (called the isthmus), so the thyroid gland resembles the letter H or a bow tie. Normally, the thyroid gland can't be seen and can barely be felt, but if it becomes enlarged, a doctor can feel it easily and a prominent bulge (goiter) may appear below or to the sides of the Adam's apple.
The thyroid gland secretes thyroid hormones, which control the speed at which the body's chemical functions proceed (metabolic rate). Thyroid hormones influence the metabolic rate in two ways: by stimulating almost every tissue in the body to produce proteins and by increasing the amount of oxygen that cells use. When the cells work harder, body organs work faster.
To produce thyroid hormones, the thyroid gland needs iodine, an element contained in food and water. The thyroid gland traps iodine and processes it into thyroid hormones. As thyroid hormones are used up, some of the iodine contained in the hormones returns to the thyroid gland and is recycled to produce more thyroid hormones.
The body has a complex mechanism for adjusting the level of thyroid hormones. First, the hypothalamus, located just above the pituitary gland in the brain, secretes thyrotropin-releasing hormone, which causes the pituitary gland to produce thyroid-stimulating hormone. Just as the name suggests, thyroid-stimulating hormone stimulates the thyroid gland to produce thyroid hormones. When the amount of thyroid hormones circulating in the blood reaches a certain level, the pituitary gland produces less thyroid-stimulating hormone; when the amount of thyroid hormones circulating in the blood decreases, the pituitary gland produces more thyroid-stimulating hormone--a negative feedback control mechanism.
Thyroid hormones are found in two forms. Thyroxine (T4), the form produced in the thyroid gland, has only a slight, if any, effect on speeding up the body's metabolic rate. Thyroxine is converted in the liver and other organs to the metabolically active form, triiodothyronine (T3). This conversion produces about 80 percent of the active form of the hormone; the remaining 20 percent is produced and secreted by the thyroid gland itself. Many factors control the conversion of T4 to T3 in the liver and other organs, including the body's needs from moment to moment. Most of the T4 and T3 is tightly bound to certain proteins in the blood and is active only when not bound to these proteins. In this remarkable way, the body maintains the correct amount of thyroid hormone needed to keep a steady metabolic rate.
For the thyroid gland to function normally, many factors must work well together: the hypothalamus, the pituitary gland, the thyroid hormone--binding proteins in the blood, and the conversion, in the liver and other tissues, of T4 to T3.
Laboratory Tests
To determine how well the thyroid gland is functioning, doctors use several laboratory tests. One of the most common is a test to measure the level of thyroid-stimulating hormone in the blood. Because this hormone stimulates the thyroid gland, blood levels are high when the thyroid gland is underactive (and thus needs more stimulation) and low when the thyroid gland is overactive (and thus needs less stimulation). If the pituitary gland isn't functioning normally (although this rarely happens), the level of thyroid-stimulating hormone alone won't accurately reflect thyroid gland function, and doctors then measure the level of free T4.
Measuring the level of thyroid-stimulating hormone and the level of free T4 circulating in the blood is usually all that is needed. However, doctors may also measure the level of a protein called thyroxine-binding globulin, because abnormal levels of this protein can lead to misinterpretation of a person's total thyroid hormone level. People who have kidney disease, some genetic disorders, or certain other diseases or who take anabolic steroids have lower levels of thyroxine-binding globulin. Conversely, levels of thyroxine-binding globulin may be increased in women who are pregnant or taking oral contraceptives or other forms of estrogen, in people in the early stages of hepatitis, and in people with some other diseases.
Some tests can be performed on the thyroid gland itself. For instance, if a doctor feels a growth in the thyroid gland, an ultrasound examination may be ordered; this procedure uses sound waves to determine whether the growth is solid or filled with fluid. A thyroid scan uses radioactive iodine or technetium and a device to produce a picture of the thyroid gland that will show any physical abnormalities. Thyroid scanning can also help the doctor determine whether the function of an area is normal, overactive, or underactive compared with the rest of the gland.
On rare occasions when a doctor isn't sure whether the problem lies in the thyroid gland or in the pituitary gland, functional stimulation tests may be ordered. One of these tests involves injecting thyrotropin-releasing hormone intravenously and then using blood tests to measure the pituitary gland's response.
Euthyroid Sick Syndrome
In the euthyroid sick syndrome, thyroid test results are abnormal even though the thyroid gland is functioning normally.
The euthyroid sick syndrome commonly occurs in people who have a severe illness other than thyroid disease. When people are sick, are malnourished, or have had surgery, the T4 form of thyroid hormone isn't converted normally to the T3 form. Large amounts of reverse T3, an inactive form of thyroid hormone, accumulate. Despite this abnormal conversion, the thyroid gland continues to function and to control the body's metabolic rate normally. Because no problem exists with the thyroid gland, no treatment is needed. Laboratory tests show normal results once the underlying illness resolves.
Symptoms of Thyroid Disease
Hyperthyroidism (too much thyroid hormone) Hypothyroidism (too little thyroid hormone)
Fast heartbeat
High blood pressure
Moist skin and increased sweat
Shakiness and tremor
Nervousness
Increased appetite with weight loss
Sleep difficulties
Frequent bowel movements and diarrhea
Weakness
Raised, thickened skin over shins
Swollen, reddened, bulging eyes
Sensitivity of eyes to light
Constant stare
Confusion Slow pulse
Hoarse voice
Slowed speech
Puffy face
Loss of eyebrows
Drooping eyelids
Intolerance to cold
Constipation
Weight gain
Sparse, coarse, dry hair
Dry, scaly, thick, coarse skin; raised, thickened skin over shins
Carpal tunnel syndrome
Confusion
Depression
Dementia
Hyperthyroidism
Hyperthyroidism--a condition in which the thyroid gland is overactive--develops when the thyroid produces too much hormone.
Hyperthyroidism has several causes, including immunologic reactions (believed to be the cause of Graves' disease). People with thyroiditis, an inflammation of the thyroid gland, typically go through a phase of hyperthyroidism. However, the inflammation may damage the thyroid gland, so that its initial overactivity is a prelude to either transient (more common) or permanent underactivity (hypothyroidism).
Toxic thyroid nodules (adenomas), areas of abnormal tissue growth within the thyroid gland, sometimes escape the mechanisms that normally control the thyroid gland and produce thyroid hormone in large quantities. A person may have one nodule or many. Toxic multinodular goiter (Plummer's disease), a disorder in which there are many nodules, is uncommon in adolescents and young adults and tends to increase with age.
