Kitabı oku: «The Healthy Thyroid: What you can do to prevent and alleviate thyroid imbalance»

CHAPTER ONE The Hidden Illness

Thyroid disease is common and affects women more frequently than men.

Many books and articles on thyroid problems for both the general public and medical profession begin with these or similar words. But this bland statement barely begins to suggest the number of women afflicted by thyroid problems or the impact of thyroid disorders on our lives. In fact, according to a review in the British Medical Journal, taken together, underactive and overactive thyroid conditions represent the most common hormonal problem – and this problem overwhelmingly affects women.

In terms of statistics alone, thyroid problems in women deserve to be taken seriously:

• Four out of five people with thyroid disorders are women.

• One in 10 women will develop a thyroid disorder at some stage in her life.

• Between one and two in 100 women in the UK will develop an underactive thyroid (hypothyroidism), a condition 10 times more common in women than men.

• Two out of every 25 women – and one in 10 past the menopause – have so-called mild thyroid failure that is considered borderline on blood tests. These ‘subclinical’ problems are linked with nagging ill health, such as fatigue, mood swings and overweight, as well as more serious medical problems such as depression, heart disease and osteoporosis.

• Overactive thyroid conditions (hyperthyroidism) are also common in women, affecting between five in every 1000 to one in 50 – or 10 times more women than men.

• One in every 100 people in the UK will develop an autoimmune thyroid disorder, when the body turns against itself to cause the thyroid to become either underactive or overactive. Autoimmune disorders, including those affecting the thyroid, are estimated to be the third biggest killer after heart disease and cancer.

• Having a personal or family history of autoimmune disorders, such as diabetes or rheumatoid arthritis, gives you a 25 per cent greater risk of developing a thyroid disease than someone without such a history.

• Hashimoto’s thyroiditis – an autoimmune disorder causing an underactive thyroid – may account for up to one-third of such cases in this country and is five times more common in women than men.

• Graves’ disease – an autoimmune condition causing an overactive thyroid – is 15 times more likely to affect you if you are a woman.

• Goitre (a swollen or enlarged thyroid gland) is four times more common in women than in men.

• Thyroid nodules or lumps are also more common in women – estimated to affect about one in 20 women.

• Thyroid cancer, although rare, is also more likely to develop if you are a woman.

Only as Healthy as Your Thyroid?

Thyroid problems can affect a woman at any age or stage in life – from the teens to retirement. Throughout this time, they are a source of much ill health and unhappiness. During the reproductive years and after the menopause, they can exacerbate other female health problems as well as create a host of debilitating symptoms that affect every system of the body:

• Thyroid problems can cause menstrual disturbances, such as heavy or absent periods, and worsen problems such as premenstrual syndrome (PMS).

• Thyroid problems are an underrecognized cause of fertility problems and miscarriage.

• During pregnancy, thyroid disorders are the most common hormonal problem.

• Even a mild shortage of thyroid hormone during pregnancy may affect the unborn child’s future IQ (intelligence quotient). Research shows that children, aged seven to nine, whose mothers had untreated hypothyroidism during pregnancy scored about seven points lower on IQ tests.

• According to US research, women with faulty thyroid function are more likely to give birth to babies with defects of the heart, brain or kidney, or have abnormalities such as a cleft lip or palate, or extra fingers.

• Babies whose mothers have an underactive thyroid have an increased risk of heart problems – even if their mothers are being treated for the condition. Yet, at the time of writing, the NHS still does not routinely test thyroid function in pregnancy.

• One in 10 young women have thyroid problems after giving birth, with symptoms such as depression, tiredness and a lack of zest that cast a shadow over the first months of parenthood. Such symptoms are often misdiagnosed as ‘the baby blues’, thus depriving women of treatment that would help.

• Later in life, thyroid disease becomes even more common. An estimated one in 10 women over 40 may have undiagnosed thyroid disease, which is particularly worrying as thyroid problems are associated with an increased risk of two very significant causes of female ill health in later life: heart disease and osteoporosis (brittle-bone disease).

• One in five women over 60 suffer thyroid problems. With the ‘baby-boomers’ reaching this age, thyroid disorders will become an increasingly major health challenge.

