Über die Autorin bzw. den Autor

M. Sara Rosenthal, M.S., is a medical journalist and educator and the author of many acclaimed health books. She is an associate of the Centre for Health Promotion, University of Toronto, a World Health Organization Collaborating Centre in health promotion.

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The Thyroid Sourcebook

The McGraw-Hill Companies, Inc.

Contents

Chapter One

A Beautiful Butterfly

An endocrinologist in South Dakota shared a story that illustrates how in the dark most of us are about our thyroid glands. He was treating a new patient for a thyroid condition and explained what the thyroid gland is, how it works, and what it does. The patient was fascinated by this new, critical body part she never realized she had and asked the doctor, "Do cows have thyroid glands? Or other animals?" "Yes," he answered, "all animals, fish, birds, and most other species have thyroid glands." As a newly practicing endocrinologist, he was surprised that this patient was completely unaware of her thyroid gland. But this is not unusual—most people are not aware of their thyroid glands unless they are diagnosed with a thyroid disorder.

Approximately 12 percent of the entire adult population worldwide suffers from some sort of thyroid disease. In the United States, this translates into roughly thirty million adults. Thyroid diseases are grouped by type, such as hypothyroidism, hyperthyroidism, and thyroid cancer. Twenty percent of all women will develop autoimmune thyroid disease, usually hypothyroidism, at some time in their life. Nearly one-fifth of all people over sixty have subclinical hypothyroidism, or mild hypothyroidism.

The purpose of this chapter is to describe where the thyroid gland is located, what it does, and how it works. It also briefly explains the most common thyroid disorders. In addition, it addresses some common misinformation that can interfere with proper decision making regarding your thyroid health. (For information on how to evaluate whether the information you find in various sources is correct, see Appendix A.) The idea is to provide you with enough information about the thyroid gland so you can better understand your diagnosis and make fully informed treatment decisions with your doctor. Since this chapter serves as a very general introduction to the thyroid gland, it will refer you to other chapters in the book for more details.

How Your Thyroid Works

The thyroid gland is often referred to as a butterfly-shaped gland, but it is also shaped like the capital letter H, specifically, the H of the Honda vehicle logo. Each side of the H or butterfly is called a lobe, while the center (the body of the butterfly) is called the isthmus. The thyroid gland is located in the lower part of your neck, in front of your windpipe, and is basically wrapped around the windpipe (see Figure 1.1). Using the butterfly analogy, the butterfly "hugs" the windpipe.

The thyroid gland makes two critical thyroid hormones. The first is thyroxine, known as T4, which has four iodine atoms for each hormone molecule. The second is triiodothyronine, or T3, which has three iodine atoms for each hormone molecule. Both hormones are referred to in the singular as thyroid hormone.

Thyroid hormone is essential for our existence, affecting every single cell in the body. Thyroid hormone serves as the speed control for our cells, controlling their "speed of life." Of the two hormones, it is thought that only T3 has a direct effect on the cells of the body; it is debatable whether T4 has any direct effect. T4 must be converted to T3 by the cells by removing one specific iodine atom before it can work. If a different iodine atom is removed from T4, it creates an inactive molecule that does not work as an effective thyroid hormone. Nearly all cells have special enzymes inside of them called deiodinases that remove the iodine atom from T4 to make it into T3. It is thought that this is one way that each cell customizes how much T3 it will get, even though the blood supply to all the cells generally provides the same level of T4 at the same time. The T3 produced in the body's cells works in the nucleus of these cells to turn some genes on and some genes off. This is how the thyroid hormone works in the body.

For example, in the cells of the pituitary gland (where thyroid-stimulating hormone (TSH) is produced), T3 turns off the gene that makes TSH. This is why TSH levels are high when thyroid hormone levels are low and TSH is decreased when thyroid hormone levels are high.

The thyroid gland usually releases around 80 percent of its thyroid hormone as T4 and 20 percent as T3. Only 0.03 percent of the total T4 traveling around in the bloodstream is in the free form, in which it can be taken up into each body cell and do the job of effective thyroid hormone. Once the free T4 passes into a body cell of any kind, it is changed into T3 by an enzyme called 5-prime deiodinase.

Once changed, this T3 is absorbed into the nucleus of the cell, where the chromosomes are found. Inside the nucleus are special T3-receptor proteins that are made to stick to distinct spots in the genes of the chromosomes and also to stick to T3. When enough T3 is taken into the nucleus, it sticks to these T3-receptors and controls the genes that they are attached to. Some genes are turned on while other genes are turned off. Thus, each cell is able to convert the correct proportion of T4 to T3 for its own needs.

Be Informed: Know the Facts About T4/T3

Many Internet sites and books falsely state that many people cannot convert T4 into T3. Converting T4 into T3 is necessary for life, and there is no evidence that this metabolic derangement can be acquired later in life. Anyone who could not convert T4 into T3 would not be alive, as this conversion is necessary for fetal development and birth.

This myth leads patients into potentially dangerous decisions about their treatment and into purchasing thyroid "support products" or T3 that they may not need. For information on how T3 levels are determined by measuring levels of the thyroid-stimulating hormone (TSH), see page 20.

The T4/T3 conversion myth is also used to support other false statements. One is that TSH tests can read as normal when you're really hypothyroid (this is false except for rare people with pituitary tumors). Another falsity is that dessicated pig thyroid hormone (sold as "natural" thyroid hormone), questionable herbal concoctions that "help" with T4/T3 conversion, or regular and chronic T3 supplementation is preferable to, or healthier than, taking levothyroxine alone at the appropriate dose.

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Be Informed: The Truth About Reverse T3

Many websites and books say that trauma and stress can create a "fake" normal T3 level, called reverse T3. The claim is that many people with so-called normal TSH levels really have reverse T3 and are actually hypothyroid. This theory takes fact and creates fiction.

T4 is converted to T3 by removing a specific iodine atom from each of the T4 molecules. If a different iodine is removed, reverse T3 is formed. Reverse T3 is inactive; it cannot do the job of T3 and therefore will not suppress TSH. It will not affect TSH levels at all. If T4 is converted to reverse T3 without making enough regular T3, your TSH levels will be high and you will be hypothyroid. If the TSH levels are normal, so are the levels of regular T3 inside of your cells.

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The Role of Iodine in the Diet

T4 and T3, as already noted, are so named because of the number of iodine atoms per hormone molecule. So what is the role of iodine in the production of thyroid hormone? The thyroid gland needs to import one key ingredient for thyroid hormone production:...