CHAPTER 1

What Is the Thyroid, and What Does It Do?

I (Ken) am often amazed, when talking with educated and articulate patients,some of whom have received years of medical treatments for a thyroid disorder,how little they know about their thyroid gland. This chapter describes thethyroid gland, covering its development, location in the body, and function. Youwill see how it produces thyroid hormone and what role this critical hormoneplays in your body. I discuss the body's natural control systems, including thepituitary "thermostat," which regulates the thyroid. In addition, you will seehow your diet and the environment affect this gland's function.

This critical knowledge enables you to understand what underlies diseases ordysfunctions of the thyroid. Rather than merely memorizing or reading long listsof signs and symptoms to learn about thyroid problems, it's helpful to gain aninsight into the causes of such problems. It is hard to know what is wrongunless you know how it should be right.

How It Formed, Where It Is, What It Does

When my youngest son was nearly four years old, a preschool teacher gave him astandardized test to assess his vocabulary and basic knowledge. Halfway througha successful test session, Jake was told, "Point to your thigh." Jaketriumphantly touched the center of his neck, just above his breastbone. "Wrong!"exclaimed the teacher. Watching the test, I was surprised. "What do you mean?" Isaid, "He thought you meant his thyroid gland." "Where is the thyroid gland?"she asked me, clearly at a loss to conceive that such a young child knewsomething about anatomy that had eluded her graduate education. Unfortunately,her situation is much too common.

In the Beginning

The first sign of the developing thyroid gland can be seen near the developingtongue in a seventeen-day-old embryo. Of course, this tiny embryo is crowdedwith many tiny little structures destined to become all of the major organs ofthe future person. The developing heart is just nearby and, over the next coupleof weeks as the heart descends in the chest, the main portion of the earlythyroid gland follows, downward from the tongue toward its eventual position atthe base of the neck. A thin stalk of thyroid material, remnant of thedescending thyroid, is left connecting the base of the tongue with the thyroidgland, which comes to be located just under the Adam's apple (the thyroidcartilage).

Although this stalk, called the thyroglossal duct, usually disintegratesby the sixth week, remnants can still remain in some adults. Sometimes diseasesthat affect the thyroid gland can also affect remnant thyroid material in thisduct, showing up as swellings or lumps anyplace between the base of the tongueand the breastbone. In some cases, the thyroid does not descend all the way,remaining near the tongue or anyplace in between there and its proper location.In very rare circumstances, the thyroid may descend too far and be found in themiddle of the chest. Any abnormal location of the gland is called an ectopicthyroid.

It is believed that 10 percent of the thyroid gland cells, the parafollicularcells that make the hormone calcitonin, come from a different part of the embryoto merge with the rest of the thyroid in the two-month-old embryo. When thefetus is at least three months old, its thyroid gland first starts makingthyroid hormone. The fetus requires thyroid hormone to permit its brain andnervous system to develop, and it appears that a small but significantcontribution from the mother's thyroid is most important before this time.Recent studies suggest that the children of hypothyroid (low- or nonfunctioningthyroid) mothers who did not take thyroid hormone during the first trimester ofpregnancy had measurably lower intelligence than children whose hypothyroidmothers received thyroid hormone treatment. For obvious reasons, it is veryimportant to be certain that a woman who may become pregnant is not hypothyroidor, if receiving thyroid hormone therapy, has proper thyroid hormone levels.

Location, Location

We like to think that the thyroid looks like a butterfly, although the ancientGreeks thought it looked like their shields, giving it the current English name(thyroid comes from the Greek word for "shield-shaped"). (See Figure1.1.) Likewise, the German name for this gland, Schilddrüse, alsomeans "shield gland." It is typically located in front of the windpipe (trachea)just above the midline bony notch in the top of the breastbone (sternal notch).Both of its "wings," the right and left thyroid lobes, wrap backward around thetrachea and are attached to the upper front of the trachea and the lower part ofthe voice box (larynx). Each lobe is around 4 centimeters (1.6 inches) from topto bottom (pole to pole). The middle part of the gland, connecting the two lobesand corresponding to the body of the butterfly, is the isthmus. Often, apencil-thin bit of thyroid, known as the pyramidal lobe (the head of the"butterfly"), comes from the middle of the isthmus. When you...