• Thyroid disease in older women is more likely to be ‘silent’, producing few or vague symptoms. But compared with, say, high blood pressure – another ‘silent’ disease with serious consequences – thyroid problems are far less likely to be suspected or tested for.

These facts and figures alone put thyroid disease on a par with conditions like diabetes, estimated to affect one to two in every 100 people, and breast cancer, which strikes one in eight women. However, thyroid problems attract only a fraction of the research funding given to these high-profile conditions, and have until only recently relatively poor media coverage. That this is now beginning to change was reflected by an editorial in the prestigious British medical journal The Lancet that declared ‘you’re only as healthy as your thyroid’.

Are Times Changing?

During the writing of the first edition of this book more than seven years ago, there was little awareness – even among journalists specializing in women’s health – of just how common thyroid problems are and of the misery they can cause. A small request for help placed in The Guardian newspaper resulted in a deluge of phone calls: 200 over two days.

Revisiting thyroid problems now, has anything changed? The good news is that there has been a shift in knowledge and attitudes. A great deal more is becoming understood in terms of how thyroid problems are caused and how they may best be treated. And certainly, many more people are now aware of thyroid disease than in 1995.

Part of this new awareness is thanks to a number of books drawing attention to the wide-ranging effects of thyroid problems and the misery they can cause. The advent of the Internet has also done much to fill the information gap. There are now several excellent websites where women with thyroid problems can get information and communicate with others who have the same condition. This is good news for the millions of women living with a faulty thyroid.

However, in other aspects, the changes have been pitifully few. Thyroid disease is still a ‘Cinderella’ disorder, despite being the cause of so much depression, tiredness, discomfort and feeling well below par. Even nowadays, women often soldier on for long periods before anyone takes their complaints seriously – hardly surprising given that the average medical student only has a lecture or two on thyroid problems, if they’re lucky. And although there are more post-training courses for interested doctors, endocrinology (the study of hormones) is still not a core subject in most basic medical courses.

Like their ‘sisters’ in 1995, many of the women interviewed for this new edition had struggled on for months, even years, with crippling symptoms before being diagnosed. Once diagnosed, they had to cope with unsympathetic doctors and endure treatments that were uncertain, took time to get right and sometimes didn’t work at all.

Just like seven years ago, many women also spoke of the dilemmas posed by treatment – how long it had taken for medications to start working, the uncertainty of their effects, the agony of deciding whether to opt for surgery or radioiodine therapy, or whether different forms of medication might be more effective. All related stories of how difficult it was to obtain relevant information and how alone they felt with this unpredictable disease.

Others told poignant stories of how the disease had affected their daily lives and relationships in the face of the pressures of holding down a job while battling overwhelming fatigue, the difficulties encountered with partners, friends and children who did not always understand why the person they loved had undergone such a major personality change, of being overweight or underweight, of the self-consciousness endured because of bulging eyes, thinning hair and thickened skin, and the effects these had on their self-esteem. Some described the heartache of not being recognized by friends they had not seen for some time.

These physical problems are often dismissed as trivial but, in a world where the pressure to look young and attractive is intense, they can become the cause of a huge amount of distress and self-loathing. They can also lead to other women’s health problems such as eating disorders.

Tip of the Iceberg?

In recent years, it has become apparent that thyroid problems may be even more common than ever imagined. The availability of more sophisticated methods of testing thyroid function has revealed that many seemingly healthy women with apparently normal thyroid function have, in fact, abnormal levels of hormones and antibodies against the thyroid gland. This also suggests that the cases of thyroid disease identified and treated may be only a fraction of what is actually out there.

Since 1995, such cases of mild or low-grade thyroid disease have received a great deal of attention in the medical and public media. Medical journals, such as the influential New England Journal of Medicine, have carried major articles on subclinical thyroid disease, while the shelves of bookshops now carry stacks of books about thyroid problems aimed at the general public. Many put forward the view that hidden thyroid problems are a factor in a host of conditions reaching epidemic proportions in the 21st century, including:

• Chronic fatigue

• Depression, anxiety and mood swings

• Difficulty in losing weight

• Eating disorders

• Menstrual problems

• Fertility problems, miscarriage and premature births

• Perimenopausal and menopausal problems

• Changes in libido

• Heart disease

• Osteoporosis

• Ageing.