In hyperthyroidism, regardless of the cause, the body's functions speed up. The heart pounds, beats more quickly, and may develop an abnormal rhythm, leading to an awareness of the heartbeat (palpitations). Blood pressure is likely to increase. Many people with hyperthyroidism feel warm even in a cool room. Their skin may become moist as they tend to sweat profusely, and their hands may develop a fine tremor. Many people feel nervous, tired, and weak, yet have an increased level of activity; have an increased appetite, yet lose weight; sleep poorly; and have frequent bowel movements, occasionally with diarrhea.
Older people with hyperthyroidism may not develop these characteristic symptoms but have what is sometimes called apathetic or masked hyperthyroidism. They simply become weak, sleepy, confused, withdrawn, and depressed. However, heart problems, especially abnormal heart rhythms, are seen more often in older people with hyperthyroidism.
Hyperthyroidism can cause changes in the eyes: puffiness around the eyes, increased tear formation, irritation, and unusual sensitivity to light. The person appears to stare. These eye symptoms disappear soon after the thyroid hormone secretion is controlled, except in people with Graves' disease, which causes special eye problems.
Hyperthyroidism may take the form of Graves' disease, toxic nodular goiter, or secondary hyperthyroidism.
Graves' Disease
Graves' disease (toxic diffuse goiter) is believed to be caused by an antibody that stimulates the thyroid to produce too much thyroid hormone. People with Graves' disease have the typical signs of hyperthyroidism and three distinctive additional symptoms. Since the entire gland is stimulated, it can become greatly enlarged, causing a bulge in the neck (goiter). People with Graves' disease may also have bulging eyes (exophthalmos) (see page 1034 in Chapter 220, Eyelid and Tear Gland Disorders) and, less commonly, raised areas of skin over the shins.
The eyes bulge outward because of a substance that builds up in the orbit. This bulging occurs in addition to the intense stare and other eye changes of hyperthyroidism. The muscles that move the eyes become unable to function properly, making it difficult or impossible to move the eyes normally or to coordinate eye movements, resulting in double vision. The eyelids may not close completely, exposing the eyes to injury from foreign particles and dryness. These eye changes may begin years before any other symptoms of hyperthyroidism, providing an early clue to Graves' disease, or may not occur until other symptoms are noticed. Eye symptoms may even appear or worsen after the excessive thyroid hormone secretion has been treated and controlled.
Eye symptoms may be helped by elevating the head of the bed, by applying eyedrops, by sleeping with the eyelids taped shut, and occasionally by taking diuretics. The double vision may be helped by using eyeglass prisms. Finally, oral corticosteroid drugs, x-ray treatment to the orbits, or eye surgery may be needed.
In Graves' disease, a substance similar to the one deposited behind the eyes may be deposited in the skin, usually over the shins. The thickened area may be itchy and red and feels hard when pressed with a finger. As with deposits behind the eyes, this problem may begin before or after other symptoms of hyperthyroidism are noticed. Corticosteroid creams or ointments can help relieve the itching and hardness. Often the problem disappears without treatment for no apparent reason months or years later.
Toxic Nodular Goiter
In toxic nodular goiter, one or more nodules in the thyroid produce too much thyroid hormone and aren't under the control of thyroid-stimulating hormone. The nodules are true hyperfunctioning benign thyroid tumors and are not associated with the bulging eyes and skin problems of Graves' disease.
Secondary Hyperthyroidism
Hyperthyroidism may (rarely) be caused by a pituitary tumor that secretes too much thyroid-stimulating hormone, which in turn stimulates the thyroid to overproduce thyroid hormones. Another rare cause of hyperthyroidism is pituitary resistance to thyroid hormone, which results in the pituitary gland secreting too much thyroid-stimulating hormone.
Women with a hydatidiform mole (see page 1113 in Chapter 239, Cancers of the Female Reproductive System) may also have hyperthyroidism because the thyroid gland is overstimulated by the high levels of human chorionic gonadotropin in the blood. The hyperthyroidism disappears after the molar pregnancy is terminated and human chorionic gonadotropin vanishes from the blood.
Complications
Thyroid storm, sudden extreme overactivity of the thyroid gland, may produce fever, extreme weakness and loss of muscle, restlessness, mood swings, confusion, altered consciousness (even coma), and an enlarged liver with mild jaundice. Thyroid storm is a life-threatening emergency requiring prompt treatment. Severe strain on the heart can lead to a life-threatening irregular heartbeat (arrhythmia) and shock.
Thyroid storm is generally caused by untreated or inadequately treated hyperthyroidism and can be triggered by infection, trauma, surgery, poorly controlled diabetes, fear, pregnancy or labor, discontinuance of thyroid medication, or other stresses. It's rare in children.
Treatment
Hyperthyroidism can usually be treated with medication, but other options include surgically removing the thyroid gland or treating it with radioactive iodine. Each treatment has advantages and disadvantages.
The thyroid gland needs a small amount of iodine to work properly, but a large amount of iodine decreases the amount of hormone the gland makes and prevents the gland from releasing excess thyroid hormone. Therefore, doctors can use large doses of iodine to stop the gland from secreting excess thyroid hormone. Iodine treatment is particularly useful when doctors need to control hyperthyroidism quickly, as during thyroid storm or before emergency surgery. However, iodine isn't used for routine or long-term treatment of hyperthyroidism.
Propylthiouracil or methimazole, the drugs most commonly used to treat hyperthyroidism, slow thyroid function by decreasing the gland's production of thyroid hormone. Both drugs are taken orally, beginning with high doses that are later adjusted according to the results of thyroid hormone blood tests. These drugs can usually control thyroid function in 6 weeks to 3 months, but larger doses of the drugs may bring it under control faster--with an increased risk of adverse effects. These adverse effects include allergic reactions (most commonly, skin rashes), nausea, loss of taste, and on rare occasions, depressed synthesis of blood cells in the bone marrow. The bone marrow depression can deplete the number of white blood cells, creating a life-threatening situation in which the person is vulnerable to infection. While these two drugs are comparable in most ways, propylthiouracil may be safer than methimazole for use in pregnant women because less of it reaches the fetus. Carbimazole, a drug that is widely used in Europe, is converted into methimazole in the body.
Beta-blocking drugs such as propranolol help control some of the symptoms of hyperthyroidism. These drugs are effective in slowing down a fast heart rate, reducing shakiness (tremor), and controlling anxiety. Doctors therefore find beta-blockers particularly useful for people with thyroid storm and for people with bothersome or dangerous symptoms whose hyperthyroidism hasn't yet been brought under control by other treatments. However, beta-blockers don't control abnormal thyroid function.
