The Healthy Thyroid: What you can do to prevent and alleviate thyroid imbalance. Patsy Westcott

      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.

      Manufacturing Thyroid Hormone

      The mineral iodine – a trace element found in soil and food – plays a central role in the manufacture of thyroid hormone. Iodine is needed for cells to work properly. To produce hormone, the thyroid absorbs iodine and, through a process involving enzymes, combines it with the amino acid tyrosine. More enzyme reactions convert this into T₄ and T₃, which are then stored by the thyroid within a protein called thyroglobulin (TG). When thyroid hormone is then needed in the body, enzymes break down this TG to release the T₄ and T3 into the bloodstream.

      Transporting Thyroid Hormone

      Most of the thyroid hormone in the body is carried around the bloodstream attached to special transport proteins, especially thyroid-binding globulin (TBG). Once the bound thyroid hormone reaches its destination, it is released from the protein binding so that T₄ can be converted to T₃ and ready for use by the cells. A tiny amount – around 0.03 per cent of T₄ and 0.3 per cent of T₃ – remains unattached to float freely about in the blood. Although only a small quantity, free-floating T₃ does not have to be released from any binding and so is immediately available for use by the cells.

      Certain conditions, such as taking the Pill, can raise the levels of protein in the blood and, in the past, thyroid tests which measured total levels of T₄ and T₃ were not always accurate because of some confusion in interpreting results. Today’s blood tests measure levels of both free-floating T₄ and bound T₄ as well as thyroid-stimulating hormone, which provide a much more accurate indication of thyroid function.

      What Can Go Wrong?

      The most common thing to go wrong with the thyroid is autoimmune thyroid disease, when the body turns against its own tissues and tries to destroy them. Over a period of time, this causes the thyroid to become either overactive (hyperthyroidism) or underactive (hypothyroidism). The result is Graves’ disease, the most common form of hyperthyroidism, and thyroiditis, the most common form of hypothyroidism.

      But why should a mechanism designed to protect our body and keep it healthy go so drastically wrong? To find the answer, it is necessary to delve deeper into the fascinating science of immunology.

      Immune Reactions

      The body’s defence system normally provides a formidable barrier against attack by ‘foreign’ invaders such as viruses, bacteria and parasites. Although the way it works is still not fully understood, what is known is that many actions depend on two kinds of lymphocytes (white blood cells) – T cells and B cells – responsible for fending off attackers from outside.

      When the body is under attack by invaders, the immune system sends T cells to the affected site to find out what is happening. There are two types of T cells: helper cells and killer cells. Helper cells help the immune system by identifying antigens, a chemical substance that marks the invaders as ‘foreign’. Once the helper cells have recognized a foreign antigen, killer cells are despatched to attack and destroy them. To protect the body against future attack by the same foreign invaders, killer T cells retain a ‘memory’ of their antigen. If the body is threatened again by the same invader, these killer T cells are quickly activated and sent in for the kill. This entire process is known as cellular immunity.

      B cells work in a similar way except that they fight off an attack by producing protein antibodies known as immunoglobulins; these are produced specific to the invader. When the immune system identifies a particular invader, B cells are stimulated to produce a large quantity of a specific immunoglobulin that will attach itself to the invading antigens and immobilize them. Once this has happened, the antigens are devoured by other white cells called phagocytes. This process is known as humoral immunity.

      The Enemy Within

      Under normal circumstances, the immune system does not turn against itself because our own cells are coded to allow our T cells and B cells to recognize them as ‘self and refrain from attacking them. Scientists still do not know exactly why this normal recognition process fails. One theory is that, in some cases, a foreign antigen – say, a protein on a virus – escapes the immune system’s surveillance by disguising itself as one the body’s own cells. As the immune system cannot distinguish this disguised protein from its own tissue, it allows the invader access to the cells.

      Inflammation From Within

      Another way in which the immune system can go awry is when an invader triggers an overzealous immune response, unleashing a flood of cell proteins called cytokines; these cause inflammation and increase the production of antibodies, which turn against the tissue in question and destroy it.

      Both Graves’ disease and Hashimoto’s thyroiditis are caused by T and B cells that infiltrate the thyroid, triggering inflammation and the production of thyroid autoantibodies. Depending on which autoantibodies are produced, this will lead to either overproduction or underproduction of thyroid hormone.

      Autoantibodies can attack virtually any of the body’s tissues or organs and not just the thyroid, causing a range of diseases in which the tissues