Thyroid disease: Understanding hypothyroidism and hyperthyroidism

By Harvard Health Publications
Content provided by the Faculty of the Harvard Medical School
Excerpted from a Harvard Special Health Report

Dear Reader,

If you are like many of my patients diagnosed with a thyroid condition, you might be surprised that such a tiny gland can have such a profound impact on your overall health and well-being. But it's no wonder, when you consider the enormous job your thyroid has. Throughout life, this busy gland is constantly producing hormones that influence your metabolism. So when disease causes your thyroid gland to slack off and underproduce thyroid hormone, or overwork and produce too much of it, you'll know something isn't right.

The symptoms of thyroid diseases are so wide-ranging—affecting your mood, energy, body temperature, weight, heart, and more—that it may be difficult to get the correct diagnosis right away. The risk of thyroid disease increases with age. Yet thyroid disease is most difficult to detect in people over 60 because it often masquerades as another illness, such as heart disease, depression, or dementia. Misleading symptoms are one reason many Americans who have thyroid disease —mostly women—do not yet know they have it.

Estimates of how many people have thyroid disease vary widely, ranging from 10 million to 30 million. 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. The survey results from 2002 showed 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.

Diagnosing thyroid conditions can be tricky, and, in certain circumstances, so is treating them. As chair of a task force composed of leading thyroid specialists for the American Thyroid Association and the American Association of Clinical Endocrinologists, and a panelist on the corresponding hyperthyroidism task force, I am happy to share information that reflects the current thinking on managing thyroid conditions. This information can help you navigate some of the key points and controversies surrounding how best to detect and treat a malfunctioning thyroid.

The good news is that with proper treatment, thyroid conditions can be successfully managed. My hope is that this report will help you work closely with your doctor to bring your thyroid hormone levels back to normal, so that you can go about living your life to its fullest.


Jeffrey R. Garber, M.D.
Medical Editor

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: How common is thyroid disease?

How common is thyroid disease?

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.

Source: Third U.S. National Health and Nutrition Examination Survey (NHANES III).

How the thyroid gland works

Just as your car engine can't run without gasoline, your thyroid needs fuel to produce thyroid hormone. This fuel is iodine. Iodine is found in such foods as iodized table salt, seafood, bread, and milk. When you eat these foods, the iodine passes into your bloodstream. Your thyroid then extracts this necessary ingredient from your blood and uses it to make two kinds of thyroid hormone: thyroxine, called T4 because it contains four iodine atoms, and triiodothyronine, or T3, which contains three iodine atoms. The thyroid's output consists primarily of T4 (see Figure 2). Most of the T3 the body needs is actually created outside the thyroid in organs and tissues that use T3, such as the liver, kidneys, and brain. These tissues convert T4 from the thyroid into T3 by removing an iodine atom.

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