Hyperthyroidism can also be treated with radioactive iodine, which destroys the thyroid gland. Radioactive iodine taken orally introduces very little radioactivity to the body as a whole but a great deal to the thyroid gland. Doctors try to adjust the dose of radioactive iodine to destroy only enough of the thyroid gland to bring its hormone production back to normal, without reducing thyroid function too much. However, most of the time, radioactive iodine treatment ultimately creates hypothyroidism (an underactive thyroid gland), a condition that requires thyroid hormone replacement therapy. People who need thyroid hormone replacement therapy take a thyroid hormone tablet daily for the rest of their lives to replace the natural hormone that's no longer being produced in sufficient quantities. About 25 percent of the people have hypothyroidism 1 year after radioactive iodine treatment, but the percentage increases steadily over the next 20 years or more. Concern that radioactive iodine may cause cancer has never been confirmed. Radioactive iodine isn't given to pregnant women, since it crosses the placenta and may destroy the fetus' thyroid gland.
In a thyroidectomy, the thyroid gland is removed surgically. Surgery is an option especially for young people with hyperthyroidism. Surgery is also an option for people who have a very large goiter, as well as for people who are allergic to or who develop severe side effects from the drugs used to treat hyperthyroidism. Hyperthyroidism is permanently controlled in more than 90 percent of those who choose this option. Some degree of hypothyroidism occurs after surgery in some people, who then have to take replacement thyroid hormone for the rest of their lives. Rare complications include paralysis of the vocal cords and damage to the parathyroid glands (the tiny glands behind the thyroid gland that control calcium levels in the blood).
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland is underactive and produces too little thyroid hormone. Very severe hypothyroidism is called myxedema.
In Hashimoto's thyroiditis, the most common cause of hypothyroidism, (see page 709 in this chapter) the thyroid gland is often enlarged, and hypothyroidism frequently results years later because the gland's functioning areas are gradually destroyed. The second most common cause of hypothyroidism is treatment of hyperthyroidism. Both radioactive iodine treatment and surgery tend to produce hypothyroidism.
A chronic lack of iodine in the diet produces an enlarged, underactive thyroid gland (goitrous hypothyroidism), the most common cause of hypothyroidism in many undeveloped countries. Since salt manufacturers began adding iodine to table salt and iodine-containing disinfectants are often used to sterilize cow's udders, this form of hypothyroidism has disappeared in the United States. Even rarer causes of hypothyroidism include some inherited disorders in which an abnormality of the enzymes in thyroid cells prevents the gland from making or secreting enough thyroid hormones. In other rare disorders, either the hypothalamus or the pituitary gland fails to secrete enough of the hormone needed to stimulate normal thyroid function.
Symptoms
Insufficient thyroid hormone causes bodily functions to slow down. In sharp contrast to hyperthyroidism, the symptoms of hypothyroidism are subtle and gradual and may be mistaken for depression. Facial expressions become dull, the voice is hoarse and speech is slow, eyelids droop, and the eyes and face become puffy and swollen. Many people with hypothyroidism gain weight, become constipated, and are unable to tolerate cold. The hair becomes sparse, coarse, and dry and the skin becomes coarse, dry, scaly, and thick. Many people develop carpal tunnel syndrome, which makes the hands tingle or hurt. (see page 336 in Chapter 70, Peripheral Nerve Disorders) The pulse may slow, the palms and soles appear slightly orange (carotenemia), and the side part of the eyebrows slowly falls out. Some people, especially older people, may appear confused, forgetful, or demented--signs that can easily be mistaken for Alzheimer's disease or other forms of dementia.
If untreated, hypothyroidism can eventually cause anemia, a low body temperature, and heart failure. This situation may progress to confusion, stupor, or coma (myxedema coma), a life-threatening complication in which breathing slows, the person has seizures, and blood flow to the brain decreases. Myxedema coma can be triggered by exposure to the cold as well as by an infection, trauma, and drugs such as sedatives and tranquilizers that depress brain function.
Treatment
Hypothyroidism is treated by replacing the deficient thyroid hormone, using one of several different oral preparations. The preferred form is synthetic thyroid hormone, T4. Another form, desiccated (dried) thyroid, is obtained from the thyroid glands of animals. In general, doctors find desiccated thyroid less satisfactory because the dose is harder to adjust and the tablets have variable amounts of T3.
Treatment in an older person begins with small doses of thyroid hormone because too large a dose can cause serious side effects. The dose is gradually increased until the person's blood levels of thyroid-stimulating hormone return to normal. Medication is usually taken for life. In emergencies, such as myxedema coma, doctors may give thyroid hormone intravenously.
Thyroiditis
Thyroiditis, an inflammation of the thyroid gland, produces transient hyperthyroidism often followed by transient hypothyroidism or no change in thyroid function at all.
The three types of thyroiditis are Hashimoto's thyroiditis, subacute granulomatous thyroiditis, and silent lymphocytic thyroiditis.
Hashimoto's Thyroiditis
Hashimoto's thyroiditis (autoimmune thyroiditis) is the most common type of thyroiditis and the most common cause of hypothyroidism. For unknown reasons, the body turns against itself in an autoimmune reaction, creating antibodies that attack the thyroid gland. (see page 816 in Chapter 168, Immunodeficiency Disorders) This type of thyroiditis is most common in elderly women and tends to run in families. The condition occurs eight times more often in women than in men and may occur in people with certain chromosomal abnormalities, including Turner's, Down, and Klinefelter's syndromes.
Hashimoto's thyroiditis often begins with a painless enlargement of the thyroid gland or a feeling of fullness in the neck. When doctors feel the gland, they usually find it enlarged, with a rubbery texture, but not tender; sometimes it feels lumpy. The thyroid gland is underactive in about 20 percent of the people when Hashimoto's thyroiditis is discovered; the rest have normal thyroid function. Many people with Hashimoto's thyroiditis have other endocrine disorders such as diabetes, an underactive adrenal gland, or underactive parathyroid glands, and other autoimmune diseases such as pernicious anemia, rheumatoid arthritis, Sjögren's syndrome, or systemic lupus erythematosus (lupus).
Doctors perform thyroid function tests on blood samples to determine whether the gland is functioning normally, but they base the diagnosis of Hashimoto's thyroiditis on the symptoms, a physical examination, and whether the person has antibodies that attack the gland (antithyroid antibodies), which can easily be measured in a blood test.
No specific treatment is available for Hashimoto's thyroiditis. Most people eventually develop hypothyroidism and must take thyroid hormone replacement therapy for the rest of their lives. Thyroid hormone may also be useful in decreasing the enlarged thyroid gland.
Subacute Granulomatous Thyroiditis
Subacute granulomatous (giant cell) thyroiditis, which is probably caused by a virus, begins much more suddenly than Hashimoto's thyroiditis. Subacute granulomatous thyroiditis often follows a viral illness and begins with what many people call a sore throat but actually proves to be neck pain localized to the thyroid. The thyroid gland becomes increasingly tender, and the person usually develops a low-grade fever (99° F. to 101° F.). The pain may shift from one side of the neck to the other, spread to the jaw and ears, and hurt more when the head is turned or when the person swallows. Subacute granulomatous thyroiditis is often mistaken at first for a dental problem or a throat or ear infection.