There is much controversy surrounding the issue of mild thyroid disease – or should it be called early or pre-thyroid disease? Despite being more widely recognized, there is little consensus on its significance and whether or how it should be treated. No one truly knows how important it is as a cause of ill health or how often it might lead to full-blown thyroid disease. Other questions remain, too: Should it be tested for in the absence of symptoms? Should women with symptoms suggestive of thyroid problems, such as tiredness and depression, be treated even if blood tests are apparently normal? Should widespread thyroid screening be introduced and, if so, at what age and how often should testing be performed?

Just as in 1995, there is a lot of debate, but no definitive answers.

Why Me?

Almost every woman included in this book wanted to know why she, in particular, had developed a thyroid disease. Unfortunately, there are no simple answers to this question. Despite its prevalence, the experts themselves still do not fully understand what causes the thyroid to misbehave. As with so many illnesses, one of the most pressing questions is whether nature or nurture lies at the root of the problem.

The cracking of the human genome, the inherited ‘database’ of some 40,000 to 50,000 genes containing all of the instructions for life, has led to an explosion of genetic research – and some interesting insights into the origin of certain kinds of thyroid problems. Some of the genes involved in certain kinds of thyroid cancer have been identified as well as other possible ‘candidate’ genes that may lead to an increased risk of developing an autoimmune thyroid disease.

However, although genes undoubtedly play a role, they are not the whole story. As with any illness with a genetic component, possessing one or more of these predisposing genes may give you a tendency to develop a particular problem – in this case, thyroid disease – but it is your environment and individual lifestyle that may yet determine whether you actually will.

The Immune Connection

The role played by the immune system in triggering a number of thyroid disorders remains a controversial topic. Autoimmune thyroid disease, which underlies both hypo- and hyperthyroidism, is caused by failure of a fundamental mechanism: the body’s ability to recognize its own organs and tissues as belonging to itself.

If the body fails to recognize itself, it produces self-attacking proteins – known as autoantibodies – to destroy its own tissue. Experts are becoming increasingly aware of a number of diverse thyroid problems due to the production of such autoantibodies, including:

• Hashimoto’s thyroiditis, which causes an underactive thyroid

• Graves’ disease, which causes an overactive thyroid

• Myxoedema, or generalized swelling of the skin and other tissue

• Subclinical hypo-/hyperthyroidism, mild or hidden thyroid under-/overactivity

• Thyroiditis, or inflammation of the thyroid

• Postpartum thyroiditis, or inflammation of the thyroid after childbirth

• Thyroid eye disease (TED).

Putting the Clues Together

Women, as we already know, are much more likely than men to develop thyroid disease. Many of those interviewed for this book added, almost as an afterthought, ‘My mother (or sister, or daughter) has thyroid problems, too.’ In particular, Graves’ disease and Hashimoto’s thyroiditis seem to cluster in families. In the past, this was dismissed as a coincidence. Recently, however, the new science of molecular genetics has led a number of researchers to look for an underlying inheritance factor in the development of autoimmunity. It is now generally agreed that as much as 10–15 per cent of us inherit an immune system with the potential to turn against itself.

Nevertheless, the development of thyroid problems is not just a matter of inheriting a faulty set of genes. Many people possess autoantibodies and do not go on to develop full-blown thyroid disease. In fact, it is estimated that only about one in 10 of those with an inherited tendency to develop thyroid antibodies will actually have thyroid problems.

One of the main aims of research, therefore, is to discover the possible triggers of thyroid problems. We know that the immune system can be damaged by many aspects of the 21st-century lifestyle that seem to have potential roles in the development of thyroid disease. Pollution, ageing, diet, stress, viral and bacterial infections, and habits like smoking and drinking are just some of the factors being explored by scientists in the hopes of finding out why the thyroid becomes faulty. Since 1995, much more information has been accrued on the roles these factors may play in triggering thyroid disease – and one important risk factor could simply lie in being female.

The Hormone Connection

The thyroid gland is involved in virtually every bodily process, including those of the reproductive system, and thyroid disease is linked to a number of specifically female problems (see Table 3.1).

