General Information: Normal and Abnormal Thyroid Function, Thyroid Diseases, Thyroid Tests, and Thyroid Treatments
Your thyroid is one of the many glands in your body that make special chemicals known as hormones. Hormones travel in your bloodstream throughout your body to affect many different parts of your system, including your brain, heart, liver, kidneys, muscles, bones, and skin. Therefore, it is not surprising that a change in any hormone level can produce abnormalities all over your body. Once they arrive at a particular tissue, hormones interact with receptors located either on the outside of the cell or inside the cell in the cytoplasm or nucleus to trigger a certain function. The particular hormone made by your thyroid gland acts on receptors located in the nucleus, affecting the rate at which many bodily processes happen.
Normally, your blood level of thyroid hormone is constant, with little day-to-day variation. However, if the gland becomes diseased, it may produce high thyroid hormone levels that may speed up body processes, causing symptoms like rapid heartbeat (palpitations), nervousness, frequent bowel movements, and weight loss as you burn up calories more rapidly. By contrast, a poorly functioning gland may produce less than a normal amount of thyroid hormone, which may slow your heartbeat and make you tired, depressed, and constipated. A low thyroid hormone level also may cause your skin, hair, and fingernails to grow more slowly, so they become rough, dry, and brittle. You may gain some weight, but usually no more than three or four pounds. This is primarily due to fluid retention rather than an increase in body fat. In short, if your thyroid is underactive, you will probably feel generally “run down.”
Figure 1-1. Your thyroid gland is normally located in the front of your neck below your Adam’s apple.
Your thyroid is normally found in the front of your neck (Figure 1-1). Its two halves, or lobes, together normally weigh about one ounce. They lie on either side of your windpipe, just below your “Adam’s apple,” and are joined together by a narrow band of thyroid tissue known as the isthmus. Occasionally, a small amount of thyroid tissue will project upward from the isthmus along your windpipe. This tissue, called the pyramidal lobe, is a reminder that, before you were born, your thyroid migrated from its place of origin at the back of your tongue down to the front of your neck.
HOW THE THYROID WORKS
Thyroid hormone production starts with iodine. Iodine is found in many foods, especially seafood, salt, bread, and milk. The thyroid takes this dietary iodine from your blood-stream and uses it to make thyroid hormones (Figure 1-2). The two most important of these hormones are triiodothyronine (T3) and thyroxine (T4). (The nicknames “T3” and “T4” refer to the number of iodine atoms contained in each hormone molecule: There are three iodine atoms in T3 and four in T4.) These hormones are stored within your thyroid until they are needed. Then they are released into your bloodstream and transported throughout your body attached to special carrier proteins. After entering the cells of your body tissues, they go into nuclei at the center of each cell, where they attach to specific receptors (Figure 1-3). The receptor-hormone complex binds to various genes in the cell nucleus and instructs these genes to become more or less active. This, in turn, causes the cell to increase or decrease the production of specific proteins that are important for growth and development and for regulating your body’s metabolism.
Figure 1-2. Manufacture, storage, and release of thyroid hormones.
Figure 1-3. The hypothalamic-pituitary-thyroid axis.
Although your thyroid has some inherent ability to produce thyroid hormone by itself, its function is governed largely by the pituitary gland, located at the base of your brain. When your level of thyroid hormone falls too low, the pituitary responds by producing thyroid stimulating hormone (TSH). If your thyroid is healthy, it responds to TSH by working harder, thereby raising the blood level of thyroid hormone back to normal.
Several factors seem to influence the way the pituitary gland controls thyroid function. A low blood level of thyroid hormone, for example, appears to influence the pituitary gland directly, provoking an increase in TSH release. On the other hand, the pituitary is itself under the control of still higher centers in the brain, including the hypothalamus and the cerebral cortex (Figure 1-3). The interactions between these higher centers of the brain and the thyroid are currently under careful study by research workers, but it is likely that the regulation of thyroid function occurs there as well.
FACTORS INVOLVED IN THYROID DISEASES
Various things can go wrong with your thyroid gland: It can produce too much hormone (hyperthyroidism) or too little (hypothyroidism), it can become infected or inflamed (thyroiditis), or it can develop a cyst or a tumor. Perhaps the most common thyroid abnormality is a goiter, a simple enlargement of the thyroid gland. More than one of these conditions may exist or develop over time in a single patient, and different but related thyroid abnormalities may occur in several members of the same family. Although researchers still do not understand all of the mechanisms that produce thyroid disease, we do know a lot about some of the factors involved.
The tendency to develop some thyroid problems appears to be inherited. The best studied of these conditions are some rare disorders that result from a failure in the production of thyroid hormone. In some of these disorders, the thyroid has difficulty getting enough iodine from the bloodstream. In others, there is a problem with the use of the iodine to make the more complex thyroid hormone molecules.
Heredity also appears to have a role in most instances in which the thyroid changes its level of function and becomes either overactive or underactive. New research suggests that there may be two or three different genes or groups of genes that may predispose an individual to these conditions. Yet, even those who have inherited the tendency to develop one of these thyroid conditions may never become ill. Therefore, thyroid disease within a family may seem to “skip” generations: You may have a thyroid problem, and, though your parents seem healthy, you may learn of a grandparent who had thyroid trouble, too. In other instances, several different thyroid problems will show up in one family; one relative may develop an overactive thyroid, another an underactive thyroid. Some may become very sick, while others may be only mildly affected or show only slight enlargement of the thyroid without a change in hormone levels.
Virtually all thyroid disorders appear to be more common in women than in men. Hyperthyroidism, for example, is two to four times more likely to occur in women than men. Thyroid cancer is twice as common in women. As of now, we don’t know why women have this predisposition, but it is likely to be influenced by estrogen. This question continues to be an important focus for research by physicians and other scientists interested in women’s health issues.
Changes in thyroid function can occur at any time from fetal life to old age. But if your thyroid is going to become overactive, you are most likely to be somewhere between the ages of twenty and forty when it happens. On the other hand, if your thyroid fails, it will be more likely to do so at a much later age—probably after you reach fifty years of age. Similarly, certain types of thyroid tumors tend to occur in young people, while others are more common in older individuals. Thyroid cancer tends to be less aggressive and more responsive to treatment in children. By contrast, thyroid cancer in adults over the age of fifty tends to be more aggressive and less responsive to treatment. The reasons for these differences in different age groups remain obscure.
Excerpted from Your Thyroid by Lawrence C. Wood, M.D., David S. Cooper, M.D., E. Chester Ridgway, M.D.. Copyright © 2005 by Lawrence C. Wood, M.D.. Excerpted by permission of Ballantine Books, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.