Inflammation usually causes the thyroid gland to release excessive thyroid hormones, resulting in hyperthyroidism, almost always followed by transient hypothyroidism. Many people with subacute granulomatous thyroiditis feel extremely tired.
Most people recover completely from this type of thyroiditis. Generally the condition goes away by itself within a few months, but sometimes it comes back or, more rarely, damages enough of the thyroid gland to cause permanent hypothyroidism.
Aspirin or other nonsteroidal anti-inflammatory drugs (such as ibuprofen) can relieve the pain and inflammation. In very severe cases, doctors may recommend corticosteroids such as prednisone, which should be tapered off over 6 to 8 weeks. When corticosteroids are stopped abruptly, symptoms often return in full force.
Silent Lymphocytic Thyroiditis
Silent lymphocytic thyroiditis occurs most often in women, typically just after childbirth, and causes the thyroid to become enlarged without becoming tender. For several weeks to several months, a person with silent lymphocytic thyroiditis has hyperthyroidism followed by hypothyroidism before eventually recovering normal thyroid function. This condition requires no specific treatment, although the hyperthyroidism or hypothyroidism may require treatment for a few weeks. Often, a beta-blocker such as propranolol is the only drug needed to control the symptoms of hyperthyroidism. During the period of hypothyroidism, a person may need to take thyroid hormone, usually for no more than a few months. Hypothyroidism becomes permanent in about 10 percent of the people with silent lymphocytic thyroiditis.
Thyroid Cancer
Thyroid cancer is any one of four main types of malignancy of the thyroid: papillary, follicular, anaplastic, or medullary.
Thyroid cancer is more common in people who have been treated with radiation to the head, neck, or chest, most often for benign conditions (although radiation treatment for benign conditions is no longer carried out). Rather than causing the whole thyroid gland to enlarge, a cancer usually causes small growths (nodules) within the thyroid. Most thyroid nodules aren't cancerous, and thyroid cancers can generally be cured. Thyroid cancers often have a limited ability to take up iodine and produce thyroid hormone, but very rarely they produce enough hormone to cause hyperthyroidism. Nodules are more likely to be cancerous if only one nodule is found rather than several, if a thyroid scan shows that the nodule isn't functioning, if the nodule is solid rather than filled with fluid (cystic), if the nodule is hard, or if the nodule is growing quickly.
A painless lump in the neck is usually the first sign of thyroid cancer. When doctors find a nodule in the thyroid gland, they request several tests. A thyroid scan determines whether the nodule is functioning, since a nonfunctioning nodule is more likely to be cancerous than a functioning one. An ultrasound scan is less helpful but may be performed to determine whether the nodule is solid or filled with fluid. A sample of the nodule is usually taken by fine-needle biopsy for examination under a microscope--the best way to determine whether the nodule is cancerous.
Papillary Cancer
Papillary cancer accounts for 60 to 70 percent of all thyroid cancers. Two to three times as many women as men have papillary cancer; however, since nodules are far more common in women, a nodule in a man is more suspicious for a cancer. Papillary cancer is more common in young people but grows and spreads more quickly in the elderly. People who have received radiation treatment to the neck, usually for a benign condition in infancy or childhood or for some other cancer in adulthood, are at greater risk of developing papillary cancer.
Surgery is the treatment for papillary cancer, which sometimes spreads to nearby lymph nodes. Nodules smaller than three quarters of an inch across are removed along with the thyroid tissue immediately surrounding them, although some experts recommend removing the entire thyroid gland. Surgery almost always cures these small cancers.
Since papillary cancer may respond to thyroid-stimulating hormone, thyroid hormone is taken in doses large enough to suppress secretion of thyroid-stimulating hormone and help prevent a recurrence. If a nodule is larger, most or all of the thyroid gland is usually removed, and radioactive iodine is often given in expectation that any remaining thyroid tissue or cancer that has spread away from the thyroid will take it up and be destroyed. Another dose of radioactive iodine may be needed to make sure the entire cancer has been destroyed. Papillary cancer is almost always cured.
Follicular Cancer
Follicular cancer accounts for about 15 percent of all thyroid cancers and is more common in the elderly. Follicular cancer is also more common in women than in men, but as with papillary cancer, a nodule in a man is more likely to be cancer. Much more aggressive than papillary cancer, follicular cancer tends to spread through the bloodstream, spreading cancerous cells to various parts of the body (metastases). Treatment for follicular cancer requires surgically removing as much of the thyroid gland as possible and destroying any remaining thyroid tissue, including the metastases, with radioactive iodine.
Anaplastic Cancer
Anaplastic cancer accounts for less than 10 percent of thyroid cancers and occurs most commonly in elderly women. This cancer grows very quickly and usually causes a large growth in the neck. About 80 percent of the people with anaplastic cancer die within 1 year. Treatment with radioactive iodine is useless because anaplastic cancers don't take up radioactive iodine. However, treatment with anticancer drugs and radiation therapy before and after surgery has resulted in some cures.
Medullary Cancer
In medullary cancer, the thyroid gland produces excessive amounts of calcitonin, a hormone produced by certain thyroid cells. Because medullary thyroid cancer can also produce other hormones, it can cause unusual symptoms. This cancer tends to spread (metastasize) through the lymphatic system to the lymph nodes and through the blood to the liver, lungs, and bones. Medullary cancer can develop along with other types of endocrine cancers in what is called multiple endocrine neoplasia syndrome. (see page 726 in Chapter 149, Multiple Endocrine Neoplasia Syndromes)
Treatment requires removing the thyroid gland completely. Additional surgery may be needed so that doctors can determine whether the cancer has spread to the lymph nodes. More than two thirds of the people whose medullary thyroid cancer is part of multiple endocrine neoplasia syndrome live for at least 10 more years. When medullary thyroid cancer occurs alone, the chances of survival are not as good.
Because medullary thyroid cancer sometimes runs in families, close blood relatives of a person with this type of cancer should be screened for a genetic abnormality that can be easily detected in blood cells. If the screening test result is negative, the relative will almost certainly not develop medullary cancer. If the screening test result is positive, then the relative has or will develop medullary cancer, and thyroid surgery should be considered even before symptoms develop and the blood level of calcitonin rises. A high blood calcitonin level or an excessive rise in the level following stimulation tests also helps a doctor predict whether someone has or will develop medullary cancer. Finding an unusually high level of calcitonin will lead the doctor to suggest removing the thyroid gland, since early treatment provides the best chance of cure.