Table 3.1 Links between thyroid disorders and the reproductive system

The first clue that something may be wrong with the thyroid gland is often when a woman consults the doctor on a ‘woman’s problem’, such as menstrual irregularities, difficulty in getting pregnant, miscarriage, postnatal depression or menopausal symptoms. It is also increasingly recognized that thyroid problems may be confused with or aggravate the symptoms of women’s problems such as PMS and the menopause.

With so many women’s problems being linked to thyroid disease and, conversely, so many thyroid problems being associated with the reproductive cycle, could it be that the female hormones play a part in susceptibility to thyroid disease? The answer is most likely yes. Research suggests that the two main female sex hormones, oestrogen and progesterone, moderate the activity of the immune system – hence the preponderance of thyroid disease in women.

The involvement of hormones and the immune system could also explain why the thyroid may misbehave for the first time during pregnancy and after birth. It also provides a reason for why so many women develop debilitating postpartum thyroiditis (PPT), which is often confused with postnatal depression.

One of the most striking developments since the first edition of this book has been the increasing awareness that the brain and nervous system, the immune system and endocrine (hormonal) system, all previously thought to be completely separate systems, do not work in isolation. This has led to the development of new fields of study such as psychoneuroendocrinology and psychoneuroimmunology, which are dedicated to exploring the body-mind connection and the way in which each ‘talks’ to the other.

With Women in Mind

This book is an exploration of these and other issues. Chapter 2 looks in detail at the thyroid gland and how it works to enable readers to understand the links between the thyroid and other body systems, and why – when it goes wrong – there may be such wide-ranging effects. The chapter also outlines some of the latest thinking on the immune system and the part it plays in thyroid problems.

Chapter 3 examines all the things that can go wrong with your thyroid, and explores some of the latest theories for how thyroid problems arise in an attempt to answer that nagging question, ‘Why me?’ There is also more detailed information on thyroid nodules (lumps) and thyroid cancer. Despite being one of the simplest forms of cancer to treat, survival rates in the UK have, until now, lagged woefully behind those of other countries.

Chapter 4 tackles the problem of getting a proper diagnosis. It includes a description of the various tests that may be performed, and explores the issue of what is normal and the difficulties involved in interpreting thyroid function tests.

Chapter 5 describes the available treatments, including medications, surgery and radiotherapy, and explains how they work, including their pros and cons. It also covers the debate over newer – and the revival of older – forms of treatment, and how you can work with your doctor to find the treatment that is right for you.

Chapter 6 covers the different ways you can help yourself, such as by paying attention to what you eat, making sure you get the right amount of exercise and managing stress, as well as how to come to terms psychologically with having thyroid disease.

Chapter 7 looks at how complementary therapies can help you manage your thyroid problems. These therapies are much more widely accepted now than when the first edition of this book was written, and many doctors and healthcare practitioners now acknowledge the part these therapies can play alongside conventional medical treatment.

Chapter 8 is devoted to thyroid eye disease, a particularly devastating condition about which too little is known, even now, and includes the still controversial issue of how it should be treated.

Chapter 9 describes how thyroid problems can affect you at different points in the female reproductive cycle, and includes important new information on how thyroid problems can affect menstruation, fertility, pregnancy and life after childbirth.

Chapter 10 looks at the problems that may be caused by thyroid disease at around the menopause and as we get older.

Chapter 11 investigates some of the major issues in thyroid disease and the advances made in our current understanding of the disorder, as well as takes a peek into the future at possible new treatments.

Finally, there is a glossary of terms relevant to thyroid disorders, and a list of books, websites and organizations that may prove helpful.

The more you know about the way your body works, the better able you will be to help yourself if something goes wrong. The objective of this book is to provide the information you need to help yourself, to work with your doctor to get the best treatment for your problems, and to feel more in control of your body and your life – something that women with thyroid problems often feel they have lost.

This book does not intend to tell you what to do or replace medical advice. There is a great deal of controversy surrounding thyroid problems – how they come about and how they should be dealt with. The main areas of debate have been outlined in this volume to give you an idea of what different experts think so that you can make up your own mind about how to live with your thyroid problems.

CHAPTER TWO Understanding Thyroid Problems

To better understand what can go wrong with your thyroid, it is necessary to know something about how the gland works. This chapter attempts to reveal why we have a thyroid gland, and looks at the way the thyroid interacts with other systems of the body, including the immune system.

