Your Thyroid (eBook)

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,...