The Merck Manual of Medical Information--Home Edition
Section 13. Hormonal Disorders
Chapter 145
Thyroid Gland Disorders
The thyroid is a small gland, measuring about 2 inches across, that lies just under the skin below the Adam's apple in the neck. The two halves (lobes) of the gland are connected in the middle (called the isthmus), so the thyroid gland resembles the letter H or a bow tie. Normally, the thyroid gland can't be seen and can barely be felt, but if it becomes enlarged, a doctor can feel it easily and a prominent bulge (goiter) may appear below or to the sides of the Adam's apple.
The thyroid gland secretes thyroid hormones, which control the speed at which the body's chemical functions proceed (metabolic rate). Thyroid hormones influence the metabolic rate in two ways: by stimulating almost every tissue in the body to produce proteins and by increasing the amount of oxygen that cells use. When the cells work harder, body organs work faster.
To produce thyroid hormones, the thyroid gland needs iodine, an element contained in food and water. The thyroid gland traps iodine and processes it into thyroid hormones. As thyroid hormones are used up, some of the iodine contained in the hormones returns to the thyroid gland and is recycled to produce more thyroid hormones.
The body has a complex mechanism for adjusting the level of thyroid hormones. First, the hypothalamus, located just above the pituitary gland in the brain, secretes thyrotropin-releasing hormone, which causes the pituitary gland to produce thyroid-stimulating hormone. Just as the name suggests, thyroid-stimulating hormone stimulates the thyroid gland to produce thyroid hormones. When the amount of thyroid hormones circulating in the blood reaches a certain level, the pituitary gland produces less thyroid-stimulating hormone; when the amount of thyroid hormones circulating in the blood decreases, the pituitary gland produces more thyroid-stimulating hormone--a negative feedback control mechanism.
Thyroid hormones are found in two forms. Thyroxine (T4), the form produced in the thyroid gland, has only a slight, if any, effect on speeding up the body's metabolic rate. Thyroxine is converted in the liver and other organs to the metabolically active form, triiodothyronine (T3). This conversion produces about 80 percent of the active form of the hormone; the remaining 20 percent is produced and secreted by the thyroid gland itself. Many factors control the conversion of T4 to T3 in the liver and other organs, including the body's needs from moment to moment. Most of the T4 and T3 is tightly bound to certain proteins in the blood and is active only when not bound to these proteins. In this remarkable way, the body maintains the correct amount of thyroid hormone needed to keep a steady metabolic rate.
For the thyroid gland to function normally, many factors must work well together: the hypothalamus, the pituitary gland, the thyroid hormone--binding proteins in the blood, and the conversion, in the liver and other tissues, of T4 to T3.
Laboratory Tests
To determine how well the thyroid gland is functioning, doctors use several laboratory tests. One of the most common is a test to measure the level of thyroid-stimulating hormone in the blood. Because this hormone stimulates the thyroid gland, blood levels are high when the thyroid gland is underactive (and thus needs more stimulation) and low when the thyroid gland is overactive (and thus needs less stimulation). If the pituitary gland isn't functioning normally (although this rarely happens), the level of thyroid-stimulating hormone alone won't accurately reflect thyroid gland function, and doctors then measure the level of free T4.
Measuring the level of thyroid-stimulating hormone and the level of free T4 circulating in the blood is usually all that is needed. However, doctors may also measure the level of a protein called thyroxine-binding globulin, because abnormal levels of this protein can lead to misinterpretation of a person's total thyroid hormone level. People who have kidney disease, some genetic disorders, or certain other diseases or who take anabolic steroids have lower levels of thyroxine-binding globulin. Conversely, levels of thyroxine-binding globulin may be increased in women who are pregnant or taking oral contraceptives or other forms of estrogen, in people in the early stages of hepatitis, and in people with some other diseases.
Some tests can be performed on the thyroid gland itself. For instance, if a doctor feels a growth in the thyroid gland, an ultrasound examination may be ordered; this procedure uses sound waves to determine whether the growth is solid or filled with fluid. A thyroid scan uses radioactive iodine or technetium and a device to produce a picture of the thyroid gland that will show any physical abnormalities. Thyroid scanning can also help the doctor determine whether the function of an area is normal, overactive, or underactive compared with the rest of the gland.
On rare occasions when a doctor isn't sure whether the problem lies in the thyroid gland or in the pituitary gland, functional stimulation tests may be ordered. One of these tests involves injecting thyrotropin-releasing hormone intravenously and then using blood tests to measure the pituitary gland's response.
Euthyroid Sick Syndrome
In the euthyroid sick syndrome, thyroid test results are abnormal even though the thyroid gland is functioning normally.
The euthyroid sick syndrome commonly occurs in people who have a severe illness other than thyroid disease. When people are sick, are malnourished, or have had surgery, the T4 form of thyroid hormone isn't converted normally to the T3 form. Large amounts of reverse T3, an inactive form of thyroid hormone, accumulate. Despite this abnormal conversion, the thyroid gland continues to function and to control the body's metabolic rate normally. Because no problem exists with the thyroid gland, no treatment is needed. Laboratory tests show normal results once the underlying illness resolves.
Symptoms of Thyroid Disease
Hyperthyroidism (too much thyroid hormone) Hypothyroidism (too little thyroid hormone)
Fast heartbeat
High blood pressure
Moist skin and increased sweat
Shakiness and tremor
Nervousness
Increased appetite with weight loss
Sleep difficulties
Frequent bowel movements and diarrhea
Weakness
Raised, thickened skin over shins
Swollen, reddened, bulging eyes
Sensitivity of eyes to light
Constant stare
Confusion Slow pulse
Hoarse voice
Slowed speech
Puffy face
Loss of eyebrows
Drooping eyelids
Intolerance to cold
Constipation
Weight gain
Sparse, coarse, dry hair
Dry, scaly, thick, coarse skin; raised, thickened skin over shins
Carpal tunnel syndrome
Confusion
Depression
Dementia
Hyperthyroidism
Hyperthyroidism--a condition in which the thyroid gland is overactive--develops when the thyroid produces too much hormone.
Hyperthyroidism has several causes, including immunologic reactions (believed to be the cause of Graves' disease). People with thyroiditis, an inflammation of the thyroid gland, typically go through a phase of hyperthyroidism. However, the inflammation may damage the thyroid gland, so that its initial overactivity is a prelude to either transient (more common) or permanent underactivity (hypothyroidism).
Toxic thyroid nodules (adenomas), areas of abnormal tissue growth within the thyroid gland, sometimes escape the mechanisms that normally control the thyroid gland and produce thyroid hormone in large quantities. A person may have one nodule or many. Toxic multinodular goiter (Plummer's disease), a disorder in which there are many nodules, is uncommon in adolescents and young adults and tends to increase with age.
