The Complete Thyroid Book 2nd Edition

By KENNETH AIN, M. Sara Rosenthal

The McGraw-Hill Companies, Inc.

Copyright ©2011 Kenneth B. Ain and M. Sara Rosenthal
All rights reserved.
ISBN: 978-0-07-174348-8

Contents

AcknowledgmentsIntroduction: Why We’re Passionate About Thyroid DiseasePART 1 The Basics1 What Is the Thyroid, and What Does It Do?2 Tests and Labs: Diagnosing Thyroid Disease3 Hypothyroidism: The Most Common Problem4 Too Much Thyroid Hormone: Thyrotoxicosis5 Hashimoto’s Thyroiditis and Other Types of Thyroiditis6 Graves’ Disease7 Goiter: An Enlarged Thyroid Gland8 Thyroid Nodules9 Thyroid Cancer10 Thyroid Hormone: The Inside Scoop11 Other Drugs Used in Thyroid Disease12 Radioactive Iodine TherapyPART 2 People in Special Circumstances13 Thyroid Disease in Pregnancy and After Delivery14 Thyroid Disease During Menopause15 Thyroid Disease in Infants and Children16 Thyroid Disease and Obesity17 Thyroid Disease in Older Individuals18 Thyroid Hormone Resistance and Rare Genetic Disorders of the ThyroidPART 3 Living Well After Treatment19 Thyroid Misconceptions and Misinformation20 Special Diets for Thyroid Disease and Healthy Eating21 Making an Informed DecisionPART 4 Complications of Thyroid Disease22 Coping with Thyroid Eye Disease23 Coping with Depression, Anxiety, and Other Mental Health Disorders24 Coping with Heart Disease25 Coping with Fatigue26 Thyroid and Public HealthAppendix A: Thyroid Links on the WebAppendix B: Links to Other Conditions Related to Thyroid DiseaseGlossarySupportive LiteratureIndex

Excerpt

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 swallow, the entirethyroid gland moves upward and back, just as the Adam’s apple (thyroidcartilage) moves.

How Does It Feel?

A normal adult thyroid weighs around 15 to 20 grams (0.5 to 0.75 ounces). Insome situations, when the gland is stimulated to grow by certain conditions(described in later chapters), the enlarged gland is called a goiter. Agoiter is considered diffuse when the entire gland is symmetricallyenlarged and nodular when there are one or more distinct lumps that canbe distinguished from the rest of the gland. Ribbonthin muscles overlie thethyroid, and a thick muscle on either side of the sternal notch runs to a pointbehind each ear (the sternocleidomastoid muscles).

I find that it is easier for me to feel the thyroid gland on another person whenI’m standing just to the side, facing the individual. I usually begin, standingto the person’s right side, with the fingers of my right hand feeling for theAdam’s apple, then moving downward over the windpipe until I feel the flattened,soft, spongy thyroid isthmus. With my fingers maintaining contact with thegland, I slide them toward the person’s left neck, pushing under thesternocleidomastoid muscle and moving upward and downward slightly to feel theentire left thyroid lobe. Often I have to place my fingers over the lowestportion of the left thyroid lobe that I can reach, then ask the person toswallow to permit me to feel the lower left thyroid pole slide upward beneath myfingers. I use the same process to feel the right thyroid lobe, facing theperson and standing at his or her left side, then using my left hand to feel theright side of the thyroid. With this technique and some practice, most thyroidglands can be felt unless the individual’s neck is extremely short and thick orthe gland is abnormally small or in an unusual location.

We encourage all readers to perform a thyroid self-exam at least once a year,which can help you find suspicious thyroid lumps (called nodules) or anenlargement of the thyroid that should be evaluated by your doctor. Table1.1 describes how to do a thyroid self-exam and what to look for. Thyroidnodules are discussed in detail in Chapter 8.

More About Structure

There are a number of additional interesting features of thyroid glandstructure. It is well supplied with blood from four major thyroid arteries.Under usual conditions, the flow through these arteries carries the equivalentof the entire body’s blood volume each hour. Under some circumstances in whichthe thyroid is stimulated to become enlarged and hyperactive, such as in Graves’disease, the entire blood volume passes through the thyroid each half minute.

The thyroid gland is also supplied with nerves that provide sensation from thegland. When the thyroid gland is irritated, you may feel discomfort in the frontof the neck; however, sometimes the sensation is felt in the ear. This is knownas referred pain, much as one may feel pain in the left arm whenexperiencing a heart attack.

