Thyroid for Dummies (9 page)

Chapter 3: Discovering How Your Thyroid Works

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Proteins that carry thyroid hormones

Three different proteins carry thyroid hormones

proteins are inactive and one theory is that

around your body. The most important by having so much tied up with proteins means far is thyroxine-binding globulin, responsible that an increase in the thyroid gland’s hormone for carrying 75 per cent of thyroid hormones in

output does not result in an increase in thyroid

your blood. Transthyretin, previously known as

activity. Another theory is that the combination

thyroxine-binding prealbumin, carries 20 per of the hormone and the protein produces a cent of your thyroid hormones, while thyroxine-large molecule, which cannot escape from the

binding albumin carries the other 5 per cent.

body through the urine, thus preserving iodine.

Exactly why proteins carry thyroid hormones is

not clear. Thyroid hormones that are bound to

more thyroid hormone to bind to these proteins, keeping the unbound thyroid hormone constant and normal. Yet the results of a total T4 blood test will be elevated. Conversely, testosterone, the male hormone, causes a decrease in the thyroxine-binding proteins. If your testosterone level rises, your thyroid makes less thyroxine and a measurement of total T4 shows a decrease (while the unbound T4 again remains normal).

Understanding the Function

of Thyroid Hormones

Your thyroid hormones affect just about every cell and organ in your body.

They perform general functions that increase the efficiency of each organ’s specific functions, whatever they are. This section tells you all about those functions and explains what too much or too little of the thyroid hormones can do to a healthy person.

Many of these changes are caused by other factors besides too little or too much thyroid hormone. For example, an infection can raise your body temperature just as having too much thyroid hormone does. Also, if you have a condition that is associated with dry skin like eczema, or the menopause, this symptom may predominate even if you have hyperthyroidism, which tends to cause moist skin. The information here shows you what classic symptoms of thyroid problems look like, but everyone is different, and each person with the condition may show individual variations.

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General functions

Thyroid hormones cause every cell in your body to make more
enzymes
, the proteins that promote your metabolism as they act as the key to help certain chemical reactions take place. If you think of your body as a machine, then adding extra thyroid hormone is like pressing harder on the accelerator of a car. The result is a revving up of the machine – like going from 2,000 revolutions per minute to 4,000 or more revolutions per minute, depending on the amount of hormone added. In the same way, when more thyroid hormone is present in your body, more chemical reactions are taking place.

Metabolism

Your
basal metabolic rate
(BMR) is an overall measure of the amount of chemical reactions taking place in your body. Increased thyroid hormone may increase your BMR, or metabolism, as much as 60 to 100 per cent. Any machine that increases its activity heats up. Likewise, your body heats up when you make more thyroid hormone, and the result is a higher body temperature. At the other end of the scale, not enough thyroid hormone results in an abnormally low body temperature.

As more metabolism takes place, more of your food intake is burned for energy, so less energy is available for storage. Your body detects the need for more energy and you get hungrier, but your faster metabolism usually more than offsets any increase in food intake. The net result is that you lose weight. However, if you take in too much extra food, you can actually gain weight.

A scientist defines an individual’s basal metabolic rate (BMR) as the energy expended when lying in bed, at complete physical and mental rest, 12–14

hours after last eating, in an ambient temperature of 26–30 oC. Although BMR is dependent on thyroid function, metabolism also varies depending on gender (usually higher in males), age (slows as you get older), lean body mass percentage (how much muscle you have), and nutritional status and genetic inheritance (which dictates the efficiency of your metabolism).

The type of food you eat also plays a role as heat released during digestion (known as
dietary-induced thermogenesis
) accounts for 10 per cent or more of the energy you get from different foods.

Muscle function

Although your muscles need thyroid hormone for proper functioning, too much of the hormone is not a good thing. Excess thyroid hormone results in muscle wasting, as muscle tissue is consumed as an emergency energy source. As you lose muscle, you become weaker. If too much thyroid hormone is present, the nerves sending signals to your muscles also show increased excitability, resulting in increased reflexes and muscle tremors.

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Some unscrupulous diet regimes give unsuspecting people thyroid hormones to speed up their metabolism and help them lose weight. Although this tactic sounds like a good idea, it definitely isn’t. An overabundance of thyroid hormone results in muscle loss, so the weight you lose is not fat but muscle, the so-called lean tissue of the body. You do not want to lose lean tissue, as this loss makes you less fit, says goodbye to a toned silhouette, and increases your general flabbiness. Do not use thyroid hormone in an attempt to lose weight.

Energy sources

As well as affecting the protein found in muscle, thyroid hormone also affects the other sources of energy in your body, namely carbohydrates and fats.

Carbohydrates are the main source of immediately available energy in the body, so they get used up faster than normal when thyroid hormone levels rise, again resulting in more heat production. Fat is also used up faster than normal. The result is a lowering of the different kinds of fat in the body, namely cholesterol and triglycerides. On the other hand, when thyroid hormone levels drop, the fats accumulate in the liver and the level of cholesterol in the blood rises.

Because chemical reactions require vitamins, minerals, and trace elements, your need for these substances increases when you have more thyroid hormone. Increased thyroid hormones cause a more rapid breakdown of the vitamins. However, multivitamin and minerals supplements have little effect upon the thyroid gland itself, except for those products that contain iodine (often in the form of kelp or seaweed). Supplements and foods containing iodine are best avoided when you have hyperthyroidism or multinodular goitre (see Chapter 9) where extra iodine is used to make too much thyroid hormone.