In hyperthyroidism, regardless of the cause, the body's functions speed up. The heart pounds, beats more quickly, and may develop an abnormal rhythm, leading to an awareness of the heartbeat (palpitations). Blood pressure is likely to increase. Many people with hyperthyroidism feel warm even in a cool room. Their skin may become moist as they tend to sweat profusely, and their hands may develop a fine tremor. Many people feel nervous, tired, and weak, yet have an increased level of activity; have an increased appetite, yet lose weight; sleep poorly; and have frequent bowel movements, occasionally with diarrhea.
Older people with hyperthyroidism may not develop these characteristic symptoms but have what is sometimes called apathetic or masked hyperthyroidism. They simply become weak, sleepy, confused, withdrawn, and depressed. However, heart problems, especially abnormal heart rhythms, are seen more often in older people with hyperthyroidism.
Hyperthyroidism can cause changes in the eyes: puffiness around the eyes, increased tear formation, irritation, and unusual sensitivity to light. The person appears to stare. These eye symptoms disappear soon after the thyroid hormone secretion is controlled, except in people with Graves' disease, which causes special eye problems.
Hyperthyroidism may take the form of Graves' disease, toxic nodular goiter, or secondary hyperthyroidism.
Graves' Disease
Graves' disease (toxic diffuse goiter) is believed to be caused by an antibody that stimulates the thyroid to produce too much thyroid hormone. People with Graves' disease have the typical signs of hyperthyroidism and three distinctive additional symptoms. Since the entire gland is stimulated, it can become greatly enlarged, causing a bulge in the neck (goiter). People with Graves' disease may also have bulging eyes (exophthalmos) (see page 1034 in Chapter 220, Eyelid and Tear Gland Disorders) and, less commonly, raised areas of skin over the shins.
The eyes bulge outward because of a substance that builds up in the orbit. This bulging occurs in addition to the intense stare and other eye changes of hyperthyroidism. The muscles that move the eyes become unable to function properly, making it difficult or impossible to move the eyes normally or to coordinate eye movements, resulting in double vision. The eyelids may not close completely, exposing the eyes to injury from foreign particles and dryness. These eye changes may begin years before any other symptoms of hyperthyroidism, providing an early clue to Graves' disease, or may not occur until other symptoms are noticed. Eye symptoms may even appear or worsen after the excessive thyroid hormone secretion has been treated and controlled.
Eye symptoms may be helped by elevating the head of the bed, by applying eyedrops, by sleeping with the eyelids taped shut, and occasionally by taking diuretics. The double vision may be helped by using eyeglass prisms. Finally, oral corticosteroid drugs, x-ray treatment to the orbits, or eye surgery may be needed.
In Graves' disease, a substance similar to the one deposited behind the eyes may be deposited in the skin, usually over the shins. The thickened area may be itchy and red and feels hard when pressed with a finger. As with deposits behind the eyes, this problem may begin before or after other symptoms of hyperthyroidism are noticed. Corticosteroid creams or ointments can help relieve the itching and hardness. Often the problem disappears without treatment for no apparent reason months or years later.
Toxic Nodular Goiter
In toxic nodular goiter, one or more nodules in the thyroid produce too much thyroid hormone and aren't under the control of thyroid-stimulating hormone. The nodules are true hyperfunctioning benign thyroid tumors and are not associated with the bulging eyes and skin problems of Graves' disease.
Secondary Hyperthyroidism
Hyperthyroidism may (rarely) be caused by a pituitary tumor that secretes too much thyroid-stimulating hormone, which in turn stimulates the thyroid to overproduce thyroid hormones. Another rare cause of hyperthyroidism is pituitary resistance to thyroid hormone, which results in the pituitary gland secreting too much thyroid-stimulating hormone.
Women with a hydatidiform mole (see page 1113 in Chapter 239, Cancers of the Female Reproductive System) may also have hyperthyroidism because the thyroid gland is overstimulated by the high levels of human chorionic gonadotropin in the blood. The hyperthyroidism disappears after the molar pregnancy is terminated and human chorionic gonadotropin vanishes from the blood.
Complications
Thyroid storm, sudden extreme overactivity of the thyroid gland, may produce fever, extreme weakness and loss of muscle, restlessness, mood swings, confusion, altered consciousness (even coma), and an enlarged liver with mild jaundice. Thyroid storm is a life-threatening emergency requiring prompt treatment. Severe strain on the heart can lead to a life-threatening irregular heartbeat (arrhythmia) and shock.
Thyroid storm is generally caused by untreated or inadequately treated hyperthyroidism and can be triggered by infection, trauma, surgery, poorly controlled diabetes, fear, pregnancy or labor, discontinuance of thyroid medication, or other stresses. It's rare in children.
Treatment
Hyperthyroidism can usually be treated with medication, but other options include surgically removing the thyroid gland or treating it with radioactive iodine. Each treatment has advantages and disadvantages.
The thyroid gland needs a small amount of iodine to work properly, but a large amount of iodine decreases the amount of hormone the gland makes and prevents the gland from releasing excess thyroid hormone. Therefore, doctors can use large doses of iodine to stop the gland from secreting excess thyroid hormone. Iodine treatment is particularly useful when doctors need to control hyperthyroidism quickly, as during thyroid storm or before emergency surgery. However, iodine isn't used for routine or long-term treatment of hyperthyroidism.
Propylthiouracil or methimazole, the drugs most commonly used to treat hyperthyroidism, slow thyroid function by decreasing the gland's production of thyroid hormone. Both drugs are taken orally, beginning with high doses that are later adjusted according to the results of thyroid hormone blood tests. These drugs can usually control thyroid function in 6 weeks to 3 months, but larger doses of the drugs may bring it under control faster--with an increased risk of adverse effects. These adverse effects include allergic reactions (most commonly, skin rashes), nausea, loss of taste, and on rare occasions, depressed synthesis of blood cells in the bone marrow. The bone marrow depression can deplete the number of white blood cells, creating a life-threatening situation in which the person is vulnerable to infection. While these two drugs are comparable in most ways, propylthiouracil may be safer than methimazole for use in pregnant women because less of it reaches the fetus. Carbimazole, a drug that is widely used in Europe, is converted into methimazole in the body.
Beta-blocking drugs such as propranolol help control some of the symptoms of hyperthyroidism. These drugs are effective in slowing down a fast heart rate, reducing shakiness (tremor), and controlling anxiety. Doctors therefore find beta-blockers particularly useful for people with thyroid storm and for people with bothersome or dangerous symptoms whose hyperthyroidism hasn't yet been brought under control by other treatments. However, beta-blockers don't control abnormal thyroid function.