Near each thyroid lobe is a special nerve that supplies the voice box and vocalcords, the recurrent laryngeal nerves. Because of their proximity to thethyroid, these nerves may be damaged during thyroid surgery or directly injuredby a thyroid cancer. If one nerve is damaged, then the vocal cord on the sameside may be permanently paralyzed, resulting in a weak or hoarse voice. If bothnerves are damaged, then both paralyzed vocal cords might block the windpipe,requiring an opening to be made in the neck so that the person can breathe (atracheostomy).

Besides blood vessels and nerves in the thyroid, tiny, thin-walled tubes, calledlymph vessels, can be found throughout the thyroid and the neck. Alongthe path of these lymph vessels are small nodules (lymph nodes)containing small white blood cells (lymphocytes). Lymph nodes function asfilters for anything carried through the lymph vessels. In this way, bacteriathat may infect the thyroid are carried first to lymph nodes, activating thelymphocytes there to stimulate the immune system to fight the infection.Likewise, in the event of thyroid cancer cells being present, some cancer cellsmay be passed through the lymph vessels and stop in these lymph node filters,growing there as lumps, which are removed during the course of thyroid cancersurgery.

The Parathyroid Glands

Four (in some cases, three to six) other glands are located very close to thethyroid gland; they’re called parathyroid glands, which means “near thethyroid.” These glands do not produce thyroid hormone but rather makeparathyroid hormone, which causes the kidneys to retain calcium in the bloodwhile releasing phosphorus into the urine. At the same time, parathyroid hormoneincreases the activation of vitamin D, which enhances the absorption of calciumand phosphorus from food and beverages.

Parathyroid glands are sometimes damaged or accidentally removed during surgeryon the thyroid gland. Although only one functioning parathyroid gland is needed,loss or damage to all four glands results in loss of parathyroid hormone. Lossof parathyroid hormone will cause calcium to be lost into the urine and reduceits absorption from the diet, causing calcium levels in the blood to fall. Thesymptoms of low calcium include muscle cramps and spasms, numbness, andsometimes seizures, if severe. With the loss of parathyroid hormone, phosphoruslevels would remain high because the kidneys would not be able to release itinto the urine.

Inside the Thyroid Cell

The smallest living unit of the human body is the cell. All of the parts of thebody are made of large numbers of these tiny cells, organized into body tissuesand organs. A single cell is so small that a powerful microscope is needed tosee it, and it takes around a million cells in a single lump before it isvisible to the eye. Cells have a “skin” called the cell membrane. Insidethe membrane is a relatively large ball (the nucleus) that contains thechromosomes, composed of genes that are responsible for controlling the cell’sfunctions and the blueprints for all of its parts, as well as many smaller balls(the organelles) that perform vital functions of energy production andmoving proteins and nutrients within the cell.

In the thyroid, the cells that produce thyroid hormone are called follicularcells. They are arranged in groups so that the follicular cells form ahollow ball (thyroid follicles), with its center (the follicularlumen) containing stored thyroid hormone (called colloid). All thesethyroid follicles are contained within the fibrous capsule of the thyroid glandlike bunches of tiny microscopic grapes. Between neighboring thyroid folliclesare tiny blood vessels, lymph vessels, and collections of other cells calledparafollicular cells. These parafollicular cells (also known as Ccells) make additional hormones, such as calcitonin and somatostatin.

Thyroid follicular cells have special proteins in their membranes that aredocking stations, or receptors, for special hormones that control thethyroid gland. One such hormone is thyroid stimulating hormone, also known asTSH. TSH (as will be discussed later in this chapter) sticks to the TSHreceptor in the membrane. This causes a signal to be sent to the nucleus of thecell to activate the genes that result in the cell making proteins that work asiodine pumps. Once these iodine pumps are placed in the follicular cellmembrane, they actively suck iodine (in the form of iodide) into the follicularcell.

The TSH receptor signal also activates the genes that control the production ofthyroglobulin. Thyroglobulin is a protein that is unique to thyroidcells and is the early form of thyroid hormone. The iodine that was pumped intothe follicular cell is joined to the thyroglobulin at certain places. Theseportions of thyroglobulin are broken off, becoming thyroid hormone(thyroxine, also known as T4 because it contains four iodineatoms for each hormone molecule). The follicular cell releases both thyroxineand thyroglobulin into the blood. The best way to think about this complexprocess is to imagine a domino effect; when TSH in the blood reaches the thyroidfollicular cell and sticks to the TSH receptor, it unleashes a chain of eventsinside the follicular cell: it manufactures iodine pumps, concentrates iodinefrom the blood, makes thyroglobulin, joins iodine to the thyroglobulin, breaksoff thyroxine, and then releases both thyroxine and thyroglobulin into thebloodstream. Some of the thyroglobulin is stored inside the lumen of the thyroidfollicles as colloid, ready to be combined with iodine, taken back into thefollicular cells, and then released into the blood, should it be needed.