The thyroid is one of 10 glands that make up the endocrine (hormonal) system. From the moment we are conceived until the time of our death, our bodies are under the influence of a cocktail of hormones produced by this system. As this system is so finely tuned, when anything happens to disturb its delicate balance, the repercussions ricochet throughout the rest of the body.

The hormones produced by glands are chemical messengers that are carried around the bloodstream to act on cells and tissues that are often far from their site of origin. Their job is to ensure that we have the correct concentrations of metabolites – vital nutrients (such as sugars and fats), vitamins and minerals (such as calcium, sodium, potassium and iodine), enzymes and other factors essential to life – in the bloodstream.

Each gland has a specific function, but also works with the other glands to keep our body in a state of chemical balance (homoeostasis). One recent, exciting discovery is that not only do hormones interact with each other, but they also exchange messages with other chemicals produced by the brain and nervous system. Research is beginning to uncover more and more links between these major interacting systems, and to throw more light on the way hormones and chemicals produced by the nervous and immune systems work together. This, in turn, is helping to clarify the connection between mind and body as reflected by a diverse number of conditions, including thyroid disease.

Key Sites

The glands themselves are situated at key locations throughout the body (see Figure 2.1). Together they produce over 50 different hormones – so-called ‘mighty molecules’ – that have widespread effects on us from cradle to grave. As hormones cannot be stored in large quantities in the glands, the brain programmes their manufacture by means of a complex biochemical cycle that uses a series of checks and balances to ensure that hormone levels are maintained according to your body’s needs.

In addition to the endocrine glands themselves, other organs contain pockets of glandular tissue that produce hormones. One of these is the hypothalamus, a region of the brain that is both part of the nervous system and a gland.

Not surprisingly, with such a complicated system, things can go wrong. Broadly speaking, when a gland ceases to function as it should, it results in two categories of problems: the gland becomes underactive and produces too few hormones; or it becomes overactive and produces too many.

Balancing the Body

The whole endocrine system is controlled by a series of ‘feedback loops’, which slow or stop a gland from working when enough hormone has been produced, and turn it back on again when more is needed – like a central-heating thermostat (see Figure 2.2). If the blood levels of any of the essential chemicals are low, special sensory cells are able to pick up a signal that

Fig 2.1 The endocrine system

Figure 2.2 The pituitary gland and the hypothalamus in the brain work together to produce a hormone that stimulates the thyroid. The thyroid gland draws iodine from the blood in order to make T3 and T4. Sensors in the TSH-secreting cells of the pituitary detect rising levels of thyroid hormones and quell further secretion. When levels fall, the pituitary releases more TSH, which stimulates the thyroid to start making more hormones.

prompts them to release hormone. This hormone, in turn, acts on other cells to release more of the needed chemical into the bloodstream. When enough chemical has been produced, the sensory cells switch the system off, which stops further hormone release. In this way, the body’s chemical balance is constantly maintained.

The system is exquisitely sensitive: food, exercise, stress, illness, changes in body chemistry such as a shortage or excess of certain nutrients, pregnancy, ageing, even the time of day or year, can affect the balancing mechanism and, with it, the amount of hormones our glands secrete.

Most hormones act only on specific tissues and not all the cells in the body. They do this by latching on to structures called ‘receptors’, which lie studded about the surface of or within cells, rather like a key fits into a lock. This enables hormones to be transported around the bloodstream to specific locations. Receptors are also important because, as we shall see, if the wrong chemical – such as an autoimmune antibody – attaches itself to a receptor, like a thief using a master key to get into your house, it can cause havoc and destruction.

The Thyroid Gland

The thyroid is a small, soft, butterfly-shaped gland that weighs just 15–20 g (¹/₂–³/₄ oz) and is about the size of a plum, yet it is also the largest pure endocrine gland in the body. It lies across the front of the windpipe (trachea) just below the larynx, or voice box (see Figure 2.3). Its two lobes, or sections, lie on either side of the Adam’s apple and are joined together by a narrow bridge of tissue called the isthmus.

You may just be able to detect its outline if you look in the mirror and stretch your neck. If you take a sip of water and swallow, you may be able to see it moving up and down. If you can’t see it, you may be able to feel it with your fingers. (But don’t worry if you can’t see or feel it – not everyone can.)