Hyperthyroidism can also be treated with radioactive iodine, which destroys the thyroid gland. Radioactive iodine taken orally introduces very little radioactivity to the body as a whole but a great deal to the thyroid gland. Doctors try to adjust the dose of radioactive iodine to destroy only enough of the thyroid gland to bring its hormone production back to normal, without reducing thyroid function too much. However, most of the time, radioactive iodine treatment ultimately creates hypothyroidism (an underactive thyroid gland), a condition that requires thyroid hormone replacement therapy. People who need thyroid hormone replacement therapy take a thyroid hormone tablet daily for the rest of their lives to replace the natural hormone that's no longer being produced in sufficient quantities. About 25 percent of the people have hypothyroidism 1 year after radioactive iodine treatment, but the percentage increases steadily over the next 20 years or more. Concern that radioactive iodine may cause cancer has never been confirmed. Radioactive iodine isn't given to pregnant women, since it crosses the placenta and may destroy the fetus' thyroid gland.
In a thyroidectomy, the thyroid gland is removed surgically. Surgery is an option especially for young people with hyperthyroidism. Surgery is also an option for people who have a very large goiter, as well as for people who are allergic to or who develop severe side effects from the drugs used to treat hyperthyroidism. Hyperthyroidism is permanently controlled in more than 90 percent of those who choose this option. Some degree of hypothyroidism occurs after surgery in some people, who then have to take replacement thyroid hormone for the rest of their lives. Rare complications include paralysis of the vocal cords and damage to the parathyroid glands (the tiny glands behind the thyroid gland that control calcium levels in the blood).
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland is underactive and produces too little thyroid hormone. Very severe hypothyroidism is called myxedema.
In Hashimoto's thyroiditis, the most common cause of hypothyroidism, (see page 709 in this chapter) the thyroid gland is often enlarged, and hypothyroidism frequently results years later because the gland's functioning areas are gradually destroyed. The second most common cause of hypothyroidism is treatment of hyperthyroidism. Both radioactive iodine treatment and surgery tend to produce hypothyroidism.
A chronic lack of iodine in the diet produces an enlarged, underactive thyroid gland (goitrous hypothyroidism), the most common cause of hypothyroidism in many undeveloped countries. Since salt manufacturers began adding iodine to table salt and iodine-containing disinfectants are often used to sterilize cow's udders, this form of hypothyroidism has disappeared in the United States. Even rarer causes of hypothyroidism include some inherited disorders in which an abnormality of the enzymes in thyroid cells prevents the gland from making or secreting enough thyroid hormones. In other rare disorders, either the hypothalamus or the pituitary gland fails to secrete enough of the hormone needed to stimulate normal thyroid function.
Symptoms
Insufficient thyroid hormone causes bodily functions to slow down. In sharp contrast to hyperthyroidism, the symptoms of hypothyroidism are subtle and gradual and may be mistaken for depression. Facial expressions become dull, the voice is hoarse and speech is slow, eyelids droop, and the eyes and face become puffy and swollen. Many people with hypothyroidism gain weight, become constipated, and are unable to tolerate cold. The hair becomes sparse, coarse, and dry and the skin becomes coarse, dry, scaly, and thick. Many people develop carpal tunnel syndrome, which makes the hands tingle or hurt. (see page 336 in Chapter 70, Peripheral Nerve Disorders) The pulse may slow, the palms and soles appear slightly orange (carotenemia), and the side part of the eyebrows slowly falls out. Some people, especially older people, may appear confused, forgetful, or demented--signs that can easily be mistaken for Alzheimer's disease or other forms of dementia.
If untreated, hypothyroidism can eventually cause anemia, a low body temperature, and heart failure. This situation may progress to confusion, stupor, or coma (myxedema coma), a life-threatening complication in which breathing slows, the person has seizures, and blood flow to the brain decreases. Myxedema coma can be triggered by exposure to the cold as well as by an infection, trauma, and drugs such as sedatives and tranquilizers that depress brain function.
Treatment
Hypothyroidism is treated by replacing the deficient thyroid hormone, using one of several different oral preparations. The preferred form is synthetic thyroid hormone, T4. Another form, desiccated (dried) thyroid, is obtained from the thyroid glands of animals. In general, doctors find desiccated thyroid less satisfactory because the dose is harder to adjust and the tablets have variable amounts of T3.
Treatment in an older person begins with small doses of thyroid hormone because too large a dose can cause serious side effects. The dose is gradually increased until the person's blood levels of thyroid-stimulating hormone return to normal. Medication is usually taken for life. In emergencies, such as myxedema coma, doctors may give thyroid hormone intravenously.
Thyroiditis
Thyroiditis, an inflammation of the thyroid gland, produces transient hyperthyroidism often followed by transient hypothyroidism or no change in thyroid function at all.
The three types of thyroiditis are Hashimoto's thyroiditis, subacute granulomatous thyroiditis, and silent lymphocytic thyroiditis.
Hashimoto's Thyroiditis
Hashimoto's thyroiditis (autoimmune thyroiditis) is the most common type of thyroiditis and the most common cause of hypothyroidism. For unknown reasons, the body turns against itself in an autoimmune reaction, creating antibodies that attack the thyroid gland. (see page 816 in Chapter 168, Immunodeficiency Disorders) This type of thyroiditis is most common in elderly women and tends to run in families. The condition occurs eight times more often in women than in men and may occur in people with certain chromosomal abnormalities, including Turner's, Down, and Klinefelter's syndromes.
Hashimoto's thyroiditis often begins with a painless enlargement of the thyroid gland or a feeling of fullness in the neck. When doctors feel the gland, they usually find it enlarged, with a rubbery texture, but not tender; sometimes it feels lumpy. The thyroid gland is underactive in about 20 percent of the people when Hashimoto's thyroiditis is discovered; the rest have normal thyroid function. Many people with Hashimoto's thyroiditis have other endocrine disorders such as diabetes, an underactive adrenal gland, or underactive parathyroid glands, and other autoimmune diseases such as pernicious anemia, rheumatoid arthritis, Sjögren's syndrome, or systemic lupus erythematosus (lupus).
Doctors perform thyroid function tests on blood samples to determine whether the gland is functioning normally, but they base the diagnosis of Hashimoto's thyroiditis on the symptoms, a physical examination, and whether the person has antibodies that attack the gland (antithyroid antibodies), which can easily be measured in a blood test.
No specific treatment is available for Hashimoto's thyroiditis. Most people eventually develop hypothyroidism and must take thyroid hormone replacement therapy for the rest of their lives. Thyroid hormone may also be useful in decreasing the enlarged thyroid gland.
Subacute Granulomatous Thyroiditis
Subacute granulomatous (giant cell) thyroiditis, which is probably caused by a virus, begins much more suddenly than Hashimoto's thyroiditis. Subacute granulomatous thyroiditis often follows a viral illness and begins with what many people call a sore throat but actually proves to be neck pain localized to the thyroid. The thyroid gland becomes increasingly tender, and the person usually develops a low-grade fever (99° F. to 101° F.). The pain may shift from one side of the neck to the other, spread to the jaw and ears, and hurt more when the head is turned or when the person swallows. Subacute granulomatous thyroiditis is often mistaken at first for a dental problem or a throat or ear infection.