An Introduction to Thyroid Hormone

Here, we’ll introduce you to thyroid hormone, a key player in understanding howthe thyroid works. In Chapter 10, we’ll discuss thyroid hormone used asreplacement hormone for treating various thyroid diseases in much greaterdetail.

Thyroid hormone is essential for our existence, affecting every single cell inthe body. In a very simplified view, thyroid hormone serves as the speed controlfor cells, controlling their “speed of life.” There are a few different forms ofthis hormone. As already discussed, it is made from portions of thethyroglobulin protein that are combined with iodine and then broken off. Iodineis the critical ingredient used by the thyroid to make thyroid hormone. In fact,without sufficient iodine in the diet, the thyroid gland is unable to produceenough thyroid hormone (see Chapter 3 for more about iodine deficiency,which causes hypothyroidism). Without sufficient iodine, a goiter (an enlargedthyroid gland) can also develop (see Chapter 7). Most people need atleast 100 micrograms of iodine each day to produce enough thyroid hormone andavoid goiter.

Thyroxine (T4) is the predominant form of thyroid hormone. It’s called T4because it contains four iodine atoms for each hormone molecule. When thyroxineis provided as a medication in pill form, it is known as levothyroxine.It is debatable whether T4 has any direct effect on the cells of the body. Whenone specific iodine atom is removed from the T4 molecule, it becomes T3(triiodothyronine, a.k.a. liothyronine), the form necessary for doing thethyroid’s job for the body’s cells.

Nearly all cells have special enzymes inside of them (deiodinases) that removean iodine atom from T4 to make it into T3. It seems likely that this is one waythat each cell customizes how much T3 it will get, even though the blood supplyto all of the cells generally provides the same level of T4 at the same time. Ifa different iodine atom is removed from T4, it makes an inactive molecule thatdoes not work as an effective thyroid hormone.

The thyroid gland usually releases around 80 percent of its thyroid hormone asT4 and 20 percent as T3. (The T3 made by the thyroid is only a tiny portion ofthe T3 that is found in the body’s cells—most is made from T4 within thecells.) When this T4 and T3 enter the blood, most of these hormones stick toblood proteins made by the liver, called thyroid hormone transportproteins. The three major thyroid hormone transport proteins arethyroxine-binding globulin (note that this is not the thyroglobulin thatis made by the thyroid, even though the words appear similar), thyroxine-bindingprealbumin, and albumin (as well as assorted other proteins, includingcholesterol-carrying lipoproteins). Of the total T4 in the blood, 99.97 percentis stuck to these proteins. Unfortunately, the T4 and T3 stuck to these proteinsgenerally do not get inside the body’s cells, making them unavailable to beeffective thyroid hormone. Only 0.03 percent of the total T4 is traveling aroundin the bloodstream in the free form, not stuck on anything. This free T4is the only portion of all of the T4 that is able to be taken up into each bodycell and do the job of effective thyroid hormone. It used to be thought thatfree T4 is passively diffused into the body’s cells. Now we know that a numberof transport proteins “pump” thyroid hormones into cells; chief among them isthe monocarboxylic transporter 8 (MCT8).

Once the free T4 passes into a body cell of any kind, it is changed into T3 bythe deiodinase enzymes. This T3 is taken into the nucleus of the cell. Insidethe nucleus are special T3-receptor proteins that are made to stick to distinctspots in the genes of the chromosomes, and also stick to T3. When enough T3 istaken into the nucleus, it sticks to these T3 receptors and controls the genesthat they are stuck to. Some genes are turned on and some genes are turned off.This shows the great power of thyroid hormones, working directly at the geneticcontrols of cells. It also shows how fundamentally important it is to haveproper amounts of thyroid hormone in the body. Lastly, it explains how givingpure levothyroxine (T4) provides all of the benefits of thyroid hormone in thebody by letting each cell convert the correct proportion to T3 for its ownneeds.

(Continues…)
(Continues…)Excerpted from The Complete Thyroid Book 2nd Edition by KENNETH AIN, M. Sara Rosenthal. Copyright © 2011 by Kenneth B. Ain and M. Sara Rosenthal. Excerpted by permission of The McGraw-Hill Companies, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.