The thyroid develops in the womb during the first weeks of life from a small piece of tissue at the root of the tongue. As the fetus grows, the tissue moves down the neck to rest at its adult position. By the time the fetus is just 12 weeks old, the thyroid has already started to work.

The thyroid is made up of two types of hormone-secreting tissue: follicular cells and parafollicular cells. The follicular cells, which make up the greater part of the thyroid, are hollow spheres surrounded by tiny capillary blood vessels, lymphatic vessels and soft connective tissue. Each follicle is filled with a yellow, semifluid, protein-containing material called thyroglobulin (TG) which, when broken down, interacts with

Figure 2.3 The thyroid gland lies across the windpipe in the throat.

iodine stored in the thyroid to produce thyroid hormone (TH). The parafollicular cells lie on their own or in small clusters in the spaces between the follicles and secrete another hormone – calcitonin.

A Tale of Two Hormones

The thyroid’s main purpose is the production, storage and release of thyroid hormone. Although referred as a single entity, there are, in fact, two thyroid hormones: thyroxine (T₄) and triiodothyronine (T₃), which carry four and three atoms of iodine, respectively.

T₃ is four times more potent than T₄ and works eight times more quickly. Yet, T₄ is about 50 times more abundant in the bloodstream than T₃. This is because, although small amounts of T₄ are converted to T₃ within the thyroid gland itself, most T₃ is produced outside of the thyroid by a process called monodeiodination, which strips away one of the iodine atoms from T₄. This allows the body to produce T₃ as needed – like changing your five-pound notes into one-pound coins for the parking meter.

All Under Control

Like all of the glands in the body, the thyroid is regulated and controlled by the pituitary gland, the small, pea-sized gland attached to the brain often referred to as the ‘master gland’. The pituitary gland orchestrates the entire hormonal symphony but is, in turn, driven by the hypothalamus, to which it is joined by a short stalk of nerve fibres. This hypothalamic – pituitary – thyroid gland connection is a key junction through which chemical messages are carried to and from the brain and the body.

Levels of thyroid hormone are regulated by a feedback loop that operates between the hypothalamus, pituitary and thyroid glands. Anything that increases the body’s need for energy – such as a fall in temperature or a bout of exercise – will provoke the hypothalamus to secrete a chemical messenger called thyrotropin-releasing hormone (TRH) to trigger the pituitary to secrete a messenger chemical called thyrotropin or thyroid-stimulating hormone (TSH), which stimulates the thyroid to secrete thyroid hormone. As the thyroid releases increasing amounts of TH, chemical messages are eventually passed on to the hypothalamus to inhibit production of TRH and, in turn, TSH.

This chemical round-robin means that TSH levels are a highly sensitive indicator of thyroid activity and can provide an early clue that the thyroid is not working as it should. This is why the TSH test is a key investigation in checking the health of your thyroid (see Chapter 4).

The Calcium Connection

The thyroid is crucial in maintaining the strength and density of our bones. The parafollicular cells of the thyroid produce the hormone calcitonin, which is involved in regulating calcium levels in the body. As well as being the main mineral used for making bone, calcium is needed to trigger impulses in nerve and muscle cells.

Calcitonin acts with another hormone – parathyroid hormone (PTH) – produced by the parathyroid glands, four tiny glands that lie behind the thyroid. Whenever calcium is needed, PTH raises the levels of calcium in the blood by stimulating the release of calcium from bone, increasing the reabsorption of calcium from the kidneys and converting vitamin D into a hormone that increases gut absorption of calcium. Once calcium levels have been increased, the thyroid releases calcitonin to suppress the release of calcium from bone.

The Incredible Thyroid

Although the thyroid is only a small gland, the hormone it produces is responsible for an incredible number of biological processes. In fact, it would be fair to say that TH is essential for the health of virtually every cell in your body. Cell growth, muscle strength, body temperature, appetite, cholesterol levels, mood and memory all depend on thyroid hormone. Likewise, your heart, liver, kidneys, reproductive organs, hair and skin all require TH to function properly.

So widespread is the activity of TH that, indeed, in Victorian times, doctors believed the thyroid was vital to life. Although it is possible to live without a thyroid, provided you receive thyroid-hormone replacement therapy, they were not that far from the truth. Thyroid hormone is unique in that, throughout the whole of our lives, it acts within almost every tissue in the body and is vital for general health and wellbeing.

