Thyroid Disease: Understanding hypothyroidism and hyperthyroidism

Unseen and often misunderstood, the thyroid gland is small but enormously important to human health. Like the director of a feature film, the thyroid works behind the scenes, influencing everything from metabolism to emotion. Many people do not realize how important thyroid health is to their overall well-being, until something goes wrong. Then a faulty performance has an impact on vital organs including the heart and brain as well as mood, strength, energy level, and more.

Normally, the thyroid works day and night producing hormones that control the pace at which each of your cells performs its functions. So when disease causes this busy gland to slack off and underproduce thyroid hormone, or overwork and produce too much of it, you'll know something isn't right. But it may take some time to figure out what. The symptoms of thyroid diseases are so wide-ranging even doctors often don't realize that the thyroid is to blame. Hypothyroidism, an underactive thyroid, leads to symptoms as diverse as depression, hair loss, weight gain, dry skin, and feeling cold and continually tired. Hyperthyroidism, an overactive thyroid, can make you lose weight and feel nervous, anxious, warm, and constantly hungry.

The risk of thyroid disease increases with age. Yet, complicating matters, thyroid disease is most difficult to detect in people over 60 because it typically masquerades as another illness, such as heart disease, depression, or dementia. Misleading symptoms may be the reason that many Americans who have thyroid disease — mostly women — do not yet know they have it.

How many people have thyroid disease? Estimates vary widely, in part because experts disagree about how to define a normal result on a blood test used to detect thyroid conditions (see "The TSH result: Should the normal range be lower?"). The most reliable number available comes from the third U.S. National Health and Nutrition Examination Survey (NHANES III) of people ages 12 and older, published in 2002. The survey reported that nearly 6% of the U.S. population, or 12.2 million people, have thyroid disease. Most of these people, about 9.6 million, have hypothyroidism. A much smaller portion, 2.6 million people, have hyperthyroidism. But the population is aging, and the proportion of people with thyroid conditions is increasing.

While many doctors do check thyroid functions periodically, routine screening for the general population is not universal. Therefore, it is up to you to ask your doctor to evaluate your thyroid if you suspect you have a problem and work closely with your doctor to bring your thyroid levels back to normal.

Your thyroid gland

If you've never been diagnosed with a thyroid problem, chances are you're not entirely sure where your thyroid gland is, let alone what it does. This small, butterfly-shaped gland weighs less than an ounce. When functioning normally, it perches unobtrusively with its wings wrapped around the front of your windpipe (trachea), below your voice box (larynx). Its slight size could easily fool you into underestimating the thyroid's importance to your health. Yet this gland controls the rate at which every cell, tissue, and organ in your body functions, from your muscles, bones, and skin to your digestive tract, brain, heart, and more. It does this primarily by secreting hormones that control how fast and efficiently cells convert nutrients into energy — a chemical activity known as metabolism — so that the cells can perform their functions.

Figure 1: Normal thyroid Normally, the hypothalamus sends a signal in the form of thyrotropin-releasing hormone (TRH) that enables the pituitary gland to secrete thyroid-stimulating hormone (TSH). In response, the thyroid gland releases T4 and a small amount of T3. These travel to the liver and other organs, where T4 is converted to T3 and enters the bloodstream. Like a heating system with a thermostat set at a constant temperature, a normally functioning thyroid operates at a steady pace without much variation.

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