Thyroid gland

Two of the body’s important hormone-producing glands are sited close together in the neck. They are the thyroid and parathyroid glands. These glands are involved in the day-to-day control of body chemistry (metabolism), which in turn affects long-term processes such as growth and maturation.

The thyroid gland

The thyroid, the larger of the two glands, is located in the front of the lower neck. It has two sections, or lobes, one on each side of the windpipe, connected by a narrow part of the gland called the isthmus. In an adult, each lobe measures about five centimetres long by three centimetres at its widest part. Normally the thyroid cannot be seen, but there are certain conditions during which it can enlarge to produce a swelling in the neck known as a goitre.

Thyroid hormones

The thyroid produces two important hormones, thyroxine (T.,) and tri-iodothyronine (T,), which have similar functions. Tri-iodothyronine is present in only very small concentrations in the blood, but weight-for-weight it is more potent than thyroxine. The main function of these two thyroid hormones is to control the rate at which chemical reactions take place inside the cells of the body. More specifically, they affect the biochemical processes that generate the energy-rich molecules a cell needs to perform its normal functions. Technically this is known as the metabolic rate. In a simple analogy, an electric motor’s speed is controlled by the voltage fed to it through a control dial; the thyroid hormones act as the hands on the dial that turn it to speed up or slow down the motor.

By controlling intracellular energy supply, thyroid hormones dictate the ‘speed’ with which most organs of the body, such as the heart, muscles and bowels, function. As a consequence they also control heat production in the body, because part of the energy generated during chemical reactions is released as heat. They are essential for normal brain development during the first years of life, for normal growth during childhood, for sexual maturation, and for normal mental functioning at all ages. Exactly how thyroid hormones exert their effects on cells has not yet been fully elucidated. As implied in the analogy above, they are not directly involved in the energy-producing process; they are the hands on the dial, rather than the voltage regulator itself. From our current knowledge two mechanisms appear to be involved. First, thyroid hormones increase metabolic rate by promoting the formation of certain substances known as enzymes, which have a direct accelerating effect on chemical reactions in cells. Usually, the more of an enzyme present, the faster the reaction it controls can proceed. Second, thyroid hormones increase the number and activity of mitochondria in a cell. Mitochondria are the sausage-shaped organelles in which oxygen reacts with glucose to generate energy-rich molecules that can be utilized elsewhere in the cell, to drive various metabolic processes. Oxygen consumption is therefore a measure of metabolic rate and an index of the physiological activity of thyroid hormones.

Scattered within the thyroid gland are small clumps of cells known as C-cells because they secrete the third thyroid hormone, calcitonin. As its name implies, calcitonin has an effect on calcium metabolism but its action is relatively weak and unimportant compared to the action of the hormone of the parathyroid glands (parathormone)and vitamin D.

Control of thyroid activity

Under normal conditions the two main thyroid hormones maintain bodily activities at the right level via a feedback system that acts through the pituitary gland, just below the brain. The secretion of thyroid hormones is controlled by another hormone, thyroid-stimulating hormone or TSH, made by the pituitary; production of TSH is in turn affected by levels of thyroid hormones circulating in the blood. Normally the system works well. However if the thyroid, or pituitary, or both, fail then thyroid hormone levels go awry. If deprived of thyroid hormones, individual organs and the body as a whole become much less active resulting from an inadequate energy supply; body temperature also falls. Conversely, if thyroid hormones are excessive, chemical reactions and energy production in cells are too fast, various organs become overactive, and body temperature rises. One factor which is important to the level of thyroid hormones is the supply of the element iodine. Iodine is a vital constituent of both main thyroid hormones, and an adequate intake of iodine in the diet is essential to ensure normal secretion by the thyroid gland. The main dietary sources of iodine are seafoods, milk, fish and ordinary drinking water. Dietary intake is usually sufficient in most parts of the world, but in some inland or mountainous areas remote from the sea, dietary iodine deficiency can be an important cause of goitre in which the thyroid enlarges in an attempt to produce more hormones. In some countries iodine is added to staple supplies such as bread, salt or drinking water, to ensure everyone has an adequate intake.

Thyroid deficiency in a newborn child, if unrecognized and untreated, leads to mental deficiency and impaired growth, sometimes known as cretinism. In the older child deficiency causes growth retardation and delayed puberty. Diagnosis is relatively simple, and can be carried out by measuring the levels of hormones in a blood sample. The two main types of thyroid disorder in adults are known medically as hyperthyroidism and hypothyroidism. Hyperthyroidism involves an overactive thyroid gland, which often swells to form a goitre. The patient is usually excitable and nervous, with a fast pulse rate and a low sleep requirement. These symptoms result from the fact that the metabolic rate is greatly increased. The patient’s eyeballs may protrude (exophthalmos), a characteristic symptom of the type of hyperthyroidism known as Graves’s disease. A common cause of the condition is a tumour of the thyroid gland, which may be treated by surgery (to remove the tumour), by radiotherapy or with drugs. Underactivity of the thyroid gland, or hypothyroidism, may result from iodine deficiency. The gland enlarges in an attempt to produce more hormones, and the result may again be the neck swelling of goitre. Inflammation of the thyroid or an accumulation within it of fibrous tissue, as in Hashimoto’s disease, are other possible causes. The latter condition may be accompanied by myxoedema, common in middle-aged women, which results in loss of hair and a swollen face lacking in expression. Hypothyroidism is generally treated by administering the missing thyroid hormone.

The parathyroid glands

These consist of four small, separate glands located behind or within the thyroid. This location is a nuisance because surgery on the thyroid gland may sometimes result in accidentally removing the glands. The hormone produced by the parathyroid glands is parathormone, whose main influence is on the amount of calcium in the blood. Because calcium performs many tasks in the body, it is obvious that this function is important. Calcium plays a role, for example, in the contraction of heart muscle, the transmission of impulses in nerves, the hardness of bones and teeth and it is necessary for a normal coagulation of blood following a wound. Parathormone heightens the blood level of calcium by lowering the amount of calcium lost in the urine and by stimulating the removal of calcium from bones. Calcium levels are also influenced by another hormone, calcitonin, produced in the thyroid glands. This hormone more-or-less opposes the calcium-heightening effects of parathormone.

This clearcut picture gets a little bit more complicated when one takes into account that vitamin D also plays a role. This vitamin partly has the same effect as parathormone (it also heightens the blood levels of calcium, because it makes the uptake of calcium in the gut more efficient), but unlike parathormone it stimulates the deposition of calcium in the bones. Excessive production of parathormone, caused by enlargement of one or more parathyroid glands, results in raised blood calcium levels. This can lead to formation of calcium stones in the kidneys and pains in the bones because of decalcification and softening. Undersecretion of parathormone leads to low blood calcium levels and symptoms such as tingling, twitching and painful cramps in the limbs.