A MATTER OF ENERGY

The main job of thyroid hormone is to regulate your metabolism – the rate of your body’s cell activity. It does this by activating mitochondria, the tiny cellular ‘powerhouses’ that produce energy. The process of metabolism – the word literally means ‘change’ – among other things, controls your appetite and maintains your body temperature, whatever the external environment.

Your metabolism determines the rate at which your cells burn oxygen, a process involved in every activity of life – from breathing and sleeping to eating, talking and moving around – as well as all the activities of your internal organs, such as the beating of your heart, the digesting of your food, the functioning of your reproductive organs and, most important of all, the working of your brain.

A LIFETIME OF ACTIVITY

Thyroid hormone is active when the embryo is still in the womb, where it plays a crucial part in helping each of the millions of cells in our bodies to become more specialized. It is this process, known as differentiation, that turns tadpoles into frogs and a human embryo to develop from a tiny cluster of cells into a fully grown baby.

At the other end of life, the thyroid is thought to play an equally important role in the control of ageing.

THE GROWTH FACTOR

In the womb and after birth, thyroid hormone is vital for both mental and physical growth. With somatotrophin (STH) from the pituitary, it determines the length and strength of your bones. During childhood, lack of TH stunts growth by preventing the bones from growing and maturing. TH is also crucial for the normal development of the brain and nervous system in both the unborn and newly born infant. During pregnancy, low levels of T₄ can affect brain development, resulting in mild-to-severe mental deficiencies. In the past, the term used to describe these defects was ‘mental cretinism’.

BREAST DEVELOPMENT

Thyroid hormone may also be involved in the development of our breasts. Studies in mice have shown that TH affects prolactin, another pituitary hormone. In breastfeeding women, this hormone is involved in stimulating the production of milk. It is also thought that breast pain that is not premenstrual breast tenderness and swelling may be linked to thyroid problems.

PROTECTION AGAINST STARVATION

When it works properly, the thyroid plays a crucial part in keeping your body weight more or less stable. Increasing the amount you eat, especially of starchy foods or carbohydrates, increases metabolism and boosts the production of the active thyroid hormone T₃. Dieting, on the other hand, decreases metabolism, causing the body to produce less T₃.

This is almost certainly a mechanism that has evolved to protect us from starvation. It is a known fact that when the body is deprived of food, it turns down the rate of metabolism. This is one of the mechanisms thought to have enabled the survival of the babies who, incredibly, were found still alive after several days trapped under the rubble of the Mexican earthquake in the 1980s. This same mechanism also explains why the thyroid becomes sluggish in women with eating disorders such as anorexia, bulimia and excessive dieting. The brain correctly perceives these states as starvation and turns down thyroid activity to conserve energy. This is how a thyroid problem can play havoc with your appetite and your weight.

PROTECTION AGAINST INFECTION

The thyroid is a vital part of the body’s immune-defence mechanism. It stimulates the production of special white blood cells, known as T cells and B cells, to help the body fight against disease. Chronic liver and kidney disease, acute and chronic illness, starvation and diets too low in carbohydrate lower the production of T3. It is thought that this may be part of an adaptive process to help the body defend itself against illness.

FLUID BALANCE

The thyroid plays a vital role in a myriad other bodily processes. It helps to maintain the body’s fluid balance by controlling the mechanisms by which water and chemicals enter and leave the cells – one reason why bloating is troublesome if you have an underactive thyroid.

VITAMIN POWER

In the liver, thyroid hormones are needed to convert beta-carotene (the pigment that gives orange, yellow and red fruits and vegetables their colour) into vitamin A. In the past few years, beta-carotene has sparked considerable interest as one of the three key antioxidant vitamins (the other two are vitamins C and E) that play a crucial role in protecting the body against degenerative diseases, such as cancer and heart disease, and those associated with ageing.

INTERACTIONS WITH OTHER HORMONES

Thyroid hormone needs to be present for other hormones to function in various parts of the body. Most important for women, it acts in concert with the female sex hormone oestrogen to modulate reproduction. This is why thyroid malfunction can sometimes be a cause of reduced fertility and other reproductive problems.