Inflammation usually causes the thyroid gland to release excessive thyroid hormones, resulting in hyperthyroidism, almost always followed by transient hypothyroidism. Many people with subacute granulomatous thyroiditis feel extremely tired.
Most people recover completely from this type of thyroiditis. Generally the condition goes away by itself within a few months, but sometimes it comes back or, more rarely, damages enough of the thyroid gland to cause permanent hypothyroidism.
Aspirin or other nonsteroidal anti-inflammatory drugs (such as ibuprofen) can relieve the pain and inflammation. In very severe cases, doctors may recommend corticosteroids such as prednisone, which should be tapered off over 6 to 8 weeks. When corticosteroids are stopped abruptly, symptoms often return in full force.
Silent Lymphocytic Thyroiditis
Silent lymphocytic thyroiditis occurs most often in women, typically just after childbirth, and causes the thyroid to become enlarged without becoming tender. For several weeks to several months, a person with silent lymphocytic thyroiditis has hyperthyroidism followed by hypothyroidism before eventually recovering normal thyroid function. This condition requires no specific treatment, although the hyperthyroidism or hypothyroidism may require treatment for a few weeks. Often, a beta-blocker such as propranolol is the only drug needed to control the symptoms of hyperthyroidism. During the period of hypothyroidism, a person may need to take thyroid hormone, usually for no more than a few months. Hypothyroidism becomes permanent in about 10 percent of the people with silent lymphocytic thyroiditis.
Thyroid Cancer
Thyroid cancer is any one of four main types of malignancy of the thyroid: papillary, follicular, anaplastic, or medullary.
Thyroid cancer is more common in people who have been treated with radiation to the head, neck, or chest, most often for benign conditions (although radiation treatment for benign conditions is no longer carried out). Rather than causing the whole thyroid gland to enlarge, a cancer usually causes small growths (nodules) within the thyroid. Most thyroid nodules aren't cancerous, and thyroid cancers can generally be cured. Thyroid cancers often have a limited ability to take up iodine and produce thyroid hormone, but very rarely they produce enough hormone to cause hyperthyroidism. Nodules are more likely to be cancerous if only one nodule is found rather than several, if a thyroid scan shows that the nodule isn't functioning, if the nodule is solid rather than filled with fluid (cystic), if the nodule is hard, or if the nodule is growing quickly.
A painless lump in the neck is usually the first sign of thyroid cancer. When doctors find a nodule in the thyroid gland, they request several tests. A thyroid scan determines whether the nodule is functioning, since a nonfunctioning nodule is more likely to be cancerous than a functioning one. An ultrasound scan is less helpful but may be performed to determine whether the nodule is solid or filled with fluid. A sample of the nodule is usually taken by fine-needle biopsy for examination under a microscope--the best way to determine whether the nodule is cancerous.
Papillary Cancer
Papillary cancer accounts for 60 to 70 percent of all thyroid cancers. Two to three times as many women as men have papillary cancer; however, since nodules are far more common in women, a nodule in a man is more suspicious for a cancer. Papillary cancer is more common in young people but grows and spreads more quickly in the elderly. People who have received radiation treatment to the neck, usually for a benign condition in infancy or childhood or for some other cancer in adulthood, are at greater risk of developing papillary cancer.
Surgery is the treatment for papillary cancer, which sometimes spreads to nearby lymph nodes. Nodules smaller than three quarters of an inch across are removed along with the thyroid tissue immediately surrounding them, although some experts recommend removing the entire thyroid gland. Surgery almost always cures these small cancers.
Since papillary cancer may respond to thyroid-stimulating hormone, thyroid hormone is taken in doses large enough to suppress secretion of thyroid-stimulating hormone and help prevent a recurrence. If a nodule is larger, most or all of the thyroid gland is usually removed, and radioactive iodine is often given in expectation that any remaining thyroid tissue or cancer that has spread away from the thyroid will take it up and be destroyed. Another dose of radioactive iodine may be needed to make sure the entire cancer has been destroyed. Papillary cancer is almost always cured.
Follicular Cancer
Follicular cancer accounts for about 15 percent of all thyroid cancers and is more common in the elderly. Follicular cancer is also more common in women than in men, but as with papillary cancer, a nodule in a man is more likely to be cancer. Much more aggressive than papillary cancer, follicular cancer tends to spread through the bloodstream, spreading cancerous cells to various parts of the body (metastases). Treatment for follicular cancer requires surgically removing as much of the thyroid gland as possible and destroying any remaining thyroid tissue, including the metastases, with radioactive iodine.
Anaplastic Cancer
Anaplastic cancer accounts for less than 10 percent of thyroid cancers and occurs most commonly in elderly women. This cancer grows very quickly and usually causes a large growth in the neck. About 80 percent of the people with anaplastic cancer die within 1 year. Treatment with radioactive iodine is useless because anaplastic cancers don't take up radioactive iodine. However, treatment with anticancer drugs and radiation therapy before and after surgery has resulted in some cures.
Medullary Cancer
In medullary cancer, the thyroid gland produces excessive amounts of calcitonin, a hormone produced by certain thyroid cells. Because medullary thyroid cancer can also produce other hormones, it can cause unusual symptoms. This cancer tends to spread (metastasize) through the lymphatic system to the lymph nodes and through the blood to the liver, lungs, and bones. Medullary cancer can develop along with other types of endocrine cancers in what is called multiple endocrine neoplasia syndrome. (see page 726 in Chapter 149, Multiple Endocrine Neoplasia Syndromes)
Treatment requires removing the thyroid gland completely. Additional surgery may be needed so that doctors can determine whether the cancer has spread to the lymph nodes. More than two thirds of the people whose medullary thyroid cancer is part of multiple endocrine neoplasia syndrome live for at least 10 more years. When medullary thyroid cancer occurs alone, the chances of survival are not as good.
Because medullary thyroid cancer sometimes runs in families, close blood relatives of a person with this type of cancer should be screened for a genetic abnormality that can be easily detected in blood cells. If the screening test result is negative, the relative will almost certainly not develop medullary cancer. If the screening test result is positive, then the relative has or will develop medullary cancer, and thyroid surgery should be considered even before symptoms develop and the blood level of calcitonin rises. A high blood calcitonin level or an excessive rise in the level following stimulation tests also helps a doctor predict whether someone has or will develop medullary cancer. Finding an unusually high level of calcitonin will lead the doctor to suggest removing the thyroid gland, since early treatment provides the best chance of cure.