Specific functions

Every organ in your body requires thyroid hormone to work normally. When that hormone is lacking, the organ tends to carry out less of its usual function, and when too much thyroid hormone is present, the organ does more than it should. In this section, we discuss the most important changes due to abnormal amounts of thyroid hormone in your body. This discussion is not complete by any means, as that needs a large book in itself, and many of the changes that occur are too subtle to result in signs or symptoms that are detectable.

The heart

Your heart needs T4 thyroid hormone for proper pumping. If not enough T4

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state can even result in heart failure when T4 is severely lacking. Conversely, when T4 levels rise too high, the heart beats too rapidly. While the heart pumps out more blood at first, if this increased pumping continues for too long, it can lead to decreased heart strength so the heart pumps out less blood than normal. This problem occurs because excessive T4 causes muscle wasting, and your heart, of course, is made of muscle.

Depending on your level of physical activity, your normal resting heart rate is usually between 60 and 80 beats per minute (assuming that you’re not taking any drugs that affect your pulse). Someone who is in exceptionally good physical condition, such as a trained athlete, may have a heart rate in the 50s or below. People with too much T4, however, often have a heart rate of 120 or faster.

The lungs

As your metabolism increases, you need more oxygen so that the chemical reactions in your body can take place. Oxygen comes into your body through the lungs. Your respiration rate, normally about 16 breaths per minute, has to speed up to bring in more oxygen if you have an overactive thyroid gland.

However, even an increased respiration rate may fail to provide you with enough oxygen if your muscular diaphragm and chest muscles are wasting from excess T4.

The stomach and intestines

Your stomach and intestines are linked with muscles that need T4 to push food along for digestion and excretion. When not enough T4 is present, intestinal movement slows, as does the rate at which you absorb nutrients from your food. And slowed bowel movements lead to – yes, you’ve guessed it – constipation. On the other hand, having too much T4 on board speeds up your bowels, which become looser, more frequent, and can lead to diarrhoea.

Great.

The skin, hair, and nails

The increased blood flow that results from a raised level of T4 is especially prominent in the skin. People with hyperthyroidism often have skin that feels warm, they may look flushed, and perspiration may increase, so it also feels moist. When T4 levels fall, the skin is more likely to feel dry and scaly and feels cold to the touch. The nails don’t achieve their proper toughness without enough thyroid hormone and may seem brittle with a tendency to break easily. Similarly, the hair is fragile, and excessive hair loss is a common complaint when not enough T4 is present.

The brain and cerebral functioning

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worry about minor things. In extreme cases, this anxiety can lead to paranoia.

In contrast, not enough T4 can lead to mental dullness and depression. Chapter 2 details the changes in mood that can occur with too much or too little thyroid hormone.

Sexual functioning and menstruation

Thyroid hormone is needed for normal sexual function. Both men and women lose interest in sex when not enough T4 is present. They don’t necessarily have increased interest in sex when T4 levels rise, however, as so many psychological and physical problems result from the increase.

The menstrual cycle also depends on adequate T4 to proceed normally.

Women with a lack of T4 may have trouble conceiving a baby. They tend to have increased menstrual flow and may develop anaemia (from losing too much blood). Too much T4 can also decrease the menstrual flow or cause missed periods.

The bones

Thyroid hormones help bones to grow normally. When too little thyroid hormone is present in early life, the bones show delayed development and do not grow to their correct length. Someone with this condition (dwarfism) is unusually short, with short arms and legs and a larger trunk. If thyroid hormone is lacking after growth has stopped, the bones appear more dense than normal because of decreased bone turnover.

When too much thyroid hormone is present, whether it’s due to taking too much thyroid hormone or inadequate treatment of hyperthyroidism, bone turnover and bone loss increases. This disorder can resemble
osteoporosis
, which is the kind of bone loss that occurs in women after the menopause and leads to brittle bones. However, bone thinning associated with excess T4 rarely results in bone fractures once the increased thyroid hormone is controlled with treatment, as the excessive bone loss then stops.

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Chapter 4

Testing Your Thyroid

In This Chapter

ᮣ Determining your thyroid hormone levels

ᮣ Using blood chemicals other than hormones to make a diagnosis

ᮣ Checking the size, shape, and content of your gland

ᮣ Investigating abnormal lumps and bumps on the thyroid These days, taking the precise measurement of thyroid function for granted is easy to do. Yet only 60 years ago, diagnosis depended more upon the physical and emotional state of the person than primitive laboratory tests. As a result, only people with obvious signs and symptoms received a diagnosis – just the tip of a huge iceberg of thyroid abnormalities.

Today, tests that measure thyroid function are getting more and more sensitive. Doctors can identify many people with
subclinical
thyroid disease, meaning their condition is not yet bad enough to trigger symptoms or signs that are apparent to the doctor or patient. These abnormalities are often picked up during routine screening tests and usually progress to become clinical sooner or later. These sensitive tests are therefore invaluable in differentiating problems due to thyroid abnormalities with the symptoms of ageing (such as general slowing down), which are so similar to those of mild hypothyroidism.

As thyroid disorders are so common, laboratories receive around 10 million requests for thyroid function tests per year in the United Kingdom, costing an estimated £30 million! Although you cannot order these tests for yourself (you request them through your doctor), this information can increase your understanding of what various tests involve and what their results mean. The material covered in this chapter can also help you have better discussions with your doctor about your diagnosis.