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The Thyroid Hormone Breakthrough
About the Thyroid and Thyroid Disease
The reasonable man adapts himself to the world;
the unreasonable one persists in trying to
adapt the world to himself.
Therefore all progress depends on the unreasonable man.
—George Bernard Shaw
The normal thyroid is a small gland weighing about an ounce that sits behind the Adam's apple in the lower part of the neck, in front of the windpipe.
It derives its name from the Greek word thyreoeides, meaning "shield-shaped." In fact, it looks like a bowtie or butterfly, with the two "wings," or lobes, of the gland connected in the middle by the isthmus.
The thyroid, like other glands, is a discrete soft body made up of a large number of vessels that produce, store, and release—or secrete—some substance. Some glands secrete their products outside the body, some inside. Those that secrete hormones and metabolic substances on the inside of the body are known as endocrine glands. The endocrine glands include the thyroid, the parathyroids, the adrenal gland, the pancreas, the pituitary gland, the pineal gland, the gonads (ovaries and testes), and the thymus.
Doctors who specialize in treating patients with endocrine problems—disorders of the endocrine glands—are called endocrinologists.
Hormones are internal secretions carried in the blood to various organs. The thyroid's main purpose is to produce, store, and release two key hormones: triiodothyronine, also called T3; and thyroxine, or T4. The numbers 3 and 4 refer to the numberof iodine molecules attached to each hormone.
Thyroid cells are the primary cells in the body capable of absorbing iodine, an essential nutrient. The thyroid takes in iodine, obtained through food, iodized salt, or supplements, and combines that iodine with the amino acid tyrosine, converting them to T3 and T4.
A healthy thyroid produces about 20 percent T3 and 80 percent T4. T3 is the biologically active hormone that is used by the cells; it is several times stronger than T4. As needed, the body converts the inactive T4 to active T3 by removing one iodine molecule. This conversion process is called monodeiodination. This conversion can take place in certain organs other than the thyroid, including the hypothalamus, a part of your brain.
T3 and T4 both exist in two forms: unbound and bound. Unbound, or free, T3 or T4 are biologically active; bound T3 and T4 are attached to the thyroid-binding globulin (TBG) protein. When measured in the blood, the free, or unbound, T3 and T4 levels tend to be most representative of the actual hormone available for use by the body.
The role of your thyroid hormones is to control your metabolism—the process by which oxygen and calories are converted to energy for use by your cells and organs. There's not a single cell in your body that doesn't depend on thyroid hormone for regulation and for energy in some form. And the thyroid hormones have a number of functions as they travel through the bloodstream. They
Enable cells to convert oxygen and calories into energy
Help the body process carbohydrates
Aid in the proper functioning of muscles
Enable proper sexual development and functioning
Help the heart pump properly and effectively
Help the body to breathe normally
Help the intestinal system digest and eliminate food
Strengthen hair, nails, and skin
Help the brain to function properly
Help with normal bone growth
Now that you have some idea of what the thyroid is and its location and function, let's go into more detail about how it fits into the overall functioning of the body.
The Thyroid Gland: Setting the Pace
When your thyroid works normally, it produces and secretes the amount of T3 and T4 necessary to keep various body functions moving at their proper pace. However, the thyroid does not do this alone. It works as part of a bigger system, one that includes the pituitary gland and the hypothalamus.
Here's how the system works. The hypothalamus constantly monitors the pace of many of the body's functions. It also monitors and reacts to a number of other factors, including environmental factors such as heat, cold, and stress. If the hypothalamus senses that certain adjustments are needed to react to any of these factors, then it produces thyrotropin-releasing hormone, known as TRH.
TRH is sent from the hypothalamus to the pituitary gland. The pituitary gland is stimulated to produce a substance called thyrotropin, but better known as thyroid-stimulating hormone, or TSH for short. The pituitary gland also monitors the body and can release TSH based on the thyroid hormone levels circulating in your blood.
TSH is sent to the thyroid gland, where it causes the thyroid to produce, store, and release more T3 and T4 thyroid hormones.
The released thyroid hormones move into the bloodstream, carried by a plasma protein known as thyroxine-binding globulin (TBG).
Now in the bloodstream, the thyroid hormone travels throughout the body, carrying orders to the various organs. Upon arriving at a particular tissue in the body, thyroid hormones interact with receptors located inside the nucleus of the cells. Interaction of the hormone and the receptor will trigger a certain function, giving directions to that tissue regarding the rate at which it should operate.
When the hypothalamus senses that the need for increased thyroid hormone production has ended, it reduces production of TRH. The reduced production of TRH in turn causes the pituitary to decrease production of TSH, and the reduced TSH levels send the message to the gland itself to slow production of thyroid hormone. By this system, many of the body's organs keep working at the proper pace.
Think of the entire feedback loop as somewhat like the thermostat in your house. It's set to maintain a particular temperature, and when it detects that your house has become too hot, it signals to either stop blowing heat (or start blowing air conditioning). And when the house becomes too cold, the heat will kick on (or the air conditioning will turn off). In similar fashion, your body is set to maintain a certain level of circulating thyroid hormone.
When thyroid disease or other conditions interfere with the system and the feedback process doesn't work, however, thyroid problems can develop.The Thyroid Hormone Breakthrough. Copyright © by Mary Shomon. Reprinted by permission of HarperCollins Publishers, Inc. All rights reserved. Available now wherever books are sold.