Kathryn Jean Lucas, MD |
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Kathryn Jean Lucas, MD |
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Thyroid Information
| inside health |
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| THYROID INFORMATION |
| By: K. Jean Lucas, M.D. |
WHAT IS THE THYROID?
The thyroid is one of the many glands in the body that make special chemicals called hormones. Hormones, substances released from glands, travel in the bloodstream to affect many different parts of the body. For example, the ovary is a gland which makes estrogen. The adrenal gland makes cortisone. The thyroid gland makes thyroid hormone.
The pituitary gland (the master gland) stimulates the thyroid to make, store, and release thyroid hormone. The pituitary gland is responsible for controlling the secretion of many of the glands in our body including ovaries, testes, and adrenal glands in addition to the thyroid. The signal sent out by the pituitary to control the thyroid is called TSH (thyroid stimulating hormone).
The thyroid uses iodine in order to make thyroid hormones. The more important of these are T4 (thyroxine) and T3 (triiodothyronine). The number after the "T" refers to the number of iodine atoms contained in each hormone molecule. These hormones are made and then stored by the thyroid until they are needed. When they are needed, they are released into the blood and transported in the blood by attaching to certain special proteins, TBG (thyroid binding globulin). Thyroid hormone then detaches from TBG in order to enter the cells and affect their functioning.
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The thyroid is located in the front and lower part of the neck. The two lobes of the thyroid are on either side of the windpipe just below the laryngeal cartilage (Adam's apple) and are joined together by a small amount of thyroid tissue called the isthmus. If your thyroid is enlarged, you may be able to see it. When you swallow, the thyroid is the only part of the neck which moves up and down. Normally, the thyroid is difficult to see, but when a nodule is present or if the thyroid is enlarged, you may be able to see it when swallowing. |
The thyroid gland, although frequently misunderstood and mysterious to most people, plays an extremely important role in the proper functioning of the entire body. The thyroid regulates energy use by the body. Overall, it determines the rate at which calories are burned for energy. The best way to understand what the thyroid does is to look at what happens when the thyroid hormone levels are too high or too low. When thyroid levels are too high, the body is "revved up". The heart beats harder and faster, more calories are burned for heat, and the body may feel warmer. The skin and hair changes and may become silkier and smoother. Appetite is stimulated; some may lose weight and some may gain weight. One may have difficulty sleeping and be tired. Some may have increased energy if sleeping is not disturbed. On the other hand, not having enough thyroid hormone is similar to an animal in hibernation. The pulse and blood pressure are much lower. The person is colder and feels sleepy, sluggish, and slowed down in body and in mind.
WHO DEVELOPS THYROID DISEASE?
Thyroid disorders may range from an abnormality in the size of the thyroid (such as a goiter or nodule) to a change in the production of thyroid hormone (hyper- and hypothyroidism). The tendency to have a thyroid disorder is inherited. However, the type of thyroid disorder may vary within the family. Some members may have a goiter, others may be hypothyroid, while other members may be hyperthyroid. If anyone in your family has a thyroid problem and you have symptoms of thyroid disease, it would be worthwhile to have it checked out by your physician.
All thyroid disorders are more common in women than in men. Hyperthyroidism is 9-10 times more likely to occur in women than in men. This occurrence may represent the fact that many thyroid disorders are autoimmune diseases in which the body makes antibodies to the thyroid. Autoimmune diseases are more common in women than in men.
In certain parts of the world (but not in the U.S. or Europe), iodine deficiency may play a role in thyroid disorders such as a goiter or hypothyroidism. In the U.S., however, iodine is added to many foods including bread and salt making iodine deficiency virtually nonexistent.
Certain medications such as lithium and amiodarone can cause a thyroid disorder. Exposure to environmental radiation plays a role in the development of thyroid nodules and thyroid cancer.
Stress may play a role in the development of autoimmune thyroid disease because of the effects of stress on the immune system.
WHAT DO THESE THYROID TESTS REALLY MEAN?
Your physician may order thyroid blood tests if you are experiencing any of the following symptoms: fatigue, weight loss or gain, sleep disturbance, skin or hair changes, swelling, cold or heat intolerance, palpitations, excessive nervousness, or change in bowel habits. Thyroid blood tests may also be done if your thyroid gland feels abnormal to your physician. For example, the gland may be enlarged or have irregularities or lumps in it.
T4 and T3, the thyroid hormones that circulate in your bloodstream, are for the most part attached to special transport proteins. To be able to be used by the body, the thyroid hormones must come off the proteins that bind them. The most frequent thyroid blood test done is the thyroid panel which has three components: T4, T3RU, and FTI. The T4 test measures the total amount of T4 circulating in the bloodstream. T3RU or RT3U does not measure any thyroid hormone but rather determines the amount of protein binding of thyroid hormone. Total amounts of T3 can be measured by the test T3RIA. Since T4 is converted into T3 before thyroid hormone can be active in the body, T3RIA can be an important measurement in suspected over activity of the thyroid hyperthyroidism). Free T4 measures the amount of thyroxine which is not on thyroid transport proteins. This test is more accurate in determining the amount of active thyroxine in the body.
Thyroid stimulating hormone (TSH) is made by the pituitary gland to control the production of thyroid hormone by the thyroid gland. If not enough thyroid hormone is being made, the TSH will increase more than normal. The TSH is often the first sign that the thyroid is not working properly; it may be abnormal even when the thyroid panel is in the normal range. If the thyroid is overactive, the TSH will be very low, at times undetectable by the lab. The TRH test which stimulates the production of TSH is used in cases in which the thyroid status (under or overactive) is unclear. If the TSH does not increase after the dose of TRH is given, the thyroid gland may be overactive. If the TSH greatly increases (more than 25) after TRH, the thyroid gland is not producing enough thyroid hormone for that individual.
Higher levels of protein binding lead to higher total levels of T4 but do not necessarily represent over activity of the thyroid gland. Higher binding protein means less thyroid hormone is free in the circulation. Less thyroid hormone is, therefore, available to be bound to a resin. The resin uptake is low in this situation. To give an estimate of the level of free T4, a calculation is done using the T4 and T3RU; its designation may be the T7 or FTI (free thyroxine index). If the T4 is elevated because of protein binding and not because of over activity of the thyroid, the calculation of FTI is usually normal. For example, women on birth control pills have increased binding protein for thyroid hormone; therefore, the T4 is elevated, the T3RU is low, and the FTI is in the normal range. The TSH is also normal. In this situation, no thyroid dysfunction exists, even though two out of three lab tests are abnormal. To determine if a woman on birth control pills or estrogens has a thyroid disorder, the history and physical are extremely important and other blood tests may need to be done. The levels of thyroid hormone not bound to binding protein can be measured (although with more difficulty). These tests are called the free T4 and free T3.
Other thyroid tests include: thyroid antibodies, thyroid stimulating immunoglobulin, thyroglobulin, I-123 uptake and scan, and thyroid ultrasound. The necessity and interpretation for these tests should be discussed on an individual basis with one's physician.
FOR FURTHER INFORMATION
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YOUR THYROID: A Home Reference, L.C. Wood, D.S.Cooper, & E.C. Ridgway, 1982. | ||||
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THE THYROID GLAND: A Book for Thyroid Patients, Joel Hamburger, 1988. | ||||
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HOW YOUR THYROID WORKS, H. Jack Baskin, 1991. | ||||
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COULD IT BE MY THYROID? Sheldon Rubenfeld, 1995 | ||||
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